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Houston Comets – Wikipedia

The Houston Comets were a Women's National Basketball Association (WNBA) team based in Houston, Texas, United States.Formed in 1997, the team was one of the original eight WNBA teams and won the first four championships of the league's existence. They are one of two teams in the WNBA that are undefeated in the WNBA Finals; the Seattle Storm are the other (however, the Storm are still in ...

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Houston Comets - Wikipedia

In Depth | Comets NASA Solar System Exploration

Overview In the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions.

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In Depth | Comets NASA Solar System Exploration

Comet – Wikipedia

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, some comets may have a higher dust content, leading them to be called "icy dirtballs".

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Comet - Wikipedia

Astronomy – Wikipedia

Not to be confused with astrology, the pseudoscience.

Astronomy (from Greek: ) is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, and comets; the phenomena also includes supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, which is the study of the Universe as a whole.[1]

Astronomy is one of the oldest of the natural sciences. The early civilizations in recorded history, such as the Babylonians, Greeks, Indians, Egyptians, Nubians, Iranians, Chinese, Maya, and many ancient indigenous peoples of the Americas, performed methodical observations of the night sky. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, and the making of calendars, but professional astronomy is now often considered to be synonymous with astrophysics.[2]

Professional astronomy is split into observational and theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, which is then analyzed using basic principles of physics. Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results.

Astronomy is one of the few sciences in which amateurs still play an active role, especially in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets.

Astronomy (from the Greek from astron, "star" and - -nomia from nomos, "law" or "culture") means "law of the stars" (or "culture of the stars" depending on the translation). Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects.[5] Although the two fields share a common origin, they are now entirely distinct.[6]

Generally, both of the terms "astronomy" and "astrophysics" may be used to refer to the same subject.[7][8][9] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties,"[10] while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena."[11] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.[12] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.[7] Some fields, such as astrometry, are purely astronomy rather than also astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,[8] and many professional astronomers have physics rather than astronomy degrees.[9] Some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, and Astronomy and Astrophysics.

In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye. In some locations, early cultures assembled massive artifacts that possibly had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year.[13]

Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye. As civilizations developed, most notably in Mesopotamia, Greece, Persia, India, China, Egypt, and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the Universe were explored philosophically. The Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Universe, or the Ptolemaic system, named after Ptolemy.[14]

A particularly important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the later astronomical traditions that developed in many other civilizations.[15] The Babylonians discovered that lunar eclipses recurred in a repeating cycle known as a saros.[16]

Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena.[17] In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, and he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model.[18] In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and invented the earliest known astronomical devices such as the astrolabe.[19] Hipparchus also created a comprehensive catalog of 1020 stars, and most of the constellations of the northern hemisphere derive from Greek astronomy.[20] The Antikythera mechanism (c. 15080 BC) was an early analog computer designed to calculate the location of the Sun, Moon, and planets for a given date. Technological artifacts of similar complexity did not reappear until the 14th century, when mechanical astronomical clocks appeared in Europe.[21]

Medieval Europe housed a number of important astronomers. Richard of Wallingford (12921336) made major contributions to astronomy and horology, including the invention of the first astronomical clock, the Rectangulus which allowed for the measurement of angles between planets and other astornomical bodies, as well as an equatorium called the Albion which could be used for astronomical calculations such as lunar, solar and planetary longitudes and could predict eclipses. Nicole Oresme (13201382) and Jean Buridan (13001361) first discussed evidence for the rotation of the Earth, furthermore, Buridan also developed the theory of impetus (predecessor of the modern scientific theory of inertia) which was able to show planets were capable of motion without the intervention of angels.[22] Georg von Peuerbach (14231461) and Regiomontanus (14361476) helped make astronomical progress instrumental to Copernicus's development of the heliocentric model decades later.

Astronomy flourished in the Islamic world and other parts of the world. This led to the emergence of the first astronomical observatories in the Muslim world by the early 9th century.[23][24][25] In 964, the Andromeda Galaxy, the largest galaxy in the Local Group, was described by the Persian Muslim astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars.[26] The SN 1006 supernova, the brightest apparent magnitude stellar event in recorded history, was observed by the Egyptian Arabic astronomer Ali ibn Ridwan and Chinese astronomers in 1006. Some of the prominent Islamic (mostly Persian and Arab) astronomers who made significant contributions to the science include Al-Battani, Thebit, Abd al-Rahman al-Sufi, Albumasar, Biruni, Ab Ishq Ibrhm al-Zarql, Al-Birjandi, and the astronomers of the Maragheh and Samarkand observatories. Persian astrologer Albumasar's practical manuals for training astrologers profoundly influenced Muslim intellectual history and, through translations, that of western Europe and Byzantium.[27] His work was probably the single most important original source for the recovery of Aristotle for medieval European scholars prior to the middle of the 12th century.[28] The criticism of Ptolemy by Averroes directly influenced the Copernicus's heliocentrism.[29] Astronomers during that time introduced many Arabic names now used for individual stars.[30][31] It is also believed that the ruins at Great Zimbabwe and Timbuktu[32] may have housed astronomical observatories.[33] Europeans had previously believed that there had been no astronomical observation in sub-Saharan Africa during the pre-colonial Middle Ages, but modern discoveries show otherwise.[34][35][36][37]

For over six centuries (from the recovery of ancient learning during the late Middle Ages into the Enlightenment), the Roman Catholic Church gave more financial and social support to the study of astronomy than probably all other institutions. Among the Church's motives was finding the date for Easter.[38]

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended by Galileo Galilei and expanded upon by Johannes Kepler. Kepler was the first to devise a system that correctly described the details of the motion of the planets around the Sun. However, Kepler did not succeed in formulating a theory behind the laws he wrote down.[39] It was Isaac Newton, with his invention of celestial dynamics and his law of gravitation, who finally explained the motions of the planets. Newton also developed the reflecting telescope.[40]

Improvements in the size and quality of the telescope led to further discoveries. The English astronomer John Flamsteed catalogued over 3000 stars,[41] More extensive star catalogues were produced by Nicolas Louis de Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found.[42] The distance to a star was announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.[43]

During the 1819th centuries, the study of the three-body problem by Leonhard Euler, Alexis Claude Clairaut, and Jean le Rond d'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Joseph-Louis Lagrange and Pierre Simon Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.[44]

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Joseph von Fraunhofer discovered about 600 bands in the spectrum of the Sun in 181415, which, in 1859, Gustav Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.[30]

The existence of the Earth's galaxy, the Milky Way, as its own group of stars was only proved in the 20th century, along with the existence of "external" galaxies. The observed recession of those galaxies led to the discovery of the expansion of the Universe.[45] Theoretical astronomy led to speculations on the existence of objects such as black holes and neutron stars, which have been used to explain such observed phenomena as quasars, pulsars, blazars, and radio galaxies. Physical cosmology made huge advances during the 20th century. In the early 1900s the model of the Big Bang theory was formulated, heavily evidenced by cosmic microwave background radiation, Hubble's law, and the cosmological abundances of elements. Space telescopes have enabled measurements in parts of the electromagnetic spectrum normally blocked or blurred by the atmosphere.[citation needed] In February 2016, it was revealed that the LIGO project had detected evidence of gravitational waves in the previous September.[46][47]

The main source of information about celestial bodies and other objects is visible light, or more generally electromagnetic radiation.[48] Observational astronomy may be categorized according to the corresponding region of the electromagnetic spectrum on which the observations are made. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere. Specific information on these subfields is given below.

Radio astronomy uses radiation with wavelengths greater than approximately one millimeter, outside the visible range.[49] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.[49]

Although some radio waves are emitted directly by astronomical objects, a product of thermal emission, most of the radio emission that is observed is the result of synchrotron radiation, which is produced when electrons orbit magnetic fields.[49] Additionally, a number of spectral lines produced by interstellar gas, notably the hydrogen spectral line at 21cm, are observable at radio wavelengths.[12][49]

A wide variety of other objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.[12][49]

Infrared astronomy is founded on the detection and analysis of infrared radiation, wavelengths longer than red light and outside the range of our vision. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets, circumstellar disks or nebulae whose light is blocked by dust. The longer wavelengths of infrared can penetrate clouds of dust that block visible light, allowing the observation of young stars embedded in molecular clouds and the cores of galaxies. Observations from the Wide-field Infrared Survey Explorer (WISE) have been particularly effective at unveiling numerous Galactic protostars and their host star clusters.[51][52]With the exception of infrared wavelengths close to visible light, such radiation is heavily absorbed by the atmosphere, or masked, as the atmosphere itself produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places on Earth or in space.[53] Some molecules radiate strongly in the infrared. This allows the study of the chemistry of space; more specifically it can detect water in comets.[54]

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.[55] Images of observations were originally drawn by hand. In the late 19th century and most of the 20th century, images were made using photographic equipment. Modern images are made using digital detectors, particularly using charge-coupled devices (CCDs) and recorded on modern medium. Although visible light itself extends from approximately 4000 to 7000 (400 nm to 700nm),[55] that same equipment can be used to observe some near-ultraviolet and near-infrared radiation.

Ultraviolet astronomy employs ultraviolet wavelengths between approximately 100 and 3200 (10 to 320nm).[49] Light at those wavelengths is absorbed by the Earth's atmosphere, requiring observations at these wavelengths to be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (OB stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.[49] However, as ultraviolet light is easily absorbed by interstellar dust, an adjustment of ultraviolet measurements is necessary.[49]

X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 107 (10million) kelvins, and thermal emission from thick gases above 107 Kelvin.[49] Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.[49]

Gamma ray astronomy observes astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.[49] The Cherenkov telescopes do not detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.[56]

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.[49]

In addition to electromagnetic radiation, a few other events originating from great distances may be observed from the Earth.

In neutrino astronomy, astronomers use heavily shielded underground facilities such as SAGE, GALLEX, and Kamioka II/III for the detection of neutrinos. The vast majority of the neutrinos streaming through the Earth originate from the Sun, but 24 neutrinos were also detected from supernova 1987A.[49] Cosmic rays, which consist of very high energy particles (atomic nuclei) that can decay or be absorbed when they enter the Earth's atmosphere, result in a cascade of secondary particles which can be detected by current observatories.[57] Some future neutrino detectors may also be sensitive to the particles produced when cosmic rays hit the Earth's atmosphere.[49]

Gravitational-wave astronomy is an emerging field of astronomy that employs gravitational-wave detectors to collect observational data about distant massive objects. A few observatories have been constructed, such as the Laser Interferometer Gravitational Observatory LIGO. LIGO made its first detection on 14 September 2015, observing gravitational waves from a binary black hole.[58] A second gravitational wave was detected on 26 December 2015 and additional observations should continue but gravitational waves require extremely sensitive instruments.[59][60]

The combination of observations made using electromagnetic radiation, neutrinos or gravitational waves and other complementary information, is known as multi-messenger astronomy.[61][62]

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation (the use of celestial objects to guide navigation) and in the making of calendars.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters or potential collisions of the Earth with those objects.[63]

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the Universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, as their properties can be compared. Measurements of the radial velocity and proper motion of stars allows astronomers to plot the movement of these systems through the Milky Way galaxy. Astrometric results are the basis used to calculate the distribution of speculated dark matter in the galaxy.[64]

During the 1990s, the measurement of the stellar wobble of nearby stars was used to detect large extrasolar planets orbiting those stars.[65]

Theoretical astronomers use several tools including analytical models and computational numerical simulations; each has its particular advantages. Analytical models of a process are generally better for giving broader insight into the heart of what is going on. Numerical models reveal the existence of phenomena and effects otherwise unobserved.[66][67]

Theorists in astronomy endeavor to create theoretical models and from the results predict observational consequences of those models. The observation of a phenomenon predicted by a model allows astronomers to select between several alternate or conflicting models as the one best able to describe the phenomena.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency between the data and model's results, the general tendency is to try to make minimal modifications to the model so that it produces results that fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.

Phenomena modeled by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale distribution of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.

A few examples of this process:

Dark matter and dark energy are the current leading topics in astronomy,[68] as their discovery and controversy originated during the study of the galaxies.

Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the astronomical objects, rather than their positions or motions in space".[69][70] Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background.[71][72] Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, astrophysicists typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.

In practice, modern astronomical research often involves a substantial amount of work in the realms of theoretical and observational physics. Some areas of study for astrophysicists include their attempts to determine the properties of dark matter, dark energy, and black holes; whether or not time travel is possible, wormholes can form, or the multiverse exists; and the origin and ultimate fate of the universe.[71] Topics also studied by theoretical astrophysicists include Solar System formation and evolution; stellar dynamics and evolution; galaxy formation and evolution; magnetohydrodynamics; large-scale structure of matter in the universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics.

Astrochemistry is the study of the abundance and reactions of molecules in the Universe, and their interaction with radiation.[73] The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.

Studies in this field contribute to the understanding of the formation of the Solar System, Earth's origin and geology, abiogenesis, and the origin of climate and oceans.

Astrobiology is an interdisciplinary scientific field concerned with the origins, early evolution, distribution, and future of life in the universe. Astrobiology considers the question of whether extraterrestrial life exists, and how humans can detect it if it does.[74] The term exobiology is similar.[75]

Astrobiology makes use of molecular biology, biophysics, biochemistry, chemistry, astronomy, physical cosmology, exoplanetology and geology to investigate the possibility of life on other worlds and help recognize biospheres that might be different from that on Earth.[76] The origin and early evolution of life is an inseparable part of the discipline of astrobiology.[77] Astrobiology concerns itself with interpretation of existing scientific data, and although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories.

This interdisciplinary field encompasses research on the origin of planetary systems, origins of organic compounds in space, rock-water-carbon interactions, abiogenesis on Earth, planetary habitability, research on biosignatures for life detection, and studies on the potential for life to adapt to challenges on Earth and in outer space.[78][79][80]

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Cosmology (from the Greek (kosmos) "world, universe" and (logos) "word, study" or literally "logic") could be considered the study of the Universe as a whole.

Observations of the large-scale structure of the Universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our Universe began at a single point in time, and thereafter expanded over the course of 13.8 billion years[81] to its present condition.[82] The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.[82]

In the course of this expansion, the Universe underwent several evolutionary stages. In the very early moments, it is theorized that the Universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early Universe.[82] (See also nucleocosmochronology.)

When the first neutral atoms formed from a sea of primordial ions, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding Universe then underwent a Dark Age due to the lack of stellar energy sources.[83]

A hierarchical structure of matter began to form from minute variations in the mass density of space. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars, the Population III stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early Universe, which, through nuclear decay, create lighter elements, allowing the cycle of nucleosynthesis to continue longer.[84]

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.[85]

Various fields of physics are crucial to studying the universe. Interdisciplinary studies involve the fields of quantum mechanics, particle physics, plasma physics, condensed matter physics, statistical mechanics, optics, and nuclear physics.

Fundamental to the structure of the Universe is the existence of dark matter and dark energy. These are now thought to be its dominant components, forming 96% of the mass of the Universe. For this reason, much effort is expended in trying to understand the physics of these components.[86]

The study of objects outside our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies, their morphology (description) and classification, the observation of active galaxies, and at a larger scale, the groups and clusters of galaxies. Finally, the latter is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.[87]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust, few star-forming regions, and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may have been formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation within which massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that emits a significant amount of its energy from a source other than its stars, dust and gas. It is powered by a compact region at the core, thought to be a super-massive black hole that is emitting radiation from in-falling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit shorter frequency, high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.[88]

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized into a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids between.[89]

The Solar System orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at its center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population I stars. The disk is surrounded by a spheroid halo of older, population II stars, as well as relatively dense concentrations of stars known as globular clusters.[90]

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as a compact pre-stellar core or dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.[91]

As the more massive stars appear, they transform the cloud into an H II region (ionized atomic hydrogen) of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually cause the cloud to disperse, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.[92]

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.[93]

The study of stars and stellar evolution is fundamental to our understanding of the Universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.[94] Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.[91]

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Astronomy - Wikipedia

astronomy | Definition & Facts | Britannica.com

Since the late 19th century astronomy has expanded to include astrophysics, the application of physical and chemical knowledge to an understanding of the nature of celestial objects and the physical processes that control their formation, evolution, and emission of radiation. In addition, the gases and dust particles around and between the stars have become the subjects of much research. Study of the nuclear reactions that provide the energy radiated by stars has shown how the diversity of atoms found in nature can be derived from a universe that, following the first few minutes of its existence, consisted only of hydrogen, helium, and a trace of lithium. Concerned with phenomena on the largest scale is cosmology, the study of the evolution of the universe. Astrophysics has transformed cosmology from a purely speculative activity to a modern science capable of predictions that can be tested.

Its great advances notwithstanding, astronomy is still subject to a major constraint: it is inherently an observational rather than an experimental science. Almost all measurements must be performed at great distances from the objects of interest, with no control over such quantities as their temperature, pressure, or chemical composition. There are a few exceptions to this limitationnamely, meteorites (most of which are from the asteroid belt, though some are from the Moon or Mars), rock and soil samples brought back from the Moon, samples of comet and asteroid dust returned by robotic spacecraft, and interplanetary dust particles collected in or above the stratosphere. These can be examined with laboratory techniques to provide information that cannot be obtained in any other way. In the future, space missions may return surface materials from Mars, or other objects, but much of astronomy appears otherwise confined to Earth-based observations augmented by observations from orbiting satellites and long-range space probes and supplemented by theory.

The solar system took shape 4.57 billion years ago, when it condensed within a large cloud of gas and dust. Gravitational attraction holds the planets in their elliptical orbits around the Sun. In addition to Earth, five major planets (Mercury, Venus, Mars, Jupiter, and Saturn) have been known from ancient times. Since then only two more have been discovered: Uranus by accident in 1781 and Neptune in 1846 after a deliberate search following a theoretical prediction based on observed irregularities in the orbit of Uranus. Pluto, discovered in 1930 after a search for a planet predicted to lie beyond Neptune, was considered a major planet until 2006, when it was redesignated a dwarf planet by the International Astronomical Union.

The average Earth-Sun distance, which originally defined the astronomical unit (AU), provides a convenient measure for distances within the solar system. The astronomical unit was originally defined by observations of the mean radius of Earths orbit but is now defined as 149,597,870.7 km (about 93 million miles). Mercury, at 0.4 AU, is the closest planet to the Sun, while Neptune, at 30.1 AU, is the farthest. Plutos orbit, with a mean radius of 39.5 AU, is sufficiently eccentric that at times it is closer to the Sun than is Neptune. The planes of the planetary orbits are all within a few degrees of the ecliptic, the plane that contains Earths orbit around the Sun. As viewed from far above Earths North Pole, all planets move in the same (counterclockwise) direction in their orbits.

Most of the mass of the solar system is concentrated in the Sun, with its 1.99 1033 grams. Together, all of the planets amount to 2.7 1030 grams (i.e., about one-thousandth of the Suns mass), and Jupiter alone accounts for 71 percent of this amount. The solar system also contains five known objects of intermediate size classified as dwarf planets and a very large number of much smaller objects collectively called small bodies. The small bodies, roughly in order of decreasing size, are the asteroids, or minor planets; comets, including Kuiper belt, Centaur, and Oort cloud objects; meteoroids; and interplanetary dust particles. Because of their starlike appearance when discovered, the largest of these bodies were termed asteroids, and that name is widely used, but, now that the rocky nature of these bodies is understood, their more descriptive name is minor planets.

The four inner, terrestrial planetsMercury, Venus, Earth, and Marsalong with the Moon have average densities in the range of 3.95.5 grams per cubic cm, setting them apart from the four outer, giant planetsJupiter, Saturn, Uranus, and Neptunewhose densities are all close to 1 gram per cubic cm, the density of water. The compositions of these two groups of planets must therefore be significantly different. This dissimilarity is thought to be attributable to conditions that prevailed during the early development of the solar system (see below Theories of origin). Planetary temperatures now range from around 170 C (330 F, 440 K) on Mercurys surface through the typical 15 C (60 F, 290 K) on Earth to 135 C (210 F, 140 K) on Jupiter near its cloud tops and down to 210 C (350 F, 60 K) near Neptunes cloud tops. These are average temperatures; large variations exist between dayside and nightside for planets closest to the Sun, except for Venus with its thick atmosphere.

The surfaces of the terrestrial planets and many satellites show extensive cratering, produced by high-speed impacts (see meteorite crater). On Earth, with its large quantities of water and an active atmosphere, many of these cosmic footprints have eroded, but remnants of very large craters can be seen in aerial and spacecraft photographs of the terrestrial surface. On Mercury, Mars, and the Moon, the absence of water and any significant atmosphere has left the craters unchanged for billions of years, apart from disturbances produced by infrequent later impacts. Volcanic activity has been an important force in the shaping of the surfaces of the Moon and the terrestrial planets. Seismic activity on the Moon has been monitored by means of seismometers left on its surface by Apollo astronauts and by Lunokhod robotic rovers. Cratering on the largest scale seems to have ceased about three billion years ago, although on the Moon there is clear evidence for a continued cosmic drizzle of small particles, with the larger objects churning (gardening) the lunar surface and the smallest producing microscopic impact pits in crystals in the lunar rocks.

All of the planets apart from the two closest to the Sun (Mercury and Venus) have natural satellites (moons) that are very diverse in appearance, size, and structure, as revealed in close-up observations from long-range space probes. The four outer dwarf planets have moons; Pluto has at least five moons, including one, Charon, fully half the size of Pluto itself. Over 200 asteroids and 80 Kuiper belt objects also have moons. Four planets (Jupiter, Saturn, Uranus, and Neptune), one dwarf planet (Haumea), and one Centaur object (Chariklo) have rings, disklike systems of small rocks and particles that orbit their parent bodies.

During the U.S. Apollo missions a total weight of 381.7 kg (841.5 pounds) of lunar material was collected; an additional 300 grams (0.66 pounds) was brought back by unmanned Soviet Luna vehicles. About 15 percent of the Apollo samples have been distributed for analysis, with the remainder stored at the NASA Johnson Space Center, Houston, Texas. The opportunity to employ a wide range of laboratory techniques on these lunar samples has revolutionized planetary science. The results of the analyses have enabled investigators to determine the composition and age of the lunar surface. Seismic observations have made it possible to probe the lunar interior. In addition, retroreflectors left on the Moons surface by Apollo astronauts have allowed high-power laser beams to be sent from Earth to the Moon and back, permitting scientists to monitor the Earth-Moon distance to an accuracy of a few centimetres. This experiment, which has provided data used in calculations of the dynamics of the Earth-Moon system, has shown that the separation of the two bodies is increasing by 4.4 cm (1.7 inches) each year. (For additional information on lunar studies, see Moon.)

Mercury is too hot to retain an atmosphere, but Venuss brilliant white appearance is the result of its being completely enveloped in thick clouds of carbon dioxide, impenetrable at visible wavelengths. Below the upper clouds, Venus has a hostile atmosphere containing clouds of sulfuric acid droplets. The cloud cover shields the planets surface from direct sunlight, but the energy that does filter through warms the surface, which then radiates at infrared wavelengths. The long-wavelength infrared radiation is trapped by the dense clouds such that an efficient greenhouse effect keeps the surface temperature near 465 C (870 F, 740 K). Radar, which can penetrate the thick Venusian clouds, has been used to map the planets surface. In contrast, the atmosphere of Mars is very thin and is composed mostly of carbon dioxide (95 percent), with very little water vapour; the planets surface pressure is only about 0.006 that of Earth. The outer planets have atmospheres composed largely of light gases, mainly hydrogen and helium.

Each planet rotates on its axis, and nearly all of them rotate in the same directioncounterclockwise as viewed from above the ecliptic. The two exceptions are Venus, which rotates in the clockwise direction beneath its cloud cover, and Uranus, which has its rotation axis very nearly in the plane of the ecliptic.

Some of the planets have magnetic fields. Earths field extends outward until it is disturbed by the solar windan outward flow of protons and electrons from the Sunwhich carries a magnetic field along with it. Through processes not yet fully understood, particles from the solar wind and galactic cosmic rays (high-speed particles from outside the solar system) populate two doughnut-shaped regions called the Van Allen radiation belts. The inner belt extends from about 1,000 to 5,000 km (600 to 3,000 miles) above Earths surface, and the outer from roughly 15,000 to 25,000 km (9,300 to 15,500 miles). In these belts, trapped particles spiral along paths that take them around Earth while bouncing back and forth between the Northern and Southern hemispheres, with their orbits controlled by Earths magnetic field. During periods of increased solar activity, these regions of trapped particles are disturbed, and some of the particles move down into Earths atmosphere, where they collide with atoms and molecules to produce auroras.

Jupiter has a magnetic field far stronger than Earths and many more trapped electrons, whose synchrotron radiation (electromagnetic radiation emitted by high-speed charged particles that are forced to move in curved paths, as under the influence of a magnetic field) is detectable from Earth. Bursts of increased radio emission are correlated with the position of Io, the innermost of the four Galilean moons of Jupiter. Saturn has a magnetic field that is much weaker than Jupiters, but it too has a region of trapped particles. Mercury has a weak magnetic field that is only about 1 percent as strong as Earths and shows no evidence of trapped particles. Uranus and Neptune have fields that are less than one-tenth the strength of Saturns and appear much more complex than that of Earth. No field has been detected around Venus or Mars.

More than 500,000 asteroids with well-established orbits are known, and thousands of additional objects are discovered each year. Hundreds of thousands more have been seen, but their orbits have not been as well determined. It is estimated that several million asteroids exist, but most are small, and their combined mass is estimated to be less than a thousandth that of Earth. Most of the asteroids have orbits close to the ecliptic and move in the asteroid belt, between 2.3 and 3.3 AU from the Sun. Because some asteroids travel in orbits that can bring them close to Earth, there is a possibility of a collision that could have devastating results (see Earth impact hazard).

Comets are considered to come from a vast reservoir, the Oort cloud, orbiting the Sun at distances of 20,00050,000 AU or more and containing trillions of icy objectslatent comet nucleiwith the potential to become active comets. Many comets have been observed over the centuries. Most make only a single pass through the inner solar system, but some are deflected by Jupiter or Saturn into orbits that allow them to return at predictable times. Halleys Comet is the best known of these periodic comets; its next return into the inner solar system is predicted for 2061. Many short-period comets are thought to come from the Kuiper belt, a region lying mainly between 30 AU and 50 AU from the Sunbeyond Neptunes orbit but including part of Plutosand housing perhaps hundreds of millions of comet nuclei. Very few comet masses have been well determined, but most are probably less than 1018 grams, one-billionth the mass of Earth.

Since the 1990s more than a thousand comet nuclei in the Kuiper belt have been observed with large telescopes; a few are about half the size of Pluto, and Pluto is the largest Kuiper belt object. Plutos orbital and physical characteristics had long caused it to be regarded as an anomaly among the planets. However, after the discovery of numerous other Pluto-like objects beyond Neptune, Pluto was seen to be no longer unique in its neighbourhood but rather a giant member of the local population. Consequently, in 2006 astronomers at the general assembly of the International Astronomical Union elected to create the new category of dwarf planets for objects with such qualifications. Pluto, Eris, and Ceres, the latter being the largest member of the asteroid belt, were given this distinction. Two other Kuiper belt objects, Makemake and Haumea, were also designated as dwarf planets.

Smaller than the observed asteroids and comets are the meteoroids, lumps of stony or metallic material believed to be mostly fragments of asteroids. Meteoroids vary from small rocks to boulders weighing a ton or more. A relative few have orbits that bring them into Earths atmosphere and down to the surface as meteorites. Most meteorites that have been collected on Earth are probably from asteroids. A few have been identified as being from the Moon, Mars, or the asteroid Vesta.

Meteorites are classified into three broad groups: stony (chondrites and achondrites; about 94 percent), iron (5 percent), and stony-iron (1 percent). Most meteoroids that enter the atmosphere heat up sufficiently to glow and appear as meteors, and the great majority of these vaporize completely or break up before they reach the surface. Many, perhaps most, meteors occur in showers (see meteor shower) and follow orbits that seem to be identical with those of certain comets, thus pointing to a cometary origin. For example, each May, when Earth crosses the orbit of Halleys Comet, the Eta Aquarid meteor shower occurs. Micrometeorites (interplanetary dust particles), the smallest meteoroidal particles, can be detected from Earth-orbiting satellites or collected by specially equipped aircraft flying in the stratosphere and returned for laboratory inspection. Since the late 1960s numerous meteorites have been found in the Antarctic on the surface of stranded ice flows (see Antarctic meteorites). Some meteorites contain microscopic crystals whose isotopic proportions are unique and appear to be dust grains that formed in the atmospheres of different stars.

The age of the solar system, taken to be close to 4.6 billion years, has been derived from measurements of radioactivity in meteorites, lunar samples, and Earths crust. Abundances of isotopes of uranium, thorium, and rubidium and their decay products, lead and strontium, are the measured quantities.

Assessment of the chemical composition of the solar system is based on data from Earth, the Moon, and meteorites as well as on the spectral analysis of light from the Sun and planets. In broad outline, the solar system abundances of the chemical elements decrease with increasing atomic weight. Hydrogen atoms are by far the most abundant, constituting 91 percent; helium is next, with 8.9 percent; and all other types of atoms together amount to only 0.1 percent.

The origin of Earth, the Moon, and the solar system as a whole is a problem that has not yet been settled in detail. The Sun probably formed by condensation of the central region of a large cloud of gas and dust, with the planets and other bodies of the solar system forming soon after, their composition strongly influenced by the temperature and pressure gradients in the evolving solar nebula. Less-volatile materials could condense into solids relatively close to the Sun to form the terrestrial planets. The abundant, volatile lighter elements could condense only at much greater distances to form the giant gas planets.

In the1990s astronomers confirmed that other stars have one or more planets revolving around them. Studies of these planetary systems have both supported and challenged astronomers theoretical models of how Earths solar system formed. Unlike the solar system, many extrasolar planetary systems have large gas giants like Jupiter orbiting very close to their stars, and in some cases these hot Jupiters are closer to their star than Mercury is to the Sun.

That so many gas giants, which form in the outer regions of their system, end up so close to their stars suggests that gas giants migrate and that such migration may have happened in the solar systems history. According to the Grand Tack hypothesis, Jupiter may have done so within a few million years of the solar systems formation. In this scenario, Jupiter is the first giant planet to form, at about 3 AU from the Sun. Drag from the protoplanetary disk causes it to fall inward to about 1.5 AU. However, by this time, Saturn begins to form at about 3 AU and captures Jupiter in a 3:2 resonance. (That is, for every three revolutions Jupiter makes, Saturn makes two.) The two planets migrate outward and clear away any material that would have gone to making Mars bigger. Mars should be bigger than Venus or Earth, but it is only half their size. The Grand Tack, in which Jupiter moves inward and then outward, explains Marss small size.

About 500 million years after the Grand Tack, according to the Nice Model (named after the French city where it was first proposed), after the four giant planetsJupiter, Saturn, Uranus, and Neptuneformed, they orbited 517 AU from the Sun. These planets were in a disk of smaller bodies called planetesimals and in orbital resonances with each other. About four billion years ago, gravitational interactions with the planetesimals increased the eccentricity of the planets orbits, driving them out of resonance. Saturn, Uranus and Neptune migrated outward, and Jupiter migrated slightly inward. (Uranus and Neptune may even have switched places.) This migration scattered the disk, causing the Late Heavy Bombardment. The final remnant of the disk became the Kuiper belt.

The origin of the planetary satellites is not entirely settled. As to the origin of the Moon, the opinion of astronomers long oscillated between theories that saw its origin and condensation as simultaneous with the formation of Earth and those that posited a separate origin for the Moon and its later capture by Earths gravitational field. Similarities and differences in abundances of the chemical elements and their isotopes on Earth and the Moon challenged each group of theories. Finally, in the 1980s a model emerged that gained the support of most lunar scientiststhat of a large impact on Earth and the expulsion of material that subsequently formed the Moon. (See Moon: Origin and evolution.) For the outer planets, with their multiple satellites, many very small and quite unlike one another, the picture is less clear. Some of these moons have relatively smooth icy surfaces, whereas others are heavily cratered; at least one, Jupiters Io, is volcanic. Some of the moons may have formed along with their parent planets, and others may have formed elsewhere and been captured.

The measurable quantities in stellar astrophysics include the externally observable features of the stars: distance, temperature, radiation spectrum and luminosity, composition (of the outer layers), diameter, mass, and variability in any of these. Theoretical astrophysicists use these observations to model the structure of stars and to devise theories for their formation and evolution. Positional information can be used for dynamical analysis, which yields estimates of stellar masses.

In a system dating back at least to the Greek astronomer-mathematician Hipparchus in the 2nd century bce, apparent stellar brightness (m) is measured in magnitudes. Magnitudes are now defined such that a first-magnitude star is 100 times brighter than a star of sixth magnitude. The human eye cannot see stars fainter than about sixth magnitude, but modern instruments used with large telescopes can record stars as faint as about 30th magnitude. By convention, the absolute magnitude (M) is defined as the magnitude that a star would appear to have if it were located at a standard distance of 10 parsecs. These quantities are related through the expression m M = 5 log10 r 5, in which r is the stars distance in parsecs.

The magnitude scale is anchored on a group of standard stars. An absolute measure of radiant power is luminosity, which is related to the absolute magnitude and usually expressed in ergs per second (ergs/sec). (Sometimes the luminosity is stated in terms of the solar luminosity, 3.86 1033 ergs/sec.) Luminosity can be calculated when m and r are known. Correction might be necessary for the interstellar absorption of starlight.

There are several methods for measuring a stars diameter. From the brightness and distance, the luminosity (L) can be calculated, and, from observations of the brightness at different wavelengths, the temperature (T) can be calculated. Because the radiation from many stars can be well approximated by a Planck blackbody spectrum (see Plancks radiation law), these measured quantities can be related through the expression L = 4R2T4, thus providing a means of calculating R, the stars radius. In this expression, is the Stefan-Boltzmann constant, 5.67 105 ergs/cm2K4sec, in which K is the temperature in kelvins. (The radius R refers to the stars photosphere, the region where the star becomes effectively opaque to outside observation.) Stellar angular diameters can be measured through interferometrythat is, the combining of several telescopes together to form a larger instrument that can resolve sizes smaller than those that an individual telescope can resolve. Alternatively, the intensity of the starlight can be monitored during occultation by the Moon, which produces diffraction fringes whose pattern depends on the angular diameter of the star. Stellar angular diameters of several milliarcseconds can be measured.

Many stars occur in binary systems (see binary star), in which the two partners orbit their mutual centre of mass. Such a system provides the best measurement of stellar masses. The period (P) of a binary system is related to the masses of the two stars (m1 and m2) and the orbital semimajor axis (mean radius; a) via Keplers third law: P2 = 42a3/G(m1 + m2). (G is the universal gravitational constant.) From diameters and masses, average values of the stellar density can be calculated and thence the central pressure. With the assumption of an equation of state, the central temperature can then be calculated. For example, in the Sun the central density is 158 grams per cubic cm; the pressure is calculated to be more than one billion times the pressure of Earths atmosphere at sea level and the temperature around 15 million K (27 million F). At this temperature, all atoms are ionized, and so the solar interior consists of a plasma, an ionized gas with hydrogen nuclei (i.e., protons), helium nuclei, and electrons as major constituents. A small fraction of the hydrogen nuclei possess sufficiently high speeds that, on colliding, their electrostatic repulsion is overcome, resulting in the formation, by means of a set of fusion reactions, of helium nuclei and a release of energy (see proton-proton cycle). Some of this energy is carried away by neutrinos, but most of it is carried by photons to the surface of the Sun to maintain its luminosity.

Other stars, both more and less massive than the Sun, have broadly similar structures, but the size, central pressure and temperature, and fusion rate are functions of the stars mass and composition. The stars and their internal fusion (and resulting luminosity) are held stable against collapse through a delicate balance between the inward pressure produced by gravitational attraction and the outward pressure supplied by the photons produced in the fusion reactions.

Stars that are in this condition of hydrostatic equilibrium are termed main-sequence stars, and they occupy a well-defined band on the Hertzsprung-Russell (H-R) diagram, in which luminosity is plotted against colour index or temperature. Spectral classification, based initially on the colour index, includes the major spectral types O, B, A, F, G, K and M, each subdivided into 10 parts (see star: Stellar spectra). Temperature is deduced from broadband spectral measurements in several standard wavelength intervals. Measurement of apparent magnitudes in two spectral regions, the B and V bands (centred on 4350 and 5550 angstroms, respectively), permits calculation of the colour index, CI = mB mV, from which the temperature can be calculated.

For a given temperature, there are stars that are much more luminous than main-sequence stars. Given the dependence of luminosity on the square of the radius and the fourth power of the temperature (R2T4 of the luminosity expression above), greater luminosity implies larger radius, and such stars are termed giant stars or supergiant stars. Conversely, stars with luminosities much less than those of main-sequence stars of the same temperature must be smaller and are termed white dwarf stars. Surface temperatures of white dwarfs typically range from 10,000 to 12,000 K (18,000 to 21,000 F), and they appear visually as white or blue-white.

The strength of spectral lines of the more abundant elements in a stars atmosphere allows additional subdivisions within a class. Thus, the Sun, a main-sequence star, is classified as G2 V, in which the V denotes main sequence. Betelgeuse, a red giant with a surface temperature about half that of the Sun but with a luminosity of about 10,000 solar units, is classified as M2 Iab. In this classification, the spectral type is M2, and the Iab indicates a giant, well above the main sequence on the H-R diagram.

The range of physically allowable masses for stars is very narrow. If the stars mass is too small, the central temperature will be too low to sustain fusion reactions. The theoretical minimum stellar mass is about 0.08 solar mass. An upper theoretical bound called the Eddington limit, of several hundred solar masses, has been suggested, but this value is not firmly defined. Stars as massive as this will have luminosities about one million times greater than that of the Sun.

A general model of star formation and evolution has been developed, and the major features seem to be established. A large cloud of gas and dust can contract under its own gravitational attraction if its temperature is sufficiently low. As gravitational energy is released, the contracting central material heats up until a point is reached at which the outward radiation pressure balances the inward gravitational pressure, and contraction ceases. Fusion reactions take over as the stars primary source of energy, and the star is then on the main sequence. The time to pass through these formative stages and onto the main sequence is less than 100 million years for a star with as much mass as the Sun. It takes longer for less massive stars and a much shorter time for those much more massive.

Once a star has reached its main-sequence stage, it evolves relatively slowly, fusing hydrogen nuclei in its core to form helium nuclei. Continued fusion not only releases the energy that is radiated but also results in nucleosynthesis, the production of heavier nuclei.

Stellar evolution has of necessity been followed through computer modeling, because the timescales for most stages are generally too extended for measurable changes to be observed, even over a period of many years. One exception is the supernova, the violently explosive finale of certain stars. Different types of supernovas can be distinguished by their spectral lines and by changes in luminosity during and after the outburst. In Type Ia, a white dwarf star attracts matter from a nearby companion; when the white dwarfs mass exceeds about 1.4 solar masses, the star implodes and is completely destroyed. Type II supernovas are not as luminous as Type Ia and are the final evolutionary stage of stars more massive than about eight solar masses. Type Ib and Ic supernovas are like Type II in that they are from the collapse of a massive star, but they do not retain their hydrogen envelope.

The nature of the final products of stellar evolution depends on stellar mass. Some stars pass through an unstable stage in which their dimensions, temperature, and luminosity change cyclically over periods of hours or days. These so-called Cepheid variables serve as standard candles for distance measurements (see above Determining astronomical distances). Some stars blow off their outer layers to produce planetary nebulas. The expanding material can be seen glowing in a thin shell as it disperses into the interstellar medium while the remnant core, initially with a surface temperature as high as 100,000 K (180,000 F), cools to become a white dwarf. The maximum stellar mass that can exist as a white dwarf is about 1.4 solar masses and is known as the Chandrasekhar limit. More-massive stars may end up as either neutron stars or black holes.

The average density of a white dwarf is calculated to exceed one million grams per cubic cm. Further compression is limited by a quantum condition called degeneracy (see degenerate gas), in which only certain energies are allowed for the electrons in the stars interior. Under sufficiently great pressure, the electrons are forced to combine with protons to form neutrons. The resulting neutron star will have a density in the range of 10141015 grams per cubic cm, comparable to the density within atomic nuclei. The behaviour of large masses having nuclear densities is not yet sufficiently understood to be able to set a limit on the maximum size of a neutron star, but it is thought to be less than three solar masses.

Still more-massive remnants of stellar evolution would have smaller dimensions and would be even denser that neutron stars. Such remnants are conceived to be black holes, objects so compact that no radiation can escape from within a characteristic distance called the Schwarzschild radius. This critical dimension is defined by Rs = 2GM/c2. (Rs is the Schwarzschild radius, G is the gravitational constant, M is the objects mass, and c is the speed of light.) For an object of three solar masses, the Schwarzschild radius would be about three kilometres. Radiation emitted from beyond the Schwarzschild radius can still escape and be detected.

Although no light can be detected coming from within a black hole, the presence of a black hole may be manifested through the effects of its gravitational field, as, for example, in a binary star system. If a black hole is paired with a normal visible star, it may pull matter from its companion toward itself. This matter is accelerated as it approaches the black hole and becomes so intensely heated that it radiates large amounts of X-rays from the periphery of the black hole before reaching the Schwarzschild radius. Some candidates for stellar black holes have been founde.g., the X-ray source Cygnus X-1. Each of them has an estimated mass clearly exceeding that allowable for a neutron star, a factor crucial in the identification of possible black holes. Supermassive black holes that do not originate as individual stars exist at the centre of active galaxies (see below Study of other galaxies and related phenomena). One such black hole, that at the center of the galaxy M87, has a mass 6.5 billion times that of the Sun and has been directly observed.

Whereas the existence of stellar black holes has been strongly indicated, the existence of neutron stars was confirmed in 1968 when they were identified with the then newly discovered pulsars, objects characterized by the emission of radiation at short and extremely regular intervals, generally between 1 and 1,000 pulses per second and stable to better than a part per billion. Pulsars are considered to be rotating neutron stars, remnants of some supernovas.

Stars are not distributed randomly throughout space. Many stars are in systems consisting of two or three members separated by less than 1,000 AU. On a larger scale, star clusters may contain many thousands of stars. Galaxies are much larger systems of stars and usually include clouds of gas and dust.

The solar system is located within the Milky Way Galaxy, close to its equatorial plane and about 8 kiloparsecs from the galactic centre. The galactic diameter is about 30 kiloparsecs, as indicated by luminous matter. There is evidence, however, for nonluminous matterso-called dark matterextending out nearly twice this distance. The entire system is rotating such that, at the position of the Sun, the orbital speed is about 220 km per second (almost 500,000 miles per hour) and a complete circuit takes roughly 240 million years. Application of Keplers third law leads to an estimate for the galactic mass of about 100 billion solar masses. The rotational velocity can be measured from the Doppler shifts observed in the 21-cm emission line of neutral hydrogen and the lines of millimetre wavelengths from various molecules, especially carbon monoxide. At great distances from the galactic centre, the rotational velocity does not drop off as expected but rather increases slightly. This behaviour appears to require a much larger galactic mass than can be accounted for by the known (luminous) matter. Additional evidence for the presence of dark matter comes from a variety of other observations. The nature and extent of the dark matter (or missing mass) constitutes one of todays major astronomical puzzles.

There are about 100 billion stars in the Milky Way Galaxy. Star concentrations within the galaxy fall into three types: open clusters, globular clusters, and associations (see star cluster). Open clusters lie primarily in the disk of the galaxy; most contain between 50 and 1,000 stars within a region no more than 10 parsecs in diameter. Stellar associations tend to have somewhat fewer stars; moreover, the constituent stars are not as closely grouped as those in the clusters and are for the most part hotter. Globular clusters, which are widely scattered around the galaxy, may extend up to about 100 parsecs in diameter and may have as many as a million stars. The importance to astronomers of globular clusters lies in their use as indicators of the age of the galaxy. Because massive stars evolve more rapidly than do smaller stars, the age of a cluster can be estimated from its H-R diagram. In a young cluster the main sequence will be well populated, but in an old cluster the heavier stars will have evolved away from the main sequence. The extent of the depopulation of the main sequence provides an index of age. In this way, the oldest globular clusters have been found to be about 12.5 billion years old, which should therefore be the minimum age for the galaxy.

The interstellar medium, composed primarily of gas and dust, occupies the regions between the stars. On average, it contains less than one atom in each cubic centimetre, with about 1 percent of its mass in the form of minute dust grains. The gas, mostly hydrogen, has been mapped by means of its 21-cm emission line. The gas also contains numerous molecules. Some of these have been detected by the visible-wavelength absorption lines that they impose on the spectra of more-distant stars, while others have been identified by their own emission lines at millimetre wavelengths. Many of the interstellar molecules are found in giant molecular clouds, wherein complex organic molecules have been discovered.

In the vicinity of a very hot O- or B-type star, the intensity of ultraviolet radiation is sufficiently high to ionize the surrounding hydrogen out to a distance as great as 100 parsecs to produce an H II region, known as a Strmgren sphere. Such regions are strong and characteristic emitters of radiation at radio wavelengths, and their dimensions are well calibrated in terms of the luminosity of the central star. Using radio interferometers, astronomers are able to measure the angular diameters of H II regions even in some external galaxies and can thereby deduce the great distances to those remote systems. This method can be used for distances up to about 30 megaparsecs. (For additional information on H II regions, see nebula: Diffuse nebulae (H II regions).)

Interstellar dust grains scatter and absorb starlight, the effect being roughly inversely proportional to wavelength from the infrared to the near ultraviolet. As a result, stellar spectra tend to be reddened. Absorption typically amounts to about one magnitude per kiloparsec but varies considerably in different directions. Some dusty regions contain silicate materials, identified by a broad absorption feature around a wavelength of 10 m. Other prominent spectral features in the infrared range have been sometimes, but not conclusively, attributed to graphite grains and polycyclic aromatic hydrocarbons (PAHs).

Starlight often shows a small degree of polarization (a few percent), with the effect increasing with stellar distance. This is attributed to the scattering of the starlight from dust grains that have been partially aligned in a weak interstellar magnetic field. The strength of this field is estimated to be a few microgauss, very close to the strength inferred from observations of nonthermal cosmic radio noise. This radio background has been identified as synchrotron radiation, emitted by cosmic-ray electrons traveling at nearly the speed of light and moving along curved paths in the interstellar magnetic field. The spectrum of the cosmic radio noise is close to what is calculated on the basis of measurements of the cosmic rays near Earth.

Cosmic rays constitute another component of the interstellar medium. Cosmic rays that are detected in the vicinity of Earth comprise high-speed nuclei and electrons. Individual particle energies, expressed in electron volts (eV; 1 eV = 1.6 1012 erg), range with decreasing numbers from about 106 eV to more than 1020 eV. Among the nuclei, hydrogen nuclei are the most plentiful at 86 percent, helium nuclei next at 13 percent, and all other nuclei together at about 1 percent. Electrons are about 2 percent as abundant as the nuclear component. (The relative numbers of different nuclei vary somewhat with kinetic energy, while the electron proportion is strongly energy-dependent.)

A minority of cosmic rays detected in Earths vicinity are produced in the Sun, especially at times of increased solar activity (as indicated by sunspots and solar flares). The origin of galactic cosmic rays has not yet been conclusively identified, but they are thought to be produced in stellar processes such as supernova explosions, perhaps with additional acceleration occurring in the interstellar regions. (For additional information on interstellar matter, see Milky Way Galaxy: The general interstellar medium.)

The central region of the Milky Way Galaxy is so heavily obscured by dust that direct observation has become possible only with the development of astronomy at nonvisual wavelengthsnamely, radio, infrared, and, more recently, X-ray and gamma-ray wavelengths. Together, these observations have revealed a nuclear region of intense activity, with a large number of separate sources of emission and a great deal of dust. Detection of gamma-ray emission at a line energy of 511,000 eV, which corresponds to the annihilation of electrons and positrons (the antimatter counterpart of electrons), along with radio mapping of a region no more than 20 AU across, points to a very compact and energetic source, designated Sagittarius A*, at the centre of the galaxy. Sagittarius A* is a supermassive black hole with a mass equivalent to 4,310,000 Suns.

Galaxies are normally classified into three principal types according to their appearance: spiral, elliptical, and irregular. Galactic diameters are typically in the tens of kiloparsecs and the distances between galaxies typically in megaparsecs.

Spiral galaxiesof which the Milky Way system is a characteristic exampletend to be flattened, roughly circular systems with their constituent stars strongly concentrated along spiral arms. These arms are thought to be produced by traveling density waves, which compress and expand the galactic material. Between the spiral arms exists a diffuse interstellar medium of gas and dust, mostly at very low temperatures (below 100 K [280 F, 170 C]). Spiral galaxies are typically a few kiloparsecs in thickness; they have a central bulge and taper gradually toward the outer edges.

Ellipticals show none of the spiral features but are more densely packed stellar systems. They range in shape from nearly spherical to very flattened and contain little interstellar matter. Irregular galaxies number only a few percent of all stellar systems and exhibit none of the regular features associated with spirals or ellipticals.

Properties vary considerably among the different types of galaxies. Spirals typically have masses in the range of a billion to a trillion solar masses, with ellipticals having values from 10 times smaller to 10 times larger and the irregulars generally 10100 times smaller. Visual galactic luminosities show similar spreads among the three types, but the irregulars tend to be less luminous. In contrast, at radio wavelengths the maximum luminosity for spirals is usually 100,000 times less than for ellipticals or irregulars.

Quasars are objects whose spectra display very large redshifts, thus implying (in accordance with the Hubble law) that they lie at the greatest distances (see above Determining astronomical distances). They were discovered in 1963 but remained enigmatic for many years. They appear as starlike (i.e., very compact) sources of radio waveshence their initial designation as quasi-stellar radio sources, a term later shortened to quasars. They are now considered to be the exceedingly luminous cores of distant galaxies. These energetic cores, which emit copious quantities of X-rays and gamma rays, are termed active galactic nuclei (AGN) and include the object Cygnus A and the nuclei of a class of galaxies called Seyfert galaxies. They are powered by the infall of matter into supermassive black holes.

The Milky Way Galaxy is one of the Local Group of galaxies, which contains about four dozen members and extends over a volume about two megaparsecs in diameter. Two of the closest members are the Magellanic Clouds, irregular galaxies about 50 kiloparsecs away. At about 740 kiloparsecs, the Andromeda Galaxy is one of the most distant in the Local Group. Some members of the group are moving toward the Milky Way system while others are traveling away from it. At greater distances, all galaxies are moving away from the Milky Way Galaxy. Their speeds (as determined from the redshifted wavelengths in their spectra) are generally proportional to their distances. The Hubble law relates these two quantities (see above Determining astronomical distances). In the absence of any other method, the Hubble law continues to be used for distance determinations to the farthest objectsthat is, galaxies and quasars for which redshifts can be measured.

Cosmology is the scientific study of the universe as a unified whole, from its earliest moments through its evolution to its ultimate fate. The currently accepted cosmological model is the big bang. In this picture, the expansion of the universe started in an intense explosion 13.8 billion years ago. In this primordial fireball, the temperature exceeded one trillion K, and most of the energy was in the form of radiation. As the expansion proceeded (accompanied by cooling), the role of the radiation diminished, and other physical processes dominated in turn. Thus, after about three minutes, the temperature had dropped to the one-billion-K range, making it possible for nuclear reactions of protons to take place and produce nuclei of deuterium and helium. (At the higher temperatures that prevailed earlier, these nuclei would have been promptly disrupted by high-energy photons.) With further expansion, the time between nuclear collisions had increased and the proportion of deuterium and helium nuclei had stabilized. After a few hundred thousand years, the temperature must have dropped sufficiently for electrons to remain attached to nuclei to constitute atoms. Galaxies are thought to have begun forming after a few million years, but this stage is very poorly understood. Star formation probably started much later, after at least a billion years, and the process continues today.

Observational support for this general model comes from several independent directions. The expansion has been documented by the redshifts observed in the spectra of galaxies. Furthermore, the radiation left over from the original fireball would have cooled with the expansion. Confirmation of this relic energy came in 1965 with one of the most striking cosmic discoveries of the 20th centurythe observation, at short radio wavelengths, of a widespread cosmic radiation corresponding to a temperature of almost 3 K (about 270 C [454 F]). The shape of the observed spectrum is an excellent fit with the theoretical Planck blackbody spectrum. (The present best value for this temperature is 2.735 K, but it is still called three-degree radiation or the cosmic microwave background.) The spectrum of this cosmic radio noise peaks at approximately a one-millimetre wavelength, which is in the far infrared, a difficult region to observe from Earth; however, the spectrum has been well mapped by the Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe, and Planck satellites. Additional support for the big bang theory comes from the observed cosmic abundances of deuterium and helium. Normal stellar nucleosynthesis cannot produce their measured quantities, which fit well with calculations of production during the early stages of the big bang.

Early surveys of the cosmic background radiation indicated that it is extremely uniform in all directions (isotropic). Calculations have shown that it is difficult to achieve this degree of isotropy unless there was a very early and rapid inflationary period before the expansion settled into its present mode. Nevertheless, the isotropy posed problems for models of galaxy formation. Galaxies originate from turbulent conditions that produce local fluctuations of density, toward which more matter would then be gravitationally attracted. Such density variations were difficult to reconcile with the isotropy required by observations of the 3 K radiation. This problem was solved when the COBE satellite was able to detect the minute fluctuations in the cosmic background from which the galaxies formed.

The very earliest stages of the big bang are less well understood. The conditions of temperature and pressure that prevailed prior to the first microsecond require the introduction of theoretical ideas of subatomic particle physics. Subatomic particles are usually studied in laboratories with giant accelerators, but the region of particle energies of potential significance to the question at hand lies beyond the range of accelerators currently available. Fortunately, some important conclusions can be drawn from the observed cosmic helium abundance, which is dependent on conditions in the early big bang. The observed helium abundance sets a limit on the number of families of certain types of subatomic particles that can exist.

The age of the universe can be calculated in several ways. Assuming the validity of the big bang model, one attempts to answer the question: How long has the universe been expanding in order to have reached its present size? The numbers relevant to calculating an answer are Hubbles constant (i.e., the current expansion rate), the density of matter in the universe, and the cosmological constant, which allows for change in the expansion rate. In 2003 a calculation based on a fresh determination of Hubbles constant yielded an age of 13.7 billion 200 million years, although the precise value depends on certain assumed details of the model used. Independent estimates of stellar ages have yielded values less than this, as would be expected, but other estimates, based on supernova distance measurements, have arrived at values of about 15 billion years, still consistent, within the errors. In the big bang model the age is proportional to the reciprocal of Hubbles constant, hence the importance of determining H as reliably as possible. For example, a value for H of 100 km/sec/Mpc would lead to an age less than that of many stars, a physically unacceptable result.

A small minority of astronomers have developed alternative cosmological theories that are seriously pursued. The overwhelming professional opinion, however, continues to support the big bang model.

Finally, there is the question of the future behaviour of the universe: Is it open? That is to say, will the expansion continue indefinitely? Or is it closed, such that the expansion will slow down and eventually reverse, resulting in contraction? (The final collapse of such a contracting universe is sometimes termed the big crunch.) The density of the universe seems to be at the critical density; that is, the universe is neither open nor closed but flat. So-called dark energy, a kind of repulsive force that is now believed to be a major component of the universe, appears to be the decisive factor in predictions of the long-term fate of the cosmos. If this energy is a cosmological constant (as proposed in 1917 by Albert Einstein to correct certain problems in his model of the universe), then the result would be a big chill. In this scenario, the universe would continue to expand, but its density would decrease. While old stars would burn out, new stars would no longer form. The universe would become cold and dark. The dark (nonluminous) matter component of the universe, whose composition remains unknown, is not considered sufficient to close the universe and cause it to collapse; it now appears to contribute only a fourth of the density needed for closure.

An additional factor in deciding the fate of the universe might be the mass of neutrinos. For decades the neutrino had been postulated to have zero mass, although there was no compelling theoretical reason for this to be so. From the observation of neutrinos generated in the Sun and other celestial sources such as supernovas, in cosmic-ray interactions with Earths atmosphere, and in particle accelerators, investigators have concluded that neutrinos have some mass, though only an extremely small fraction of the mass of an electron. Although there are vast numbers of neutrinos in the universe, the sum of such small neutrino masses appears insufficient to close the universe.

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astronomy | Definition & Facts | Britannica.com

Comet – Wikipedia

Icy small Solar System body

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30 (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.[1] Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3 (PANSTARRS).[5] 27 Manx comets were found from 2013 to 2017.[6]

As of July2018[update] there are 6,339 known comets,[7] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[8][9] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[10] Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet,[11] and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek ("wearing long hair"), and the Oxford English Dictionary notes that the term () already meant "long-haired star, comet" in Greek. was derived from ("to wear the hair long"), which was itself derived from ("the hair of the head") and was used to mean "the tail of a comet".[12][13]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[14]

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia.[15] As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.[16] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".[17] Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[18]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[19][20] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.[21] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[22][23]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it,[24] and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%;[24] by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.[25]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[26] but ascertaining their exact size is difficult.[27] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[28] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[29] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[30] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[31]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[32] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/ChuryumovGerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[33][34] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1km (0.62mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[35][36] Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[37][38][39]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.[48]

The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000km; 280,000,000 to 370,000,000mi) of the Sun.[49] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[49] Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.[50]

Although the solid nucleus of comets is generally less than 60 kilometres (37mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[51] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[52] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[53] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[53] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[53] Ion tails have been observed to extend one astronomical unit (150million km) or more.[52]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation.[54] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[55] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[56]

In 1996, comets were found to emit X-rays.[57] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.[58]

Bow shocks form at as a result of the interaction between the solar wind and the cometary ionosphere, which is created by ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.

The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/GiacobiniZinner,[59] 1P/Halley,[60] and 26P/GriggSkjellerup.[61] It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.

The Rosetta spacecraft observed the bow shock at comet 67P/ChuryumovGerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.[62]

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[63][64] but these detections have been questioned.[65][66] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail.[54] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[67] On occasionssuch as when the Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.[68]

The observation of antitails contributed significantly to the discovery of solar wind.[69] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.[70]

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".[70] This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[71]

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."[72][73]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.[74] These streams of gas and dust can cause the nucleus to spin, and even split apart.[74] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[75] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[76]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[77] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.[78] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[79] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family".[80] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[81]

At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs).[82][83] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[84][85] As of 2019[update], 85 HTCs have been observed,[86] compared with 664 identified JFCs.[87]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[88]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[89] Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group.[83] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[90][91]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[92] a disk of objects in the trans-Neptunian regionwhereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[93] Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.[94]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[95] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[96] For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[95] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.[97] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[98] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[99] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

As of 2018, 1I/Oumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While Oumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectorywhich suggests outgassingindicate that it is indeed a comet.[100] Comet C/1980 E1 had an orbital period of roughly 7.1million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[101] Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),[102] whereas others use it to mean exclusively short-period comets.[95] Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.[103]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[105] to as far as 50,000AU (0.79ly)[84] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[105] The region can be subdivided into a spherical outer Oort cloud of 20,00050,000AU (0.320.79ly), and a doughnut-shaped inner cloud, the Hills cloud, of 2,00020,000AU (0.030.32ly).[106] The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune.[84] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[107] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[107][108][109] it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[110]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[111] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[112][113] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[111][112]

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[114] If the Earth's orbit sends it through that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle.[115] Halley's Comet is the source of the Orionid shower in October.[115]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea.[116] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[18] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[117] In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.[118] In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.[119]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.[120] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[121]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[122] The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near.[123] He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmund Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on the Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters.[122] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet,[124] causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[125]

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such they are called hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.[126] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[127]

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[32] Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[128] Some asteroids in elliptical orbits are now identified as extinct comets.[129][130][131][132] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[32]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[133] A significant cometary disruption was that of Comet ShoemakerLevy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmospherethe first time astronomers had observed a collision between two objects in the Solar System.[134][135] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[136] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[137] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[138]

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.[139]

Some comets have been observed to break up during their perihelion passage, including great comets West and IkeyaSeki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.[140]

Some comets meet a more spectacular end either falling into the Sun[141] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet ShoemakerLevy 9 broke up into pieces and collided with Jupiter.[142]

Ghost tail of C/2015 D1 (SOHO) after passage at the Sun

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910".

After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet.[144] Similarly, the second and third known periodic comets, Encke's Comet[145] and Biela's Comet,[146] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.[147]

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.[147]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[148] Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.[149][150]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[151] Pliny the Elder believed that comets were connected with political unrest and death.[152]

In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varhamihira and Bhadrabahu, and the 10th-century astronomer Bhaotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.[153]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[154][155]

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[156]

In 1705, Edmond Halley (16561742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 17589.[157] Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[158][159] When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.[160]

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.[161]

From his huge vapouring train perhaps to shakeReviving moisture on the numerous orbs,Thro' which his long ellipsis winds; perhapsTo lend new fuel to declining suns,To light up worlds, and feed th' ethereal fire.

James Thomson The Seasons (1730; 1748)[162]

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[163] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.[164]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[165] This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.[166]

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt.[167] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[168] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."[168] On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[169][170]

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets.[137] Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions.[179] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[180] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[181]

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick successionComet Hyakutake in 1996, followed by HaleBopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.[182]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[183] Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.[184]

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.[185] The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.[186]

Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/SchwassmannWachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.[187] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[188] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[189] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[190] Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active,[191] and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.[192]

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.[193] SOHO's 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[194] and both discoverers of HaleBopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[195] There are at least 18 comets that fit this category.[196]

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.[197] Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910)[197] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".[197]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[198] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven's Gate cult.[199]

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984).[197] In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.[200]

NASA is developing a comet harpoon for returning samples to Earth

Continued here:

Comet - Wikipedia

Comet | Definition of Comet by Merriam-Webster

Recent Examples on the Web. And the Oort Cloud may not be just comets and debris. John Wenz, Popular Mechanics, "Why Voyager 2 Is In Interstellar Space But Not Out of the Solar System," 13 Dec. 2018 Some iconographic details of this mythical apocalypse that emerged around 1000 AD may have been influenced by astronomical eventsnotably comets and total eclipses.

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Comet | Definition of Comet by Merriam-Webster

comet | Definition, Composition, & Facts | Britannica.com

HistoryAncient Greece to the 19th century

The Greek philosopher Aristotle thought that comets were dry exhalations of Earth that caught fire high in the atmosphere or similar exhalations of the planets and stars. However, the Roman philosopher Seneca thought that comets were like the planets, though in much larger orbits. He wrote:

The man will come one day who will explain in what regions the comets move, why they diverge so much from the other stars, what is their size and their nature.

Aristotles view won out and persisted until 1577, when Danish astronomer Tycho Brahe attempted to use parallax to triangulate the distance to a bright comet. Because he could not measure any parallax, Brahe concluded that the comet was very far away, at least four times farther than the Moon.

Brahes student, German astronomer Johannes Kepler, devised his three laws of planetary motion using Brahes meticulous observations of Mars but was unable to fit his theory to the very eccentric orbits of comets. Kepler believed that comets traveled in straight lines through the solar system. The solution came from English scientist Isaac Newton, who used his new law of gravity to calculate a parabolic orbit for the comet of 1680. A parabolic orbit is open, with an eccentricity of exactly 1, meaning the comet would never return. (A circular orbit has an eccentricity of 0.) Any less-eccentric orbits are closed ellipses, which means a comet would return.

Newton was friends with English astronomer Edmond Halley, who used Newtons methods to determine the orbits for 24 observed comets, which he published in 1705. All the orbits were fit with parabolas because the quality of the observations at that time was not good enough to determine elliptical or hyperbolic orbits (eccentricities greater than 1). But Halley noted that the comets of 1531, 1607, and 1682 had remarkably similar orbits and had appeared at approximately 76-year intervals. He suggested that it was really one comet in an approximately 76-year orbit that returned at regular intervals. Halley predicted that the comet would return again in 1758. He did not live to see his prediction come true, but the comet was recovered on Christmas Day, 1758, and passed closest to the Sun on March 13, 1759. The comet was the first recognized periodic comet and was named in Halleys honour, Comet Halley.

Halley also speculated whether comets were members of the solar system or not. Although he could only calculate parabolic orbits, he suggested that the orbits were actually eccentric and closed, writing:

For so their Number will be determinate and, perhaps, not so very great. Besides, the Space between the Sun and the fixd Stars is so immense that there is Room enough for a Comet to revolve tho the period of its Revolution be vastly long.

The German astronomer Johann Encke was the second person to recognize a periodic comet. He determined that a comet discovered by French astronomer Jean-Louis Pons in 1818 did not seem to follow a parabolic orbit. He found that the orbit was indeed a closed ellipse. Moreover, he showed that the orbital period of the comet around the Sun was only 3.3 years, still the shortest orbital period of any comet on record. Encke also showed that the same comet had been observed by French astronomer Pierre Mchain in 1786, by British astronomer Caroline Herschel in 1795, and by Pons in 1805. The comet was named in Enckes honour, as Comet Halley was named for the astronomer who described its orbit.

Enckes Comet soon presented a new problem for astronomers. Because it returned so often, its orbit could be predicted precisely based on Newtons law of gravity, with effects from gravitational perturbations by the planets taken into account. But Enckes Comet repeatedly arrived about 2.5 hours too soon. Its orbit was slowly shrinking. The problem became even more complex when it was discovered that other periodic comets arrived too late. Those include the comets 6P/DArrest, 14P/Wolf 1, and even 1P/Halley, which typically returns about four days later than a purely gravitational orbit would predict.

Several explanations were suggested for this phenomenon, such as a resisting interplanetary medium that caused the comet to slowly lose orbital energy. However, that idea could not explain comets whose orbits were growing, not shrinking. German mathematician and astronomer Friedrich Bessel suggested that expulsion of material from a comet near perihelion was acting like a rocket motor and propelling the comet into a slightly shorter- (or longer-) period orbit each time it passed close to the Sun. History would prove Bessel right.

As the quality of the observations and mathematical techniques to calculate orbits improved, it became obvious that most comets were on elliptical orbits and thus were members of the solar system. Many were recognized to be periodic. But some orbit solutions for long-period comets suggested that they were slightly hyperbolic, suggesting that they came from interstellar space. That problem would not be solved until the 20th century.

Another interesting problem for astronomers was a comet discovered in 1826 by the Austrian military officer and astronomer Wilhelm, Freiherr (baron) von Biela. Calculation of its orbit showed that it, like Enckes Comet, was a short-period comet; it had a period of about 6.75 years. It was only the third periodic comet to be confirmed. It was identified with a comet observed by French astronomers Jacques Lebaix Montaigne and Charles Messier in 1772 and by Pons in 1805, and it returned, as predicted, in 1832. In 1839 the comet was too close in the sky to the Sun and could not be observed, but it was seen again on schedule in November 1845. On January 13, 1846, American astronomer Matthew Maury found that there was no longer a single comet: there were two, following each other closely around the Sun. The comets returned as a pair in 1852 but were never seen again. Searches for the comets in 1865 and 1872 were unsuccessful, but a brilliant meteor shower appeared in 1872 coming from the same direction from which the comets should have appeared. Astronomers concluded that the meteor shower was the debris of the disrupted comets. However, they were still left with the question as to why the comet broke up. That recurring meteor shower is now known as the Andromedids, named for the constellation in the sky where it appears to radiate from, but is also sometimes referred to as the Bielids.

The study of meteor showers received a huge boost on November 12 and 13, 1833, when observers saw an incredible meteor shower, with rates of hundreds and perhaps thousands of meteors per hour. That shower was the Leonids, so named because its radiant (or origin) is in the constellation Leo. It was suggested that Earth was encountering interplanetary debris spread along the Earth-crossing orbits of yet unknown bodies in the solar system. Further analysis showed that the orbits of the debris were highly eccentric.

American mathematician Hubert Newton published a series of papers in the 1860s in which he examined historical records of major Leonid meteor showers and found that they occurred about every 33 years. That showed that the Leonid particles were not uniformly spread around the orbit. He predicted another major shower for November 1866. As predicted, a large Leonid meteor storm occurred on November 13, 1866. In the same year, Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteor shower, usually observed on August 1012 each year, and noted its strong similarity to the orbit of Comet Swift-Tuttle (109P/1862 O1) discovered in 1862. Soon after, the Leonids were shown to have an orbit very similar to Comet Tempel-Tuttle (55P/1865 Y1), discovered in 1865. Since then the parent comets of many meteoroid streams have been identified, though the parent comets of some streams remains a mystery.

Meanwhile, the study of comets benefitted greatly from the improvement in the quality and size of telescopes and the technology for observing comets. In 1858 English portrait artist William Usherwood took the first photograph of a comet, Comet Donati (C/1858 L1), followed by American astronomer George Bond the next night. The first photographic discovery of a comet was made by American astronomer Edward Barnard in 1892, while he was photographing the Milky Way. The comet, which was in a short-period orbit, was known as D/Barnard 3 because it was soon lost, but it was recovered by Italian astronomer Andrea Boattini in 2008 and is now known as Comet Barnard/Boattini (206P/2008 T3). In 1864 Italian astronomer Giovanni Donati was the first to look at a comet through a spectroscope, and he discovered three broad emission bands that are now known to be caused by long-chain carbon molecules in the coma. The first spectrogram (a spectrum recorded on film) was of Comet Tebbutt (C/1881 K1), taken by English astronomer William Huggins on June 24, 1881. Later the same night, an American doctor and amateur astronomer, Henry Draper, took spectra of the same comet. Both men later became professional astronomers.

Some years before the appearance of Comet Halley in 1910, the molecule cyanogen was identified as one of the molecules in the spectra of cometary comae. Cyanogen is a poisonous gas derived from hydrogen cyanide (HCN), a well-known deadly poison. It was also detected in Halleys coma as that comet approached the Sun in 1910. That led to great consternation as Earth was predicted to pass through the tail of the comet. People panicked, bought comet pills, and threw end-of-the-world parties. But when the comet passed by only 0.15 AU away on the night of May 1819, 1910, there were no detectable effects.

The 20th century saw continued progress in cometary science. Spectroscopy revealed many of the molecules, radicals, and ions in the comae and tails of comets. An understanding began to develop about the nature of cometary tails, with the ion (Type I) tails resulting from the interaction of ionized molecules with some form of corpuscular radiation, possibly electrons and protons, from the Sun, and the dust (Type II) tails coming from solar radiation pressure on the fine dust particles emitted from the comet.

Astronomers continued to ask, Where do the comets come from? There were three schools of thought: (1) that comets were captured from interstellar space, (2) that comets were erupted out of the giant planets, or (3) that comets were primeval matter that had not been incorporated into the planets. The first idea had been suggested by French mathematician and astronomer Pierre Laplace in 1813, while the second came from another French mathematician-astronomer, Joseph Lagrange. The third came from English astronomer George Chambers in 1910.

The idea of an interstellar origin for comets ran into some serious problems. First, astronomers showed that capture of an interstellar comet by Jupiter, the most massive planet, was a highly unlikely event and probably could not account for the number of short-period comets then known. Also, no comets had ever been observed on truly hyperbolic orbits. Some long-period comets did have orbit solutions that were slightly hyperbolic, barely above an eccentricity of 1.0. But a truly hyperbolic comet approaching the solar system with the Suns velocity relative to the nearby stars of about 20 km (12 miles) per second would have an eccentricity of 2.0.

In 1914 Swedish-born Danish astronomer Elis Strmgren published a special list of cometary orbits. Strmgren took the well-determined orbits of long-period comets and projected them backward in time to before the comets had entered the planetary region. He then referenced the orbits to the barycentre (the centre of mass) of the entire solar system. He found that most of the apparently hyperbolic orbits became elliptical. That proved that the comets were members of the solar system. Orbits of that type are referred to as original orbits, whereas the orbit of a comet as it passes through the planetary region is called the osculating (or instantaneous) orbit, and the orbit after the comet has left the planetary region is called the future orbit.

The idea of comets erupting from giant planets was favoured by the Soviet astronomer Sergey Vsekhsvyatsky based on similar molecules having been discovered in both the atmospheres of the giant planets and in cometary comae. The idea helped to explain the many short-period comets that regularly encountered Jupiter. But the giant planets have very large escape velocities, about 60 km (37 miles) per second in the case of Jupiter, and it was difficult to understand what physical process could achieve those velocities. So Vsekhsvyatsky moved the origin sites to the satellites of the giant planets, which had far lower escape velocities. However, most scientists still did not believe in the eruption model. The discovery of volcanos on Jupiters large satellite Io by the Voyager 1 spacecraft in 1979 briefly resurrected the idea, but Ios composition proved to be a very poor match to the composition of comets.

Another idea about cometary origins was promoted by the English astronomer Raymond Lyttleton in a research paper in 1951 and a book, The Comets and Their Origin, in 1953. Because it was known that some comets were associated with meteor showers observed on Earth, the sandbank model suggested that a comet was simply a cloud of meteoritic particles held together by its own gravity. Interplanetary gases were adsorbed on the surfaces of the dust grains and escaped when the comet came close to the Sun and the particles were heated. Lyttleton went on to explain that comets were formed when the Sun and solar system passed through an interstellar dust cloud. The Suns gravity focused the passing dust in its wake, and these subclouds then collapsed under their own gravity to form the cometary sandbanks.

One problem with that theory was that Lyttleton estimated that the gravitational focusing by the Sun would bring the particles together only about 150 AU behind the Sun and solar system. But that did not agree well with the known orbits of long-period comets, which showed no concentration of comets that would have formed at that distance or in that direction. In addition, the total amount of gases that could be adsorbed on a sandbank cloud was not sufficient to explain the measured gas production rates of many observed comets.

In 1948 Dutch astronomer Adrianus van Woerkom, as part of his Ph.D. thesis work at the University of Leiden, examined the role of Jupiters gravity in changing the orbits of comets as they passed through the planetary system. He showed that Jupiter could scatter the orbits in energy, leading to either longer or shorter orbital periods and correspondingly to larger or smaller orbits. In some cases the gravitational perturbations from Jupiter were sufficient to change the previously elliptical orbits of the comets to hyperbolic, ejecting them from the solar system and sending them into interstellar space. Van Woerkom also showed that because of Jupiter, repeated passages of comets through the solar system would lead to a uniform distribution in orbital energy for the long-period comets, with as many long-period comets ending in very long-period orbits as in very short-period orbits. Finally, van Woerkom showed that Jupiter would eventually eject all the long-period comets to interstellar space over a time span of about one million years. Thus, the comets needed to be resupplied somehow.

Van Woerkoms thesis adviser was the Dutch astronomer Jan Oort, who had become famous in the 1920s for his work on the structure and rotation of the Milky Way Galaxy. Oort became interested in the problem of where the long-period comets came from. Building on van Woerkoms work, Oort closely examined the energy distribution of long-period comet original orbits as determined by Strmgren. He found that, as van Woerkom had predicted, there was a uniform distribution of orbital energies for most energy values. But, surprisingly, there was also a large excess of comets with orbital semimajor axes (half of the long axis of the comets elliptical orbit) larger than 20,000 AU.

Oort suggested that the excess of orbits at very large distances could only be explained if the long-period comets came from there. He proposed that the solar system was surrounded by a vast cloud of comets that stretched halfway to the nearest stars. He showed that gravitational perturbations by random passing stars would perturb the orbits in the comet cloud, occasionally sending a comet into the planetary region where it could be observed. Oort referred to those comets making their first passage through the planetary region as new comets. As the new comets pass through the planetary region, Jupiters gravity takes control of their orbits, spreading them in orbital energy, and either capturing them to shorter periods or ejecting them to interstellar space.

Based on the number of comets seen each year, Oort estimated that the cloud contained 190 billion comets; today that number is thought to be closer to one trillion comets. Oorts hypothesis was all the more impressive because it was based on accurate original orbits for only 19 comets. In his honour, the cloud of comets surrounding the solar system is called the Oort cloud.

Oort noticed that the number of long-period comets returning to the planetary system was far less than what his model predicted. To account for that, he suggested that the comets were physically lost by disruption (as had happened to Bielas Comet). Oort proposed two values for the disruption rate of comets on each perihelion passage, 0.3 and 1.9 percent, which both gave reasonably good results when comparing his predictions with the actual energy distribution, except for an excess of new comets at near-zero energy.

In 1979 American astronomer Paul Weissman (the author of this article) published computer simulations of the Oort cloud energy distribution using planetary perturbations by Jupiter and Saturn and physical models of loss mechanisms such as random disruption and formation of a nonvolatile crust, based on actual observations of comets. He showed that a very good agreement with the observed energy distribution could be obtained if new comets were disrupted about 10 percent of the time on the first perihelion passage from the Oort cloud and about 4 percent of the time on subsequent passages. Also, comet nuclei developed nonvolatile crusts, cutting off all coma activity, after about 10100 returns, on average.

In 1981 American astronomer Jack Hills suggested that in addition to the Oort cloud there was also an inner cloud extending inward toward the planetary region to about 1,000 AU from the Sun. Comets are not seen coming from this region because their orbits are too tightly bound to the Sun; stellar perturbations are typically not strong enough to change their orbits significantly. Hills hypothesized that only if a star came very close, even penetrating through the Oort cloud, could it excite the orbits of the comets in the inner cloud, sending a shower of comets into the planetary system.

But where did the Oort cloud come from? At large distances on the order of 104105 AU from the Sun, the solar nebula would have been too thin to form large bodies like comets that are several kilometres in diameter. The comets had to have formed much closer to the planetary region. Oort suggested that the comets were thrown out of the asteroid belt by close encounters with Jupiter. At that time it was not known that most asteroids are rocky, carbonaceous, or iron bodies and that only a fraction contain any water.

Oorts work was preceded in part by that of the Estonian astronomer Ernst pik. In 1932 pik published a paper examining what happened to meteors or comets scattered to very large distances from the Sun, where they could be perturbed by random passing stars. He showed that the gravitational tugs from the stars would raise the perihelion distances of most objects to beyond the most distant planet. Thus, he predicted that there would be a cloud of comets surrounding the solar system. However, pik said little about the comets returning to the planetary region, other than that some comets could be thrown into the Sun by the stars during their evolution outward to the cloud. Indeed, pik concluded:

comets of an aphelion distance exceeding 10,000 a.u., are not very likely to occur among the observable objects, because of the rapid increase of the average perihelion distance due to stellar perturbations.

pik also failed to make any comparison between his results and the known original orbits of the long-period comets.

Oorts paper, published in 1950, revolutionized the field of cometary dynamics. Two months later a paper on the nature of the cometary nucleus by Fred Whipple would do the same for cometary physics. Whipple combined many of the ideas of the day and suggested that the cometary nucleus was a solid body made up of volatile ices and meteoritic material. That was called the icy conglomerate model but also became more popularly known as the dirty snowball.

Whipple provided proof for his model in the form of the shrinking orbit of Enckes Comet. Whipple believed that, as Bessel had suggested, rocket forces from sublimating ices on the sunlit side of the nucleus would alter the comets orbit. For a nonrotating solid nucleus, the force would push the nucleus away from the Sun, appearing to lessen the effect of gravity. But if the comet nucleus was rotating (as most solar system bodies do) and if the rotation pole was not perpendicular to the plane of the comets orbit, both tangential forces (forward or backward along the comets direction of motion) and out-of-plane forces (up or down) could result. The effect was helped by the thermal lag caused by the Sun continuing to heat the nucleus surface after local noontime, just as temperatures on Earth are usually at their maximum a few hours after local noon.

Thus, Whipple explained the slow shrinking of Enckes orbit as the result of tangential forces that were pointed opposite to the comets direction of motion, causing the comet nucleus to slow down, slowly shrinking the orbit. That model also explained periodic comets whose orbits were growing, such as DArrest and Wolf 1, depending on the direction of the nucleis rotation poles and the direction in which the nuclei were rotating. Because the rocket force results from the high activity of the comet nucleus near perihelion, the force does not change the perihelion distance but rather the aphelion distance, either raising or lowering it.

Whipple also pointed out that the loss of cometary ices would leave a layer of nonvolatile material on the surface of the nucleus, making sublimation more difficult, as the heat from the Sun needed to filter down through multiple layers to where there were fresh ices. Furthermore, Whipple suggested that the solar systems zodiacal dust cloud came from dust released by comets as they passed through the planetary system.

Whipples ideas set off an intense debate over whether the nucleus was a solid body or not. Many scientists still advocated Lyttletons idea of a sandbank nucleus, simply a cloud of meteoritic material with adsorbed gases. The question would not be put definitively to rest until the first spacecraft encounters with Halleys Comet in 1986.

Solid proof for Whipples nongravitational force model came from English astronomer Brian Marsden, a colleague of Whipples at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. Marsden was an expert on comet and asteroid orbits and tested Whipples icy conglomerate model against the orbits of many known comets. Using a computer program that determined the orbits of comets and asteroids from observations, Marsden added a term for the expected rocket effect when the comet was active. In this he was aided by Belgian astronomer Armand Delsemme, who carefully calculated the rate of water ice sublimation as a function of a comets distance from the Sun.

When one calculates an orbit for an object, the calculation usually does not fit all the observed positions of the object perfectly. Small errors creep into the observed positions for many reasons, such as not knowing the exact time of the observations or finding the positions using an out-of-date star catalog. So every orbit fit has a mean residual, which is the average difference between the observations and the comets predicted position based on the newly determined orbit. Mean residuals of less than about 1.5 arc seconds are considered a good fit.

When Marsden calculated the comet orbits, he found that he could obtain smaller mean residuals if he included the rocket force in his calculations. Marsden found that for a short-period comet, the magnitude of the rocket force was typically only a few hundred-thousandths of the solar gravitational attraction, but that was enough to change the time when the comet would return. Later, Marsden and colleagues computed the rocket forces for long-period comets and found that there too the mean residuals were reduced. For the long-period comets, the rocket force was typically a few ten-thousandths of the solar gravitational attraction. Long-period comets tend to be far more active than short-period comets, and thus for them the force is larger.

A further interesting result of Marsdens work was that when he performed his calculations on apparently hyperbolic comet orbits, the resulting eccentricities often changed from hyperbolic to elliptical. Very few comets were left with hyperbolic original orbits, and all of those were only slightly hyperbolic. Marsden had provided further proof that all long-period comets were members of the solar system.

In 1951 the Dutch American astronomer Gerard Kuiper published an important paper on where the comets had formed. Kuiper was studying the origin of the solar system and suggested that the volatile molecules, radicals, and ions observed in cometary comae and tails (e.g., CH, NH, OH, CN, CO+, CO2+, N2+) must come from ices frozen in the solid nucleus (e.g., CH4, NH3, H2O, HCN, CO, CO2, and N2). But those ices could only condense in the solar nebula where it was very cold. So he suggested that comets had formed at 3850 AU from the Sun, where mean temperatures were only about 3045 K (243 to 228 C, or 406 to 379 F).

Kuiper suggested that the solar nebula did not end at the orbit of what was then considered the most distant planet, Pluto, at about 39 AU, but that it continued on to about 50 AU. He believed that at those large distances from the Sun neither the density of solar nebula material nor the time was enough to form another planet. Rather, he suggested that there would be a belt of smaller bodiesi.e., cometsbetween 38 and 50 AU. He also suggested that Pluto would dynamically eject comets from that region to distant orbits, forming the Oort cloud.

Astronomers have since discovered that Pluto is too small to have done that job (or even to be considered a planet), and it is really Neptune at 30 AU that defines the outer boundary of the planetary system. Neptune is large enough to slowly scatter comets both inward to short-period orbits and outward to the Oort cloud, along with some help from the other giant planets.

Kuipers 1951 paper did not achieve the same fame as those by Oort and Whipple in 1950, but astronomers occasionally followed up his ideas. In 1968 Egyptian astronomer Salah Hamid worked with Whipple and Marsden to study the orbits of seven comets that passed near the region of Kuipers hypothetical comet belt beyond Neptune. They found no evidence of gravitational perturbations from the belt and set upper limits on the mass of the belt of 0.5 Earth masses out to 40 AU and 1.3 Earth masses out to 50 AU.

The situation changed in 1980 when Uruguayan astronomer Julio Fernndez suggested that a comet belt beyond Neptune would be a good source for the short-period comets. Up until that time it was thought that short-period comets were long-period comets from the Oort cloud that had dynamically evolved to short-period orbits because of planetary perturbations, primarily by Jupiter. But astronomers who tried to simulate that process on computers found that it was very inefficient and likely could not supply new short-period comets fast enough to replace the existing ones that either were disrupted, faded away, or were perturbed out of the planetary region.

Fernndez recognized that a key element in understanding the short-period comets was their relatively low-inclination orbits. Typical short-period comets have orbital inclinations up to about 35, whereas long-period comets have completely random orbital inclinations from 0 to 180. Fernndez suggested that the easiest way to produce a low-inclination short-period comet population was to start with a source that had a relatively low inclination. Kuipers hypothesized comet belt beyond Neptune fit this requirement. Fernndez used dynamical simulations to show how comets could be perturbed by larger bodies in the comet belt, on the order of the size of Ceres, the largest asteroid (diameter of about 940 km [580 miles]), and be sent into orbits that could encounter Neptune. Neptune then could pass about half of the comets inward to Uranus, with the other half being sent outward to the Oort cloud. In that manner, comets could be handed down to each giant planet and finally to Jupiter, which placed the comets in short-period orbits.

Fernndezs paper renewed interest in a possible comet belt beyond Neptune. In 1988 American astronomer Martin Duncan and Canadian astronomers Thomas Quinn and Scott Tremaine built a more complex computer simulation of the trans-Neptunian comet belt and again showed that it was the likely source of the short-period comets. They also proposed that the belt be named in honour of Gerard Kuiper, based on the predictions of his 1951 paper. As fate would have it, the distant comet belt had also been predicted in two lesser-known papers in 1943 and 1949 by a retired Irish army officer and astronomer, Kenneth Edgeworth. Therefore, some scientists refer to the comet belt as the Kuiper belt, while others call it the Edgeworth-Kuiper belt.

Astronomers at observatories began to search for the distant objects. In 1992 they were finally rewarded when British astronomer David Jewitt and Vietnamese American astronomer Jane Luu found an object well beyond Neptune in an orbit with a semimajor axis of 43.9 AU, an eccentricity of only 0.0678, and an inclination of only 2.19. The object, officially designated (15760) 1992 QB1, has a diameter of about 200 km (120 miles). Since 1992 more than 1,500 objects have been found in the Kuiper belt, some almost as large as Pluto. In fact, it was the discovery of that swarm of bodies beyond Neptune that led to Pluto being recognized in 2006 as simply one of the largest bodies in the swarm and no longer a planet. (The same thing happened to the largest asteroid Ceres in the mid-19th century when it was recognized as simply the largest body in the asteroid belt and not a true planet.)

In 1977 American astronomer Charles Kowal discovered an unusual object orbiting the Sun among the giant planets. Named 2060 Chiron, it is about 200 km (120 miles) in diameter and has a low-inclination orbit that stretches from 8.3 AU (inside the orbit of Saturn) to 18.85 AU (just inside the orbit of Uranus). Because it can make close approaches to those two giant planets, the orbit is unstable on a time span of several million years. Thus, Chiron likely came from somewhere else. Even more interesting, several years later Chiron began to display a cometary coma even though it was still very far from the Sun. Chiron is one of a few objects that appear in both asteroid and comet catalogs; in the latter it is designated 95 P/Chiron.

Chiron was the first of a new class of objects in giant-planet-crossing orbits to be discovered. The searches for Kuiper belt objects have also led to the discovery of many similar objects orbiting the Sun among the giant planets. Collectively they are now known as the Centaur objects. About 300 such objects have now been found, and more than a few also show sporadic cometary activity.

The Centaurs appear to be objects that are slowly diffusing into the planetary region from the Kuiper belt. Some will eventually be seen as short-period comets, while most others will be thrown into long-period orbits or even ejected to interstellar space.

In 1996 European astronomers Eric Elst and Guido Pizarro found a new comet, which was designated 133P/Elst-Pizarro. But when the orbit of the comet was determined, it was found to lie in the outer asteroid belt with a semimajor axis of 3.16 AU, an eccentricity of 0.162, and an inclination of only 1.39. A search of older records showed that 133P had been observed previously in 1979 as an inactive asteroid. So it is another object that was catalogued as both a comet and an asteroid.

The explanation for 133P was that, given its position in the asteroid belt, where maximum solar surface temperatures are only about 48 C (54 F), it likely acquired some water in the form of ice from the solar nebula. Like in comets, the ices near the surface of 133P sublimated early in its history, leaving an insulating layer of nonvolatile material covering the ice at depth. Then a random impact from a piece of asteroidal debris punched through the insulating layer and exposed the buried ice. Comet 133P has shown regular activity at the same location in its orbit for at least three orbits since it was discovered.

Twelve additional objects in asteroidal orbits have been discovered since that time, most of them also in the outer main belt. They are sometimes referred to as main belt comets, though the more recently accepted term is active asteroids.

The latter half of the 20th century saw a massive leap forward in the understanding of the solar system as a result of spacecraft visits to the planets and their satellites. Those spacecraft collected a wealth of scientific data close up and in situ. The anticipated return of Halleys Comet in 1986 provided substantial motivation to begin using spacecraft to study comets.

The first comet mission (of a sort) was the International Cometary Explorer (ICE) spacecrafts encounter with Comet 21P/Giacobini-Zinner on September 11, 1985. The mission had originally been launched as part of a joint project by the U.S. National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) known as the International Sun-Earth Explorer (ISEE). The mission consisted of three spacecraft, two of them, ISEE-1 and -2, in Earth orbit and the third, ISEE-3, positioned in a heliocentric orbit between Earth and the Sun, studying the solar wind in Earths vicinity.

In 1982 and 1983 engineers maneuvered ISEE-3 to accomplish several gravity-assist encounters with the Moon, which put it on a trajectory to encounter 21P/Giacobini-Zinner. The spacecraft was targeted to pass through the ion tail of the comet, about 7,800 km (4,800 miles) behind the nucleus at a relative velocity of 21 km (13 miles) per second, and returned the first in situ measurements of the magnetic field, plasma, and energetic particle environment inside a comets tail. Those measurements confirmed the model of the comets ion tail first put forward in 1957 by the Swedish physicist (and later Nobel Prize winner) Hannes Alfvn. It also showed that H2O+ was the most common ion in the plasma tail, consistent with the Whipple model of an icy conglomerate nucleus. However, ICE carried no instruments to study the nucleus or coma of the comet.

In 1986 five spacecraft were sent to encounter Halleys Comet. They were informally known as the Halley Armada and consisted of two Japanese spacecraft, Suisei and Sakigake (Japanese for comet and pioneer, respectively); two Soviet spacecraft, Vega 1 and 2 (a contraction of Venus-Halley using Cyrillic spelling); and an ESA spacecraft, Giotto (named after the Italian painter who depicted the Star of Bethlehem as a comet in a fresco painted in 130506).

Suisei flew by Halley on March 8, 1986, at a distance of 151,000 km (94,000 miles) on the sunward side and produced ultraviolet images of the comets hydrogen corona, an extension of the visible coma seen only in ultraviolet light. It also measured the energetic particle environment in the solar wind ahead of the comet. Sakigakes closest approach to the comet was on March 11, 1986, at a distance of 6.99 million km (4.34 million miles), and it made additional measurements of the solar wind.

Before flying past Halleys Comet, the two Soviet spacecraft had flown by Venus and had each dropped off landers and balloons to study that planet. Vega 1 flew through the Halley coma on March 6, 1986, to within 8,889 km (5,523 miles) of the nucleus and made numerous measurements of the coma gas and dust composition, plasma and energetic particles, and magnetic field environment. It also returned the first picture ever of a solid cometary nucleus. Unfortunately, the camera was slightly out of focus and had other technical problems that required considerable image processing to see the nucleus. Vega 2 fared much better when it flew through the Halley coma on March 9 to within 8,030 km (4,990 miles) of the nucleus, and its images clearly showed a peanut-shaped nucleus about 16 by 8 km (10 by 5 miles) in diameter. The nucleus was also very dark, reflecting only about 4 percent of the incident sunlight, which had already been established from Earth-based observations.

Both Vega spacecraft carried infrared spectrometers designed to measure the temperature of the Halley nucleus. They found quite warm temperatures between 320 and 400 K (47 and 127 C [116 and 260 F]). That surprised many scientists who had predicted that the effect of water ice sublimation would be to cool the nucleuss surface; water ice requires a great deal of heat to sublimate. The high temperatures suggested that much of the nucleuss surface was not sublimating, but why?

Whipples classic paper in 1950 had suggested that as comets lost material from the surface, some particles were too heavy to escape the weak gravity of the nucleus and fell back onto the surface, forming a lag deposit. That idea was later studied by American astronomer and author David Brin in his thesis work with his adviser, Sri Lankan physicist Asoka Mendis, in 1979. As the lag deposit built up, it would effectively insulate the icy materials below it from sunlight. Calculations showed that a layer only 10100 cm (439 inches) in thickness could completely turn off sublimation from the surface. Brin and Mendis predicted that Halley would be so active that it would blow away any lag deposit, but that was not the case. Only about 30 percent of Halleys sunlit hemisphere was active. Bright dust jets could be seen coming from specific areas on the nucleus surface, but much of the surface showed no visible activity.

Giotto flew through Halleys coma on March 14, 1986, and passed only 596 km (370 miles) from the nucleus. It returned the highest-resolution images of the nucleus and showed a very rugged terrain with mountain peaks jutting up hundreds of metres from the surface. It also showed the same peanut shape that Vega 2 saw but from a different viewing angle and with much greater visible detail. Discrete dust jets were coming off the nucleus surface, but the resolution was not good enough to reveal the source of the jets.

Giotto and both Vega spacecraft obtained numerous measurements of the dust and gas in the coma. Dust particles came in two types: silicate and organic. The silicate grains were typical of rocks found on Earth such as forsterite (Mg2SiO4), a high-temperature mineralthat is, one which would be among the first to condense out of the hot solar nebula. Analyses of other grains showed that the comet was far richer in magnesium relative to iron. The organic grains were composed solely of the elements carbon, hydrogen, oxygen, and nitrogen and were called CHON grains based on the chemical symbol for each of those elements. Larger grains were also detected that were combinations of silicate and CHON grains, supporting the view that comet nuclei had accreted from the slow aggregation of tiny particles in the solar nebula.

The three spacecraft also measured gases in the coma, water being the dominant molecule but also carbon monoxide accounting for about 7 percent of the gas relative to water. Formaldehyde, carbon dioxide, and hydrogen cyanide were also detected at a few percent relative to water.

The Halley Armada was a rousing success and resulted from international cooperation by many nations. Its success is even more impressive when one considers that the spacecraft all flew by the Halley nucleus at velocities ranging from 68 to 79 km per second (152,000 to 177,000 miles per hour). (The velocities were so high because Halleys retrograde orbit had it going around the Sun in the opposite direction from the spacecraft.)

Giotto was later retargeted using assists from Earths gravity to pass within about 200 km (120 miles) of the nucleus of the comet 26P/Grigg-Skjellrup. The flyby was successful, but some of the scientific instruments, including the camera, were no longer working after being sandblasted at Halley.

The next comet mission was not until 1998, when NASA launched Deep Space 1, a spacecraft designed to test a variety of new technologies. After flying past the asteroid 9969 Braille in 1999, Deep Space 1 was retargeted to fly past the comet 19P/Borrelly on September 22, 2001. Images of the Borrelly nucleus showed it to be shaped like a bowling pin, with very rugged terrain on parts of its surface and mesa-like formations over a large area of it. Individual dust and gas jets were seen emanating from the surface, but the activity was far less than that of Halleys Comet.

The NASA Stardust mission was launched in 1999 with the goal of collecting samples of dust from the coma of Comet 81P/Wild 2. At a flyby speed of 6.1 km per second (13,600 miles per hour), the dust samples would be completely destroyed by impact with a hard collector. Therefore, Stardust used a material made of silica (sand) called aerogel that had a very low density, approaching that of air. The idea was that the aerogel would slow the dust particles without destroying them, much as a detective might shoot a bullet into a box full of cotton in order to collect the undamaged bullet. It worked, and thousands of fine dust particles were returned to Earth in 2006. Perhaps the biggest surprise was that the sample contained high-temperature materials that must have formed much closer to the Sun than where the comets formed in the outer solar system. That unexpected result meant that material in the solar nebula had been mixed, at least from the inside outward, during the formation of the planets.

Stardusts images of the nucleus of Wild 2 showed a surface that was radically different from either Halley or Borrelly. The surface appeared to be covered with large flat-floored depressions. Those were likely not impact craters, as they did not have the correct morphology and there were far too many large ones. There was some suggestion that it was a very new cometary surface on a nucleus that had not been close to the Sun before. Support for that was the fact that Wild 2 had been placed into its current orbit by a close Jupiter approach in 1974, reducing the perihelion distance to about 1.5 AU (224 million km, or 139 million miles). Before the Jupiter encounter, its perihelion was 4.9 AU (733 million km, or 455 million miles), beyond the region where water ice sublimation is significant.

In 2002 NASA launched a mission called Contour (Comet Nucleus Tour) that was to fly by Enckes Comet and 73P/Schwassman-Wachmann 3 and possibly continue on to 6P/DArrest. Unfortunately, the spacecraft structure failed when leaving Earth orbit.

In 2005 NASA launched yet another comet mission, called Deep Impact. It consisted of two spacecraft, a mother spacecraft that would fly by Comet 9P/Tempel 1 and a daughter spacecraft that would be deliberately crashed into the comet nucleus. The mother spacecraft would take images of the impact. The daughter spacecraft contained its own camera system to image the nucleus surface up to the moment of impact. To maximize the effect of the impact, the daughter spacecraft contained 360 kg (794 pounds) of solid copper. The predicted impact energy was equivalent to 4.8 tonnes of TNT.

The two spacecraft encountered Tempel 1 on July 4, 2005. The impactor produced the highest-resolution pictures of a nucleus surface ever, imaging details less than 10 metres (33 feet) in size. The mother spacecraft watched the explosion and saw a huge cloud of dust and gas emitted from the nucleus. One of the mission goals was to image the crater made by the explosion, but the dust cloud was so thick that the nucleus surface could not be seen through it. Because the mission was a flyby, the mother spacecraft could not wait around for the dust to clear.

Images of the Tempel 1 nucleus were very different from what had been seen before. The surface appeared to be old, with examples of geologic processes having occurred. There was evidence of dust flows across the nucleus surface and what appeared to be two modest-sized impact craters. There was evidence of material having been eroded away. For the first time, icy patches were discovered in some small areas of the nucleus surface.

For the first time, a mission was also able to measure the mass and density of a cometary nucleus. Typically, the nuclei are too small and their gravity too weak to affect the trajectory of the flyby spacecraft. The same was true for Tempel 1, but observations of the expanding dust cloud from the impact could be modeled so as to solve for the nucleus gravity. When combined with the volume of the nucleus as obtained from the camera images, it was shown that the Tempel 1 nucleus had a bulk density between 0.2 and 1.0 gram per cubic centimetre with a preferred value of 0.4 gram per cubic centimetre, less than half that of water ice. The measurement clearly confirmed ideas from telescopic research that comets were not very dense.

After the great success of Stardust and Deep Impact, NASA had additional plans for the spacecraft. Stardust was retargeted to go to Tempel 1 and image the crater from the Deep Impact explosion as well as more of the nucleus surface not seen on the first flyby. Deep Impact was retargeted to fly past 103P/Hartley 2, a small but very active comet.

Deep Impact, in its postimpact EPOXI mission, flew past Comet Hartley 2 on November 4, 2010. It imaged a small nucleus about 2.3 km (1.4 miles) in length and 0.9 km (0.6 mile) wide. As with Halley and Borrelly, the nucleus appeared to be two bodies stuck together, each having rough terrain but covered with very fine, smooth material at the neck where they came together. The most amazing result was that the smaller of the two bodies making up the nucleus was far more active than the larger one. The activity on the smaller body appeared to be driven by CO2 sublimationan unexpected result, given that short-period comets are expected to lose their near-surface CO2 early during their many passages close to the Sun. The other half of the nucleus was far less active and only showed evidence of water ice sublimation. The active half of the comet also appeared to be flinging baseball- to basketball-sized chunks of water ice into the coma, further enhancing the gas production from the comet as they sublimated away.

The EPOXI images also showed that the nucleus was not rotating smoothly but was in complex rotationa state where the comet nucleus rotates but the direction of the rotation pole precesses rapidly, drawing a large circle on the sky. Hartley 2 was the first encountered comet to exhibit complex rotation. It was likely driven by the very high activity from the smaller half of the nucleus, putting large torques on the nucleus rotation.

Stardust/NExT (New Exploration of Tempel 1) flew past Tempel 1 on February 14, 2011, and it imaged the spot where the Deep Impact daughter spacecraft had struck the nucleus. Some scientists believed that they saw evidence of a crater about 150 metres (500 feet) in diameter, but other scientists looked at the same images and saw no clear evidence of a crater. Some of the ambiguity was due to the fact that the Stardust camera was not as sharp as the Deep Impact cameras, and some of it was also due to the fact that sunlight was illuminating the nucleus from a different direction. The debate over whether there was a recognizable crater lingers on.

Among the new areas observed by Stardust-NeXT there was further evidence of geologic processes, including layered terrains. Using stereographic imaging, the scientists traced dust jets observed in the coma back to the nucleus surface, and they appeared to originate from some of the layered terrain. Again, the resolution of the images was not good enough to understand why the jets were coming from that area.

In 2004 ESA launched Rosetta (named after the Rosetta Stone, which had unlocked the secret of Egyptian hieroglyphics) on a trajectory to Comet 67P/Churyumov-Gerasimenko (67P). Rendezvous with 67P took place on August 6, 2014. Along the way, Rosetta successfully flew by the asteroids 2849 Steins and 21 Lutetia and obtained considerable scientific data. Rosetta uses 11 scientific instruments to study the nucleus, coma, and solar wind interaction. Unlike previous comet missions, Rosetta will orbit the nucleus until December 2015, providing a complete view of the comet as activity begins, reaches a maximum at perihelion, and then wanes. Rosetta carried a spacecraft called Philae that landed on the nucleus surface on November 12, 2014. Philae drilled into the nucleus surface to collect samples of the nucleus and analyze them in situ. As the first mission to orbit and land on a cometary nucleus, Rosetta is expected to answer many questions about the sources of cometary activity.

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comet | Definition, Composition, & Facts | Britannica.com

Comet – definition of comet by The Free Dictionary

Almost in the center of it, above the Prechistenka Boulevard, surrounded and sprinkled on all sides by stars but distinguished from them all by its nearness to the earth, its white light, and its long uplifted tail, shone the enormous and brilliant comet of 18l2- the comet which was said to portend all kinds of woes and the end of the world.For instance, some worthy persons maintained that the moon was an ancient comet which, in describing its elongated orbit round the sun, happened to pass near the earth, and became confined within her circle of attraction.Their apprehensions arise from several changes they dread in the celestial bodies: for instance, that the earth, by the continual approaches of the sun towards it, must, in course of time, be absorbed, or swallowed up; that the face of the sun, will, by degrees, be encrusted with its own effluvia, and give no more light to the world; that the earth very narrowly escaped a brush from the tail of the last comet, which would have infallibly reduced it to ashes; and that the next, which they have calculated for one-and-thirty years hence, will probably destroy us.Comets, out of question, have likewise power and effect, over the gross and mass of things; but they are rather gazed upon, and waited upon in their journey, than wisely observed in their effects; specially in, their respective effects; that is, what kind of comet, for magnitude, color, version of the beams, placing in the reign of heaven, or lasting, produceth what kind of effects.It was Kitty Comet, the prettiest of all the pussies, and Comet evidently had a mission to perform, for a pink bow adorned her neck, and a bit of paper was pinned to it bearing the words, "For Miss Rose, from Frank.She'd read that interview of mine with the Comet man," Mr.At last a steady twilight brooded over the earth, a twilight only broken now and then when a comet glared across the darkling sky.No one is obliged to discover either a planet, a comet, or a satellite; and whoever makes a mistake in such a case exposes himself justly to the derision of the mass.It was already nearly seventeen years since he had received from the king, on November 7, 1465, the comet year,* that fine charge of the provostship of Paris, which was reputed rather a seigneury than an office.There was only one sofa; it was against the wall; there was only one chair where a body could get at it--I had been revolving around it like a planet, and colliding with it like a comet half the night.Then there was a wild yelp of agony and the poodle went sailing up the aisle; the yelps continued, and so did the dog; he crossed the house in front of the altar; he flew down the other aisle; he crossed before the doors; he clamored up the home-stretch; his anguish grew with his progress, till presently he was but a woolly comet moving in its orbit with the gleam and the speed of light.And, making a digression at this stage on the subject of light, I expounded at considerable length what the nature of that light must be which is found in the sun and the stars, and how thence in an instant of time it traverses the immense spaces of the heavens, and how from the planets and comets it is reflected towards the earth.

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Comet - definition of comet by The Free Dictionary

In Depth | Comets Solar System Exploration: NASA Science

OverviewIn the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions. These historic comet annals have proven to be a valuable resource for later astronomers.

We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as "dirty snowballs." They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.

Where Do Comets Come From?

As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits the Sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the Sun, become the so-called short-period comets. Taking less than 200 years to orbit the Sun, in many cases their appearance is predictable because they have passed by before. Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is, about 100,000 times the distance between Earth and the Sun) from the Sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.

Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The Sun's heat causes the comet's ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the Sun, sometimes forming a long, bright tail. Comets actually have two tailsa dust tail and an ion (gas) tail.

Most comets travel a safe distance from the Suncomet Halley comes no closer than 89 million kilometers (55 million miles). However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.

Exploration of Comets

Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley's nucleus. NASA's Deep Space 1 spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 kilometers (5 miles) long.

NASA's Stardust mission successfully flew within 236 kilometers (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006. The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.

Another NASA mission, Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet's surface. En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.

After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys. Deep Impact's mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprised two projects: the Deep Impact Extended Investigation (DIXI), which encountered comet Hartley 2 in November 2010, and the Extrasolar Planet Observation and Characterization (EPOCh) investigation, which searched for Earth-size planets around other stars on route to Hartley 2. NASA returned to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission observed changes in the nucleus since Deep Impact's 2005 encounter.

How Comets Get Their Names

Comet naming can be complicated. Comets are generally named for their discoverereither a person or a spacecraft. This International Astronomical Union guideline was developed only in the last century. For example, comet Shoemaker-Levy 9 was so named because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Since spacecraft are very effective at spotting comets many comets have LINEAR, SOHO or WISE in their names.

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In Depth | Comets Solar System Exploration: NASA Science

Overview | Comets Solar System Exploration: NASA Science

Comets are cosmic snowballs of frozen gases, rock and dust that orbit the Sun. When frozen, they are the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles. There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort Cloud.

The current number of known comets is:

Go farther. Explore Comets in Depth

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Kid-Friendly Comets

Kid-Friendly Comets

Comets orbit the Sun just like planets and asteroids do, except a comet usually has a very elongated orbit.

As the comet gets closer to the Sun, some of the ice starts to melt and boil off, along with particles of dust. These particles and gases make a cloud around the nucleus, called a coma.

The coma is lit by the Sun. The sunlight also pushes this material into the beautiful brightly lit tail of the comet.

Visit NASA Space Place for more kid-friendly facts.

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Overview | Comets Solar System Exploration: NASA Science

In Depth | Comets Solar System Exploration: NASA Science

OverviewIn the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions. These historic comet annals have proven to be a valuable resource for later astronomers.

We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as "dirty snowballs." They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.

Where Do Comets Come From?

As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits the Sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the Sun, become the so-called short-period comets. Taking less than 200 years to orbit the Sun, in many cases their appearance is predictable because they have passed by before. Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is, about 100,000 times the distance between Earth and the Sun) from the Sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.

Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The Sun's heat causes the comet's ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the Sun, sometimes forming a long, bright tail. Comets actually have two tailsa dust tail and an ion (gas) tail.

Most comets travel a safe distance from the Suncomet Halley comes no closer than 89 million kilometers (55 million miles). However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.

Exploration of Comets

Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley's nucleus. NASA's Deep Space 1 spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 kilometers (5 miles) long.

NASA's Stardust mission successfully flew within 236 kilometers (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006. The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.

Another NASA mission, Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet's surface. En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.

After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys. Deep Impact's mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprised two projects: the Deep Impact Extended Investigation (DIXI), which encountered comet Hartley 2 in November 2010, and the Extrasolar Planet Observation and Characterization (EPOCh) investigation, which searched for Earth-size planets around other stars on route to Hartley 2. NASA returned to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission observed changes in the nucleus since Deep Impact's 2005 encounter.

How Comets Get Their Names

Comet naming can be complicated. Comets are generally named for their discoverereither a person or a spacecraft. This International Astronomical Union guideline was developed only in the last century. For example, comet Shoemaker-Levy 9 was so named because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Since spacecraft are very effective at spotting comets many comets have LINEAR, SOHO or WISE in their names.

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In Depth | Comets Solar System Exploration: NASA Science

Comet – Wikipedia

icy small Solar System body

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30 (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.[1] Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets. They are still classified as comets, such as C/2014 S3 (PANSTARRS).[5] 27 Manx comets were found from 2013 to 2017.[6]

As of July2018[update] there are 6,339 known comets,[7] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[8][9] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[10] Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet,[11] and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek ("wearing long hair"), and the Oxford English Dictionary notes that the term () already meant "long-haired star, comet" in Greek. was derived from ("to wear the hair long"), which was itself derived from ("the hair of the head") and was used to mean "the tail of a comet".[12][13]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[14]

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia.[15] As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.[16] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".[17] Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[18]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[19][20] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.[21] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[22][23]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it,[24] and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%;[24] by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.[25]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[26] but ascertaining their exact size is difficult.[27] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[28] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[29] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[30] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[31]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[32] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/ChuryumovGerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[33][34] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1km (0.62mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[35][36] Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[37][38][39]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.[48]

The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000km; 280,000,000 to 370,000,000mi) of the Sun.[49] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[49] Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.[50]

Although the solid nucleus of comets is generally less than 60 kilometres (37mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[51] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[52] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[53] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[53] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[53] Ion tails have been observed to extend one astronomical unit (150million km) or more.[52]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation.[54] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[55] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[56]

In 1996, comets were found to emit X-rays.[57] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.[58]

Bow shocks form at as a result of the interaction between the solar wind and the cometary ionosphere, which is created by ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.

The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/GiacobiniZinner,[59] 1P/Halley,[60] and 26P/GriggSkjellerup.[61] It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.

The Rosetta spacecraft observed the bow shock at comet 67P/ChuryumovGerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.[62]

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[63][64] but these detections have been questioned.[65][66] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail.[54] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[67] On occasionssuch as when the Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.[68]

The observation of antitails contributed significantly to the discovery of solar wind.[69] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.[70]

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".[70] This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[71]

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."[72][73]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.[74] These streams of gas and dust can cause the nucleus to spin, and even split apart.[74] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[75] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[76]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[77] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.[78] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[79] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family".[80] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[81]

At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs).[82][83] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[84][85] As of 2019[update], 85 HTCs have been observed,[86] compared with 664 identified JFCs.[87]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[88]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[89] Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group.[83] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[90][91]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[92] a disk of objects in the trans-Neptunian regionwhereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[93] Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.[94]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[95] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[96] For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[95] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.[97] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[98] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[99] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

As of 2018, 1I/Oumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While Oumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectorywhich suggests outgassingindicate that it is indeed a comet.[100] Comet C/1980 E1 had an orbital period of roughly 7.1million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[101] Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),[102] whereas others use it to mean exclusively short-period comets.[95] Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.[103]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[105] to as far as 50,000AU (0.79ly)[84] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[105] The region can be subdivided into a spherical outer Oort cloud of 20,00050,000AU (0.320.79ly), and a doughnut-shaped inner cloud, the Hills cloud, of 2,00020,000AU (0.030.32ly).[106] The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune.[84] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[107] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[107][108][109] it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[110]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[111] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[112][113] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[111][112]

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[114] If the Earth's orbit sends it through that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle.[115] Halley's Comet is the source of the Orionid shower in October.[115]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea.[116] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[18] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[117] In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.[118] In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.[119]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.[120] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[121]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[122] The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near.[123] He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmund Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on the Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters.[122] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet,[124] causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[125]

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such they are called hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.[126] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[127]

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[32] Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[128] Some asteroids in elliptical orbits are now identified as extinct comets.[129][130] [131] [132] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[32]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[133] A significant cometary disruption was that of Comet ShoemakerLevy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmospherethe first time astronomers had observed a collision between two objects in the Solar System.[134][135] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[136] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[137] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[138]

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.[139]

Some comets have been observed to break up during their perihelion passage, including great comets West and IkeyaSeki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.[140]

Some comets meet a more spectacular end either falling into the Sun[141] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet ShoemakerLevy 9 broke up into pieces and collided with Jupiter.[142]

Ghost tail of C/2015 D1 (SOHO) after passage at the Sun

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910".

After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet.[144] Similarly, the second and third known periodic comets, Encke's Comet[145] and Biela's Comet,[146] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.[147]

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.[147]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[148] Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.[149][150]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[151] Pliny the Elder believed that comets were connected with political unrest and death.[152]

In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varhamihira and Bhadrabahu, and the 10th-century astronomer Bhaotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.[153]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[154][155]

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[156]

In 1705, Edmond Halley (16561742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 17589.[157] Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[158][159] When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.[160]

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.[161]

From his huge vapouring train perhaps to shakeReviving moisture on the numerous orbs,Thro' which his long ellipsis winds; perhapsTo lend new fuel to declining suns,To light up worlds, and feed th' ethereal fire.

James Thomson The Seasons (1730; 1748)[162]

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[163] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.[164]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[165] This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.[166]

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt.[167] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[168] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."[168] On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[169][170]

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets.[137] Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions.[179] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[180] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[181]

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick successionComet Hyakutake in 1996, followed by HaleBopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.[182]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[183] Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.[184]

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.[185] The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.[186]

Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/SchwassmannWachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.[187] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[188] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[189] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[190] Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active,[191] and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.[192]

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.[193] SOHO's 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[194] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[195] There are at least 18 comets that fit this category.[196]

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.[197] Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910)[197] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".[197]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[198] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven's Gate cult.[199]

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984).[197] In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.[200]

NASA is developing a comet harpoon for returning samples to Earth

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Comet - Wikipedia

Overview | Comets Solar System Exploration: NASA Science

Comets are cosmic snowballs of frozen gases, rock and dust that orbit the Sun. When frozen, they are the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles. There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort Cloud.

The current number of known comets is:

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Kid-Friendly Comets

Kid-Friendly Comets

Comets orbit the Sun just like planets and asteroids do, except a comet usually has a very elongated orbit.

As the comet gets closer to the Sun, some of the ice starts to melt and boil off, along with particles of dust. These particles and gases make a cloud around the nucleus, called a coma.

The coma is lit by the Sun. The sunlight also pushes this material into the beautiful brightly lit tail of the comet.

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Comet | Definition of Comet by Merriam-Webster

Recent Examples on the Web. Astronomy images span a phenomenal range scale, from things that would fit neatly on Earth (like comets or features on local bodies) to the mind-bogglingly large (like stellar nurseries or entire galaxies). John Timmer, Ars Technica, "Some good came out of 2018: Astronomy photos," 11 Nov. 2018 Earth will pass through the thickest part of the comet's trail during ...

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Comet | Definition of Comet by Merriam-Webster

Comet – Wikipedia

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30 (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.[1] Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. Recent years, the discovery of some minor bodies that has a long-period comet orbit but has the characteristics of a inner solar system asteroid sometimes is called Manx Object (It will still be classified as Comet, such as C/2014 S3 (PANSTARRS)).[5] 27 Manxes were found from 2013-2017. [6]

As of July2018[update] there are 6,339 known comets,[7] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[8][9] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[10] Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet,[11] and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek ("wearing long hair"), and the Oxford English Dictionary notes that the term () already meant "long-haired star, comet" in Greek. was derived from ("to wear the hair long"), which was itself derived from ("the hair of the head") and was used to mean "the tail of a comet".[12][13]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[14]

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia.[15] As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.[16] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".[17] Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[18]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[19][20] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.[21] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[22][23]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it,[24] and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%;[24] by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.[25]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[26] but ascertaining their exact size is difficult.[27] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[28] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[29] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[30] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[31]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[32] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/ChuryumovGerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[33][34] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1km (0.62mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[35][36] Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[37][38][39]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.[48]

The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000km; 280,000,000 to 370,000,000mi) of the Sun.[49] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[49] Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.[50]

Although the solid nucleus of comets is generally less than 60 kilometres (37mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[51] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[52] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[53] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[53] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[53] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[52]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation.[54] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[55] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[56]

In 1996, comets were found to emit X-rays.[57] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.[58]

Bow shocks form at as a result of the interaction between the solar wind and the cometary ionosphere, which is created by ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.

The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/GiacobiniZinner,[59] 1P/Halley,[60] and 26P/GriggSkjellerup.[61] It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.

The Rosetta spacecraft observed the bow shock at comet 67P/ChuryumovGerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.[62]

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[63][64] but these detections have been questioned.[65][66] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail.[54] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[67] On occasionssuch as when the Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.[68]

The observation of antitails contributed significantly to the discovery of solar wind.[69] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.[70]

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".[70] This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[71]

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."[72][73]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.[74] These streams of gas and dust can cause the nucleus to spin, and even split apart.[74] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[75] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[76]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[77] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.[78] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[79] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family".[80] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[81]

At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs).[82][83] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[84][85] As of 2018[update], 85 HTCs have been observed,[86] compared with 660 identified JFCs.[87]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[88]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[89] Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group.[83] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[90][91]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[92] a disk of objects in the trans-Neptunian regionwhereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[93] Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.[94]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[95] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[96] For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[95] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.[97] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[98] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[99] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

As of 2018, 1I/Oumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While Oumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectorywhich suggests outgassingindicate that it is indeed a comet.[100] Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[101] Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),[102] whereas others use it to mean exclusively short-period comets.[95] Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.[103]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[105] to as far as 50,000AU (0.79ly)[84] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[105] The region can be subdivided into a spherical outer Oort cloud of 20,00050,000AU (0.320.79ly), and a doughnut-shaped inner cloud, the Hills cloud, of 2,00020,000AU (0.030.32ly).[106] The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune.[84] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[107] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[107][108][109] it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[110]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[111] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[112][113] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[111][112]

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[114] If the Earth's orbit sends it through that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle.[115] Halley's Comet is the source of the Orionid shower in October.[115]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea.[116] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[18] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[117] In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.[118] In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.[119]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.[120] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[121]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[122] The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near.[123] He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmund Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on the Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters.[122] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet,[124] causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[125]

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such they are called hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.[126] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[127]

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[32] Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[128] Some asteroids in elliptical orbits are now identified as extinct comets.[129] [130] [131] [132] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[32]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[133] A significant cometary disruption was that of Comet ShoemakerLevy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmospherethe first time astronomers had observed a collision between two objects in the Solar System.[134][135] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[136] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[137] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[138]

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.[139]

Some comets have been observed to break up during their perihelion passage, including great comets West and IkeyaSeki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.[140]

Some comets meet a more spectacular end either falling into the Sun[141] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet ShoemakerLevy 9 broke up into pieces and collided with Jupiter.[142]

Ghost tail of C/2015 D1 (SOHO) after passage at the sun

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910".

After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet.[144] Similarly, the second and third known periodic comets, Encke's Comet[145] and Biela's Comet,[146] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.[147]

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.[147]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[148] Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.[149][150]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[151] Pliny the Elder believed that comets were connected with political unrest and death.[152]

In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varhamihira and Bhadrabahu, and the 10th-century astronomer Bhaotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.[153]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[154][155]

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[156]

In 1705, Edmond Halley (16561742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 17589.[157] Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[158][159] When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.[160]

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.[161]

From his huge vapouring train perhaps to shakeReviving moisture on the numerous orbs,Thro' which his long ellipsis winds; perhapsTo lend new fuel to declining suns,To light up worlds, and feed th' ethereal fire.

James Thomson The Seasons (1730; 1748)[162]

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[163] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.[164]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[165] This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.[166]

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt.[167] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[168] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."[168] On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[169][170]

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets.[137] Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions.[179] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[180] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[181]

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick successionComet Hyakutake in 1996, followed by HaleBopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.[182]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[183] Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.[184]

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.[185] The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.[186]

Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/SchwassmannWachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.[187] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[188] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[189] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[190] Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active,[191] and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.[192]

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.[193] SOHO's 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[194] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[195] There are at least 18 comets that fit this category.[196]

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.[197] Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910)[197] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".[197]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[198] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven's Gate cult.[199]

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984).[197] In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.[200]

NASA is developing a comet harpoon for returning samples to Earth

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Comet - Wikipedia

Comet – Wikipedia

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30 (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.[1] Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. Recent years, the discovery of some minor bodies that has a long-period comet orbit but has the characteristics of a inner solar system asteroid sometimes is called Manx Object (It will still be classified as Comet, such as C/2014 S3 (PANSTARRS)).[5] 27 Manxes were found from 2013-2017. [6]

As of July2018[update] there are 6,339 known comets,[7] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[8][9] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[10] Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet,[11] and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek ("wearing long hair"), and the Oxford English Dictionary notes that the term () already meant "long-haired star, comet" in Greek. was derived from ("to wear the hair long"), which was itself derived from ("the hair of the head") and was used to mean "the tail of a comet".[12][13]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[14]

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia.[15] As such, they are popularly described as "dirty snowballs" after Fred Whipple's model.[16] However, some comets may have a higher dust content, leading them to be called "icy dirtballs".[17] Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[18]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[19][20] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.[21] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[22][23]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it,[24] and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%;[24] by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.[25]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[26] but ascertaining their exact size is difficult.[27] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[28] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[29] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[30] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[31]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[32] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/ChuryumovGerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[33][34] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1km (0.62mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[35][36] Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[37][38][39]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.[48]

The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000km; 280,000,000 to 370,000,000mi) of the Sun.[49] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[49] Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.[50]

Although the solid nucleus of comets is generally less than 60 kilometres (37mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[51] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[52] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[53] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[53] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[53] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[52]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation.[54] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[55] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[56]

In 1996, comets were found to emit X-rays.[57] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.[58]

Bow shocks form at as a result of the interaction between the solar wind and the cometary ionosphere, which is created by ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears.

The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/GiacobiniZinner,[59] 1P/Halley,[60] and 26P/GriggSkjellerup.[61] It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed.

The Rosetta spacecraft observed the bow shock at comet 67P/ChuryumovGerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.[62]

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[63][64] but these detections have been questioned.[65][66] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail.[54] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[67] On occasionssuch as when the Earth passes through a comet's orbital plane, the antitail, pointing in the opposite direction to the ion and dust tails, may be seen.[68]

The observation of antitails contributed significantly to the discovery of solar wind.[69] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.[70]

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event".[70] This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[71]

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."[72][73]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.[74] These streams of gas and dust can cause the nucleus to spin, and even split apart.[74] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[75] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[76]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[77] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.[78] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[79] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family".[80] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[81]

At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs).[82][83] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[84][85] As of 2018[update], 85 HTCs have been observed,[86] compared with 660 identified JFCs.[87]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[88]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[89] Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group.[83] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[90][91]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[92] a disk of objects in the trans-Neptunian regionwhereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[93] Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.[94]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[95] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[96] For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[95] when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun.[97] The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[98] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[99] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

As of 2018, 1I/Oumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While Oumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectorywhich suggests outgassingindicate that it is indeed a comet.[100] Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[101] Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS).

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets),[102] whereas others use it to mean exclusively short-period comets.[95] Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years).

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.[103]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[105] to as far as 50,000AU (0.79ly)[84] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[105] The region can be subdivided into a spherical outer Oort cloud of 20,00050,000AU (0.320.79ly), and a doughnut-shaped inner cloud, the Hills cloud, of 2,00020,000AU (0.030.32ly).[106] The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune.[84] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[107] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[107][108][109] it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.[110]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[111] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[112][113] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[111][112]

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[114] If the Earth's orbit sends it through that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle.[115] Halley's Comet is the source of the Orionid shower in October.[115]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea.[116] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[18] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[117] In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis.[118] In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.[119]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice.[120] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[121]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[122] The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near.[123] He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmund Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on the Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters.[122] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet,[124] causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[125]

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such they are called hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.[126] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[127]

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[32] Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[128] Some asteroids in elliptical orbits are now identified as extinct comets.[129] [130] [131] [132] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[32]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[133] A significant cometary disruption was that of Comet ShoemakerLevy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmospherethe first time astronomers had observed a collision between two objects in the Solar System.[134][135] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[136] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[137] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[138]

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.[139]

Some comets have been observed to break up during their perihelion passage, including great comets West and IkeyaSeki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.[140]

Some comets meet a more spectacular end either falling into the Sun[141] or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet ShoemakerLevy 9 broke up into pieces and collided with Jupiter.[142]

Ghost tail of C/2015 D1 (SOHO) after passage at the sun

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910".

After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet.[144] Similarly, the second and third known periodic comets, Encke's Comet[145] and Biela's Comet,[146] were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.[147]

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.[147]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[148] Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.[149][150]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[151] Pliny the Elder believed that comets were connected with political unrest and death.[152]

In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varhamihira and Bhadrabahu, and the 10th-century astronomer Bhaotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.[153]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[154][155]

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.[156]

In 1705, Edmond Halley (16561742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 17589.[157] Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy.[158][159] When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.[160]

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air.[161]

From his huge vapouring train perhaps to shakeReviving moisture on the numerous orbs,Thro' which his long ellipsis winds; perhapsTo lend new fuel to declining suns,To light up worlds, and feed th' ethereal fire.

James Thomson The Seasons (1730; 1748)[162]

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion.[163] In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.[164]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[165] This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.[166]

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt.[167] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[168] The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."[168] On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[169][170]

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets.[137] Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions.[179] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[180] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[181]

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick successionComet Hyakutake in 1996, followed by HaleBopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.[182]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[183] Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.[184]

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System.[185] The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.[186]

Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/SchwassmannWachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.[187] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[188] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[189] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[190] Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active,[191] and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.[192]

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO.[193] SOHO's 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[194] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[195] There are at least 18 comets that fit this category.[196]

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.[197] Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910)[197] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".[197]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[198] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven's Gate cult.[199]

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984).[197] In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.[200]

NASA is developing a comet harpoon for returning samples to Earth

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Comet - Wikipedia

comet | Definition, Composition, & Facts | Britannica.com

HistoryAncient Greece to the 19th century

The Greek philosopher Aristotle thought that comets were dry exhalations of Earth that caught fire high in the atmosphere or similar exhalations of the planets and stars. However, the Roman philosopher Seneca thought that comets were like the planets, though in much larger orbits. He wrote:

The man will come one day who will explain in what regions the comets move, why they diverge so much from the other stars, what is their size and their nature.

Aristotles view won out and persisted until 1577, when Danish astronomer Tycho Brahe attempted to use parallax to triangulate the distance to a bright comet. Because he could not measure any parallax, Brahe concluded that the comet was very far away, at least four times farther than the Moon.

Brahes student, German astronomer Johannes Kepler, devised his three laws of planetary motion using Brahes meticulous observations of Mars but was unable to fit his theory to the very eccentric orbits of comets. Kepler believed that comets traveled in straight lines through the solar system. The solution came from English scientist Isaac Newton, who used his new law of gravity to calculate a parabolic orbit for the comet of 1680. A parabolic orbit is open, with an eccentricity of exactly 1, meaning the comet would never return. (A circular orbit has an eccentricity of 0.) Any less-eccentric orbits are closed ellipses, which means a comet would return.

Newton was friends with English astronomer Edmond Halley, who used Newtons methods to determine the orbits for 24 observed comets, which he published in 1705. All the orbits were fit with parabolas because the quality of the observations at that time was not good enough to determine elliptical or hyperbolic orbits (eccentricities greater than 1). But Halley noted that the comets of 1531, 1607, and 1682 had remarkably similar orbits and had appeared at approximately 76-year intervals. He suggested that it was really one comet in an approximately 76-year orbit that returned at regular intervals. Halley predicted that the comet would return again in 1758. He did not live to see his prediction come true, but the comet was recovered on Christmas Day, 1758, and passed closest to the Sun on March 13, 1759. The comet was the first recognized periodic comet and was named in Halleys honour, Comet Halley.

Halley also speculated whether comets were members of the solar system or not. Although he could only calculate parabolic orbits, he suggested that the orbits were actually eccentric and closed, writing:

For so their Number will be determinate and, perhaps, not so very great. Besides, the Space between the Sun and the fixd Stars is so immense that there is Room enough for a Comet to revolve tho the period of its Revolution be vastly long.

The German astronomer Johann Encke was the second person to recognize a periodic comet. He determined that a comet discovered by French astronomer Jean-Louis Pons in 1818 did not seem to follow a parabolic orbit. He found that the orbit was indeed a closed ellipse. Moreover, he showed that the orbital period of the comet around the Sun was only 3.3 years, still the shortest orbital period of any comet on record. Encke also showed that the same comet had been observed by French astronomer Pierre Mchain in 1786, by British astronomer Caroline Herschel in 1795, and by Pons in 1805. The comet was named in Enckes honour, as Comet Halley was named for the astronomer who described its orbit.

Enckes Comet soon presented a new problem for astronomers. Because it returned so often, its orbit could be predicted precisely based on Newtons law of gravity, with effects from gravitational perturbations by the planets taken into account. But Enckes Comet repeatedly arrived about 2.5 hours too soon. Its orbit was slowly shrinking. The problem became even more complex when it was discovered that other periodic comets arrived too late. Those include the comets 6P/DArrest, 14P/Wolf 1, and even 1P/Halley, which typically returns about four days later than a purely gravitational orbit would predict.

Several explanations were suggested for this phenomenon, such as a resisting interplanetary medium that caused the comet to slowly lose orbital energy. However, that idea could not explain comets whose orbits were growing, not shrinking. German mathematician and astronomer Friedrich Bessel suggested that expulsion of material from a comet near perihelion was acting like a rocket motor and propelling the comet into a slightly shorter- (or longer-) period orbit each time it passed close to the Sun. History would prove Bessel right.

As the quality of the observations and mathematical techniques to calculate orbits improved, it became obvious that most comets were on elliptical orbits and thus were members of the solar system. Many were recognized to be periodic. But some orbit solutions for long-period comets suggested that they were slightly hyperbolic, suggesting that they came from interstellar space. That problem would not be solved until the 20th century.

Another interesting problem for astronomers was a comet discovered in 1826 by the Austrian military officer and astronomer Wilhelm, Freiherr (baron) von Biela. Calculation of its orbit showed that it, like Enckes Comet, was a short-period comet; it had a period of about 6.75 years. It was only the third periodic comet to be confirmed. It was identified with a comet observed by French astronomers Jacques Lebaix Montaigne and Charles Messier in 1772 and by Pons in 1805, and it returned, as predicted, in 1832. In 1839 the comet was too close in the sky to the Sun and could not be observed, but it was seen again on schedule in November 1845. On January 13, 1846, American astronomer Matthew Maury found that there was no longer a single comet: there were two, following each other closely around the Sun. The comets returned as a pair in 1852 but were never seen again. Searches for the comets in 1865 and 1872 were unsuccessful, but a brilliant meteor shower appeared in 1872 coming from the same direction from which the comets should have appeared. Astronomers concluded that the meteor shower was the debris of the disrupted comets. However, they were still left with the question as to why the comet broke up. That recurring meteor shower is now known as the Andromedids, named for the constellation in the sky where it appears to radiate from, but is also sometimes referred to as the Bielids.

The study of meteor showers received a huge boost on November 12 and 13, 1833, when observers saw an incredible meteor shower, with rates of hundreds and perhaps thousands of meteors per hour. That shower was the Leonids, so named because its radiant (or origin) is in the constellation Leo. It was suggested that Earth was encountering interplanetary debris spread along the Earth-crossing orbits of yet unknown bodies in the solar system. Further analysis showed that the orbits of the debris were highly eccentric.

American mathematician Hubert Newton published a series of papers in the 1860s in which he examined historical records of major Leonid meteor showers and found that they occurred about every 33 years. That showed that the Leonid particles were not uniformly spread around the orbit. He predicted another major shower for November 1866. As predicted, a large Leonid meteor storm occurred on November 13, 1866. In the same year, Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteor shower, usually observed on August 1012 each year, and noted its strong similarity to the orbit of Comet Swift-Tuttle (109P/1862 O1) discovered in 1862. Soon after, the Leonids were shown to have an orbit very similar to Comet Tempel-Tuttle (55P/1865 Y1), discovered in 1865. Since then the parent comets of many meteoroid streams have been identified, though the parent comets of some streams remains a mystery.

Meanwhile, the study of comets benefitted greatly from the improvement in the quality and size of telescopes and the technology for observing comets. In 1858 English portrait artist William Usherwood took the first photograph of a comet, Comet Donati (C/1858 L1), followed by American astronomer George Bond the next night. The first photographic discovery of a comet was made by American astronomer Edward Barnard in 1892, while he was photographing the Milky Way. The comet, which was in a short-period orbit, was known as D/Barnard 3 because it was soon lost, but it was recovered by Italian astronomer Andrea Boattini in 2008 and is now known as Comet Barnard/Boattini (206P/2008 T3). In 1864 Italian astronomer Giovanni Donati was the first to look at a comet through a spectroscope, and he discovered three broad emission bands that are now known to be caused by long-chain carbon molecules in the coma. The first spectrogram (a spectrum recorded on film) was of Comet Tebbutt (C/1881 K1), taken by English astronomer William Huggins on June 24, 1881. Later the same night, an American doctor and amateur astronomer, Henry Draper, took spectra of the same comet. Both men later became professional astronomers.

Some years before the appearance of Comet Halley in 1910, the molecule cyanogen was identified as one of the molecules in the spectra of cometary comae. Cyanogen is a poisonous gas derived from hydrogen cyanide (HCN), a well-known deadly poison. It was also detected in Halleys coma as that comet approached the Sun in 1910. That led to great consternation as Earth was predicted to pass through the tail of the comet. People panicked, bought comet pills, and threw end-of-the-world parties. But when the comet passed by only 0.15 AU away on the night of May 1819, 1910, there were no detectable effects.

The 20th century saw continued progress in cometary science. Spectroscopy revealed many of the molecules, radicals, and ions in the comae and tails of comets. An understanding began to develop about the nature of cometary tails, with the ion (Type I) tails resulting from the interaction of ionized molecules with some form of corpuscular radiation, possibly electrons and protons, from the Sun, and the dust (Type II) tails coming from solar radiation pressure on the fine dust particles emitted from the comet.

Astronomers continued to ask, Where do the comets come from? There were three schools of thought: (1) that comets were captured from interstellar space, (2) that comets were erupted out of the giant planets, or (3) that comets were primeval matter that had not been incorporated into the planets. The first idea had been suggested by French mathematician and astronomer Pierre Laplace in 1813, while the second came from another French mathematician-astronomer, Joseph Lagrange. The third came from English astronomer George Chambers in 1910.

The idea of an interstellar origin for comets ran into some serious problems. First, astronomers showed that capture of an interstellar comet by Jupiter, the most massive planet, was a highly unlikely event and probably could not account for the number of short-period comets then known. Also, no comets had ever been observed on truly hyperbolic orbits. Some long-period comets did have orbit solutions that were slightly hyperbolic, barely above an eccentricity of 1.0. But a truly hyperbolic comet approaching the solar system with the Suns velocity relative to the nearby stars of about 20 km (12 miles) per second would have an eccentricity of 2.0.

In 1914 Swedish-born Danish astronomer Elis Strmgren published a special list of cometary orbits. Strmgren took the well-determined orbits of long-period comets and projected them backward in time to before the comets had entered the planetary region. He then referenced the orbits to the barycentre (the centre of mass) of the entire solar system. He found that most of the apparently hyperbolic orbits became elliptical. That proved that the comets were members of the solar system. Orbits of that type are referred to as original orbits, whereas the orbit of a comet as it passes through the planetary region is called the osculating (or instantaneous) orbit, and the orbit after the comet has left the planetary region is called the future orbit.

The idea of comets erupting from giant planets was favoured by the Soviet astronomer Sergey Vsekhsvyatsky based on similar molecules having been discovered in both the atmospheres of the giant planets and in cometary comae. The idea helped to explain the many short-period comets that regularly encountered Jupiter. But the giant planets have very large escape velocities, about 60 km (37 miles) per second in the case of Jupiter, and it was difficult to understand what physical process could achieve those velocities. So Vsekhsvyatsky moved the origin sites to the satellites of the giant planets, which had far lower escape velocities. However, most scientists still did not believe in the eruption model. The discovery of volcanos on Jupiters large satellite Io by the Voyager 1 spacecraft in 1979 briefly resurrected the idea, but Ios composition proved to be a very poor match to the composition of comets.

Another idea about cometary origins was promoted by the English astronomer Raymond Lyttleton in a research paper in 1951 and a book, The Comets and Their Origin, in 1953. Because it was known that some comets were associated with meteor showers observed on Earth, the sandbank model suggested that a comet was simply a cloud of meteoritic particles held together by its own gravity. Interplanetary gases were adsorbed on the surfaces of the dust grains and escaped when the comet came close to the Sun and the particles were heated. Lyttleton went on to explain that comets were formed when the Sun and solar system passed through an interstellar dust cloud. The Suns gravity focused the passing dust in its wake, and these subclouds then collapsed under their own gravity to form the cometary sandbanks.

One problem with that theory was that Lyttleton estimated that the gravitational focusing by the Sun would bring the particles together only about 150 AU behind the Sun and solar system. But that did not agree well with the known orbits of long-period comets, which showed no concentration of comets that would have formed at that distance or in that direction. In addition, the total amount of gases that could be adsorbed on a sandbank cloud was not sufficient to explain the measured gas production rates of many observed comets.

In 1948 Dutch astronomer Adrianus van Woerkom, as part of his Ph.D. thesis work at the University of Leiden, examined the role of Jupiters gravity in changing the orbits of comets as they passed through the planetary system. He showed that Jupiter could scatter the orbits in energy, leading to either longer or shorter orbital periods and correspondingly to larger or smaller orbits. In some cases the gravitational perturbations from Jupiter were sufficient to change the previously elliptical orbits of the comets to hyperbolic, ejecting them from the solar system and sending them into interstellar space. Van Woerkom also showed that because of Jupiter, repeated passages of comets through the solar system would lead to a uniform distribution in orbital energy for the long-period comets, with as many long-period comets ending in very long-period orbits as in very short-period orbits. Finally, van Woerkom showed that Jupiter would eventually eject all the long-period comets to interstellar space over a time span of about one million years. Thus, the comets needed to be resupplied somehow.

Van Woerkoms thesis adviser was the Dutch astronomer Jan Oort, who had become famous in the 1920s for his work on the structure and rotation of the Milky Way Galaxy. Oort became interested in the problem of where the long-period comets came from. Building on van Woerkoms work, Oort closely examined the energy distribution of long-period comet original orbits as determined by Strmgren. He found that, as van Woerkom had predicted, there was a uniform distribution of orbital energies for most energy values. But, surprisingly, there was also a large excess of comets with orbital semimajor axes (half of the long axis of the comets elliptical orbit) larger than 20,000 AU.

Oort suggested that the excess of orbits at very large distances could only be explained if the long-period comets came from there. He proposed that the solar system was surrounded by a vast cloud of comets that stretched halfway to the nearest stars. He showed that gravitational perturbations by random passing stars would perturb the orbits in the comet cloud, occasionally sending a comet into the planetary region where it could be observed. Oort referred to those comets making their first passage through the planetary region as new comets. As the new comets pass through the planetary region, Jupiters gravity takes control of their orbits, spreading them in orbital energy, and either capturing them to shorter periods or ejecting them to interstellar space.

Based on the number of comets seen each year, Oort estimated that the cloud contained 190 billion comets; today that number is thought to be closer to one trillion comets. Oorts hypothesis was all the more impressive because it was based on accurate original orbits for only 19 comets. In his honour, the cloud of comets surrounding the solar system is called the Oort cloud.

Oort noticed that the number of long-period comets returning to the planetary system was far less than what his model predicted. To account for that, he suggested that the comets were physically lost by disruption (as had happened to Bielas Comet). Oort proposed two values for the disruption rate of comets on each perihelion passage, 0.3 and 1.9 percent, which both gave reasonably good results when comparing his predictions with the actual energy distribution, except for an excess of new comets at near-zero energy.

In 1979 American astronomer Paul Weissman (the author of this article) published computer simulations of the Oort cloud energy distribution using planetary perturbations by Jupiter and Saturn and physical models of loss mechanisms such as random disruption and formation of a nonvolatile crust, based on actual observations of comets. He showed that a very good agreement with the observed energy distribution could be obtained if new comets were disrupted about 10 percent of the time on the first perihelion passage from the Oort cloud and about 4 percent of the time on subsequent passages. Also, comet nuclei developed nonvolatile crusts, cutting off all coma activity, after about 10100 returns, on average.

In 1981 American astronomer Jack Hills suggested that in addition to the Oort cloud there was also an inner cloud extending inward toward the planetary region to about 1,000 AU from the Sun. Comets are not seen coming from this region because their orbits are too tightly bound to the Sun; stellar perturbations are typically not strong enough to change their orbits significantly. Hills hypothesized that only if a star came very close, even penetrating through the Oort cloud, could it excite the orbits of the comets in the inner cloud, sending a shower of comets into the planetary system.

But where did the Oort cloud come from? At large distances on the order of 104105 AU from the Sun, the solar nebula would have been too thin to form large bodies like comets that are several kilometres in diameter. The comets had to have formed much closer to the planetary region. Oort suggested that the comets were thrown out of the asteroid belt by close encounters with Jupiter. At that time it was not known that most asteroids are rocky, carbonaceous, or iron bodies and that only a fraction contain any water.

Oorts work was preceded in part by that of the Estonian astronomer Ernst pik. In 1932 pik published a paper examining what happened to meteors or comets scattered to very large distances from the Sun, where they could be perturbed by random passing stars. He showed that the gravitational tugs from the stars would raise the perihelion distances of most objects to beyond the most distant planet. Thus, he predicted that there would be a cloud of comets surrounding the solar system. However, pik said little about the comets returning to the planetary region, other than that some comets could be thrown into the Sun by the stars during their evolution outward to the cloud. Indeed, pik concluded:

comets of an aphelion distance exceeding 10,000 a.u., are not very likely to occur among the observable objects, because of the rapid increase of the average perihelion distance due to stellar perturbations.

pik also failed to make any comparison between his results and the known original orbits of the long-period comets.

Oorts paper, published in 1950, revolutionized the field of cometary dynamics. Two months later a paper on the nature of the cometary nucleus by Fred Whipple would do the same for cometary physics. Whipple combined many of the ideas of the day and suggested that the cometary nucleus was a solid body made up of volatile ices and meteoritic material. That was called the icy conglomerate model but also became more popularly known as the dirty snowball.

Whipple provided proof for his model in the form of the shrinking orbit of Enckes Comet. Whipple believed that, as Bessel had suggested, rocket forces from sublimating ices on the sunlit side of the nucleus would alter the comets orbit. For a nonrotating solid nucleus, the force would push the nucleus away from the Sun, appearing to lessen the effect of gravity. But if the comet nucleus was rotating (as most solar system bodies do) and if the rotation pole was not perpendicular to the plane of the comets orbit, both tangential forces (forward or backward along the comets direction of motion) and out-of-plane forces (up or down) could result. The effect was helped by the thermal lag caused by the Sun continuing to heat the nucleus surface after local noontime, just as temperatures on Earth are usually at their maximum a few hours after local noon.

Thus, Whipple explained the slow shrinking of Enckes orbit as the result of tangential forces that were pointed opposite to the comets direction of motion, causing the comet nucleus to slow down, slowly shrinking the orbit. That model also explained periodic comets whose orbits were growing, such as DArrest and Wolf 1, depending on the direction of the nucleis rotation poles and the direction in which the nuclei were rotating. Because the rocket force results from the high activity of the comet nucleus near perihelion, the force does not change the perihelion distance but rather the aphelion distance, either raising or lowering it.

Whipple also pointed out that the loss of cometary ices would leave a layer of nonvolatile material on the surface of the nucleus, making sublimation more difficult, as the heat from the Sun needed to filter down through multiple layers to where there were fresh ices. Furthermore, Whipple suggested that the solar systems zodiacal dust cloud came from dust released by comets as they passed through the planetary system.

Whipples ideas set off an intense debate over whether the nucleus was a solid body or not. Many scientists still advocated Lyttletons idea of a sandbank nucleus, simply a cloud of meteoritic material with adsorbed gases. The question would not be put definitively to rest until the first spacecraft encounters with Halleys Comet in 1986.

Solid proof for Whipples nongravitational force model came from English astronomer Brian Marsden, a colleague of Whipples at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. Marsden was an expert on comet and asteroid orbits and tested Whipples icy conglomerate model against the orbits of many known comets. Using a computer program that determined the orbits of comets and asteroids from observations, Marsden added a term for the expected rocket effect when the comet was active. In this he was aided by Belgian astronomer Armand Delsemme, who carefully calculated the rate of water ice sublimation as a function of a comets distance from the Sun.

When one calculates an orbit for an object, the calculation usually does not fit all the observed positions of the object perfectly. Small errors creep into the observed positions for many reasons, such as not knowing the exact time of the observations or finding the positions using an out-of-date star catalog. So every orbit fit has a mean residual, which is the average difference between the observations and the comets predicted position based on the newly determined orbit. Mean residuals of less than about 1.5 arc seconds are considered a good fit.

When Marsden calculated the comet orbits, he found that he could obtain smaller mean residuals if he included the rocket force in his calculations. Marsden found that for a short-period comet, the magnitude of the rocket force was typically only a few hundred-thousandths of the solar gravitational attraction, but that was enough to change the time when the comet would return. Later, Marsden and colleagues computed the rocket forces for long-period comets and found that there too the mean residuals were reduced. For the long-period comets, the rocket force was typically a few ten-thousandths of the solar gravitational attraction. Long-period comets tend to be far more active than short-period comets, and thus for them the force is larger.

A further interesting result of Marsdens work was that when he performed his calculations on apparently hyperbolic comet orbits, the resulting eccentricities often changed from hyperbolic to elliptical. Very few comets were left with hyperbolic original orbits, and all of those were only slightly hyperbolic. Marsden had provided further proof that all long-period comets were members of the solar system.

In 1951 the Dutch American astronomer Gerard Kuiper published an important paper on where the comets had formed. Kuiper was studying the origin of the solar system and suggested that the volatile molecules, radicals, and ions observed in cometary comae and tails (e.g., CH, NH, OH, CN, CO+, CO2+, N2+) must come from ices frozen in the solid nucleus (e.g., CH4, NH3, H2O, HCN, CO, CO2, and N2). But those ices could only condense in the solar nebula where it was very cold. So he suggested that comets had formed at 3850 AU from the Sun, where mean temperatures were only about 3045 K (243 to 228 C, or 406 to 379 F).

Kuiper suggested that the solar nebula did not end at the orbit of what was then considered the most distant planet, Pluto, at about 39 AU, but that it continued on to about 50 AU. He believed that at those large distances from the Sun neither the density of solar nebula material nor the time was enough to form another planet. Rather, he suggested that there would be a belt of smaller bodiesi.e., cometsbetween 38 and 50 AU. He also suggested that Pluto would dynamically eject comets from that region to distant orbits, forming the Oort cloud.

Astronomers have since discovered that Pluto is too small to have done that job (or even to be considered a planet), and it is really Neptune at 30 AU that defines the outer boundary of the planetary system. Neptune is large enough to slowly scatter comets both inward to short-period orbits and outward to the Oort cloud, along with some help from the other giant planets.

Kuipers 1951 paper did not achieve the same fame as those by Oort and Whipple in 1950, but astronomers occasionally followed up his ideas. In 1968 Egyptian astronomer Salah Hamid worked with Whipple and Marsden to study the orbits of seven comets that passed near the region of Kuipers hypothetical comet belt beyond Neptune. They found no evidence of gravitational perturbations from the belt and set upper limits on the mass of the belt of 0.5 Earth masses out to 40 AU and 1.3 Earth masses out to 50 AU.

The situation changed in 1980 when Uruguayan astronomer Julio Fernndez suggested that a comet belt beyond Neptune would be a good source for the short-period comets. Up until that time it was thought that short-period comets were long-period comets from the Oort cloud that had dynamically evolved to short-period orbits because of planetary perturbations, primarily by Jupiter. But astronomers who tried to simulate that process on computers found that it was very inefficient and likely could not supply new short-period comets fast enough to replace the existing ones that either were disrupted, faded away, or were perturbed out of the planetary region.

Fernndez recognized that a key element in understanding the short-period comets was their relatively low-inclination orbits. Typical short-period comets have orbital inclinations up to about 35, whereas long-period comets have completely random orbital inclinations from 0 to 180. Fernndez suggested that the easiest way to produce a low-inclination short-period comet population was to start with a source that had a relatively low inclination. Kuipers hypothesized comet belt beyond Neptune fit this requirement. Fernndez used dynamical simulations to show how comets could be perturbed by larger bodies in the comet belt, on the order of the size of Ceres, the largest asteroid (diameter of about 940 km [580 miles]), and be sent into orbits that could encounter Neptune. Neptune then could pass about half of the comets inward to Uranus, with the other half being sent outward to the Oort cloud. In that manner, comets could be handed down to each giant planet and finally to Jupiter, which placed the comets in short-period orbits.

Fernndezs paper renewed interest in a possible comet belt beyond Neptune. In 1988 American astronomer Martin Duncan and Canadian astronomers Thomas Quinn and Scott Tremaine built a more complex computer simulation of the trans-Neptunian comet belt and again showed that it was the likely source of the short-period comets. They also proposed that the belt be named in honour of Gerard Kuiper, based on the predictions of his 1951 paper. As fate would have it, the distant comet belt had also been predicted in two lesser-known papers in 1943 and 1949 by a retired Irish army officer and astronomer, Kenneth Edgeworth. Therefore, some scientists refer to the comet belt as the Kuiper belt, while others call it the Edgeworth-Kuiper belt.

Astronomers at observatories began to search for the distant objects. In 1992 they were finally rewarded when British astronomer David Jewitt and Vietnamese American astronomer Jane Luu found an object well beyond Neptune in an orbit with a semimajor axis of 43.9 AU, an eccentricity of only 0.0678, and an inclination of only 2.19. The object, officially designated (15760) 1992 QB1, has a diameter of about 200 km (120 miles). Since 1992 more than 1,500 objects have been found in the Kuiper belt, some almost as large as Pluto. In fact, it was the discovery of that swarm of bodies beyond Neptune that led to Pluto being recognized in 2006 as simply one of the largest bodies in the swarm and no longer a planet. (The same thing happened to the largest asteroid Ceres in the mid-19th century when it was recognized as simply the largest body in the asteroid belt and not a true planet.)

In 1977 American astronomer Charles Kowal discovered an unusual object orbiting the Sun among the giant planets. Named 2060 Chiron, it is about 200 km (120 miles) in diameter and has a low-inclination orbit that stretches from 8.3 AU (inside the orbit of Saturn) to 18.85 AU (just inside the orbit of Uranus). Because it can make close approaches to those two giant planets, the orbit is unstable on a time span of several million years. Thus, Chiron likely came from somewhere else. Even more interesting, several years later Chiron began to display a cometary coma even though it was still very far from the Sun. Chiron is one of a few objects that appear in both asteroid and comet catalogs; in the latter it is designated 95 P/Chiron.

Chiron was the first of a new class of objects in giant-planet-crossing orbits to be discovered. The searches for Kuiper belt objects have also led to the discovery of many similar objects orbiting the Sun among the giant planets. Collectively they are now known as the Centaur objects. About 300 such objects have now been found, and more than a few also show sporadic cometary activity.

The Centaurs appear to be objects that are slowly diffusing into the planetary region from the Kuiper belt. Some will eventually be seen as short-period comets, while most others will be thrown into long-period orbits or even ejected to interstellar space.

In 1996 European astronomers Eric Elst and Guido Pizarro found a new comet, which was designated 133P/Elst-Pizarro. But when the orbit of the comet was determined, it was found to lie in the outer asteroid belt with a semimajor axis of 3.16 AU, an eccentricity of 0.162, and an inclination of only 1.39. A search of older records showed that 133P had been observed previously in 1979 as an inactive asteroid. So it is another object that was catalogued as both a comet and an asteroid.

The explanation for 133P was that, given its position in the asteroid belt, where maximum solar surface temperatures are only about 48 C (54 F), it likely acquired some water in the form of ice from the solar nebula. Like in comets, the ices near the surface of 133P sublimated early in its history, leaving an insulating layer of nonvolatile material covering the ice at depth. Then a random impact from a piece of asteroidal debris punched through the insulating layer and exposed the buried ice. Comet 133P has shown regular activity at the same location in its orbit for at least three orbits since it was discovered.

Twelve additional objects in asteroidal orbits have been discovered since that time, most of them also in the outer main belt. They are sometimes referred to as main belt comets, though the more recently accepted term is active asteroids.

The latter half of the 20th century saw a massive leap forward in the understanding of the solar system as a result of spacecraft visits to the planets and their satellites. Those spacecraft collected a wealth of scientific data close up and in situ. The anticipated return of Halleys Comet in 1986 provided substantial motivation to begin using spacecraft to study comets.

The first comet mission (of a sort) was the International Cometary Explorer (ICE) spacecrafts encounter with Comet 21P/Giacobini-Zinner on September 11, 1985. The mission had originally been launched as part of a joint project by the U.S. National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) known as the International Sun-Earth Explorer (ISEE). The mission consisted of three spacecraft, two of them, ISEE-1 and -2, in Earth orbit and the third, ISEE-3, positioned in a heliocentric orbit between Earth and the Sun, studying the solar wind in Earths vicinity.

In 1982 and 1983 engineers maneuvered ISEE-3 to accomplish several gravity-assist encounters with the Moon, which put it on a trajectory to encounter 21P/Giacobini-Zinner. The spacecraft was targeted to pass through the ion tail of the comet, about 7,800 km (4,800 miles) behind the nucleus at a relative velocity of 21 km (13 miles) per second, and returned the first in situ measurements of the magnetic field, plasma, and energetic particle environment inside a comets tail. Those measurements confirmed the model of the comets ion tail first put forward in 1957 by the Swedish physicist (and later Nobel Prize winner) Hannes Alfvn. It also showed that H2O+ was the most common ion in the plasma tail, consistent with the Whipple model of an icy conglomerate nucleus. However, ICE carried no instruments to study the nucleus or coma of the comet.

In 1986 five spacecraft were sent to encounter Halleys Comet. They were informally known as the Halley Armada and consisted of two Japanese spacecraft, Suisei and Sakigake (Japanese for comet and pioneer, respectively); two Soviet spacecraft, Vega 1 and 2 (a contraction of Venus-Halley using Cyrillic spelling); and an ESA spacecraft, Giotto (named after the Italian painter who depicted the Star of Bethlehem as a comet in a fresco painted in 130506).

Suisei flew by Halley on March 8, 1986, at a distance of 151,000 km (94,000 miles) on the sunward side and produced ultraviolet images of the comets hydrogen corona, an extension of the visible coma seen only in ultraviolet light. It also measured the energetic particle environment in the solar wind ahead of the comet. Sakigakes closest approach to the comet was on March 11, 1986, at a distance of 6.99 million km (4.34 million miles), and it made additional measurements of the solar wind.

Before flying past Halleys Comet, the two Soviet spacecraft had flown by Venus and had each dropped off landers and balloons to study that planet. Vega 1 flew through the Halley coma on March 6, 1986, to within 8,889 km (5,523 miles) of the nucleus and made numerous measurements of the coma gas and dust composition, plasma and energetic particles, and magnetic field environment. It also returned the first picture ever of a solid cometary nucleus. Unfortunately, the camera was slightly out of focus and had other technical problems that required considerable image processing to see the nucleus. Vega 2 fared much better when it flew through the Halley coma on March 9 to within 8,030 km (4,990 miles) of the nucleus, and its images clearly showed a peanut-shaped nucleus about 16 by 8 km (10 by 5 miles) in diameter. The nucleus was also very dark, reflecting only about 4 percent of the incident sunlight, which had already been established from Earth-based observations.

Both Vega spacecraft carried infrared spectrometers designed to measure the temperature of the Halley nucleus. They found quite warm temperatures between 320 and 400 K (47 and 127 C [116 and 260 F]). That surprised many scientists who had predicted that the effect of water ice sublimation would be to cool the nucleuss surface; water ice requires a great deal of heat to sublimate. The high temperatures suggested that much of the nucleuss surface was not sublimating, but why?

Whipples classic paper in 1950 had suggested that as comets lost material from the surface, some particles were too heavy to escape the weak gravity of the nucleus and fell back onto the surface, forming a lag deposit. That idea was later studied by American astronomer and author David Brin in his thesis work with his adviser, Sri Lankan physicist Asoka Mendis, in 1979. As the lag deposit built up, it would effectively insulate the icy materials below it from sunlight. Calculations showed that a layer only 10100 cm (439 inches) in thickness could completely turn off sublimation from the surface. Brin and Mendis predicted that Halley would be so active that it would blow away any lag deposit, but that was not the case. Only about 30 percent of Halleys sunlit hemisphere was active. Bright dust jets could be seen coming from specific areas on the nucleus surface, but much of the surface showed no visible activity.

Giotto flew through Halleys coma on March 14, 1986, and passed only 596 km (370 miles) from the nucleus. It returned the highest-resolution images of the nucleus and showed a very rugged terrain with mountain peaks jutting up hundreds of metres from the surface. It also showed the same peanut shape that Vega 2 saw but from a different viewing angle and with much greater visible detail. Discrete dust jets were coming off the nucleus surface, but the resolution was not good enough to reveal the source of the jets.

Giotto and both Vega spacecraft obtained numerous measurements of the dust and gas in the coma. Dust particles came in two types: silicate and organic. The silicate grains were typical of rocks found on Earth such as forsterite (Mg2SiO4), a high-temperature mineralthat is, one which would be among the first to condense out of the hot solar nebula. Analyses of other grains showed that the comet was far richer in magnesium relative to iron. The organic grains were composed solely of the elements carbon, hydrogen, oxygen, and nitrogen and were called CHON grains based on the chemical symbol for each of those elements. Larger grains were also detected that were combinations of silicate and CHON grains, supporting the view that comet nuclei had accreted from the slow aggregation of tiny particles in the solar nebula.

The three spacecraft also measured gases in the coma, water being the dominant molecule but also carbon monoxide accounting for about 7 percent of the gas relative to water. Formaldehyde, carbon dioxide, and hydrogen cyanide were also detected at a few percent relative to water.

The Halley Armada was a rousing success and resulted from international cooperation by many nations. Its success is even more impressive when one considers that the spacecraft all flew by the Halley nucleus at velocities ranging from 68 to 79 km per second (152,000 to 177,000 miles per hour). (The velocities were so high because Halleys retrograde orbit had it going around the Sun in the opposite direction from the spacecraft.)

Giotto was later retargeted using assists from Earths gravity to pass within about 200 km (120 miles) of the nucleus of the comet 26P/Grigg-Skjellrup. The flyby was successful, but some of the scientific instruments, including the camera, were no longer working after being sandblasted at Halley.

The next comet mission was not until 1998, when NASA launched Deep Space 1, a spacecraft designed to test a variety of new technologies. After flying past the asteroid 9969 Braille in 1999, Deep Space 1 was retargeted to fly past the comet 19P/Borrelly on September 22, 2001. Images of the Borrelly nucleus showed it to be shaped like a bowling pin, with very rugged terrain on parts of its surface and mesa-like formations over a large area of it. Individual dust and gas jets were seen emanating from the surface, but the activity was far less than that of Halleys Comet.

The NASA Stardust mission was launched in 1999 with the goal of collecting samples of dust from the coma of Comet 81P/Wild 2. At a flyby speed of 6.1 km per second (13,600 miles per hour), the dust samples would be completely destroyed by impact with a hard collector. Therefore, Stardust used a material made of silica (sand) called aerogel that had a very low density, approaching that of air. The idea was that the aerogel would slow the dust particles without destroying them, much as a detective might shoot a bullet into a box full of cotton in order to collect the undamaged bullet. It worked, and thousands of fine dust particles were returned to Earth in 2006. Perhaps the biggest surprise was that the sample contained high-temperature materials that must have formed much closer to the Sun than where the comets formed in the outer solar system. That unexpected result meant that material in the solar nebula had been mixed, at least from the inside outward, during the formation of the planets.

Stardusts images of the nucleus of Wild 2 showed a surface that was radically different from either Halley or Borrelly. The surface appeared to be covered with large flat-floored depressions. Those were likely not impact craters, as they did not have the correct morphology and there were far too many large ones. There was some suggestion that it was a very new cometary surface on a nucleus that had not been close to the Sun before. Support for that was the fact that Wild 2 had been placed into its current orbit by a close Jupiter approach in 1974, reducing the perihelion distance to about 1.5 AU (224 million km, or 139 million miles). Before the Jupiter encounter, its perihelion was 4.9 AU (733 million km, or 455 million miles), beyond the region where water ice sublimation is significant.

In 2002 NASA launched a mission called Contour (Comet Nucleus Tour) that was to fly by Enckes Comet and 73P/Schwassman-Wachmann 3 and possibly continue on to 6P/DArrest. Unfortunately, the spacecraft structure failed when leaving Earth orbit.

In 2005 NASA launched yet another comet mission, called Deep Impact. It consisted of two spacecraft, a mother spacecraft that would fly by Comet 9P/Tempel 1 and a daughter spacecraft that would be deliberately crashed into the comet nucleus. The mother spacecraft would take images of the impact. The daughter spacecraft contained its own camera system to image the nucleus surface up to the moment of impact. To maximize the effect of the impact, the daughter spacecraft contained 360 kg (794 pounds) of solid copper. The predicted impact energy was equivalent to 4.8 tonnes of TNT.

The two spacecraft encountered Tempel 1 on July 4, 2005. The impactor produced the highest-resolution pictures of a nucleus surface ever, imaging details less than 10 metres (33 feet) in size. The mother spacecraft watched the explosion and saw a huge cloud of dust and gas emitted from the nucleus. One of the mission goals was to image the crater made by the explosion, but the dust cloud was so thick that the nucleus surface could not be seen through it. Because the mission was a flyby, the mother spacecraft could not wait around for the dust to clear.

Images of the Tempel 1 nucleus were very different from what had been seen before. The surface appeared to be old, with examples of geologic processes having occurred. There was evidence of dust flows across the nucleus surface and what appeared to be two modest-sized impact craters. There was evidence of material having been eroded away. For the first time, icy patches were discovered in some small areas of the nucleus surface.

For the first time, a mission was also able to measure the mass and density of a cometary nucleus. Typically, the nuclei are too small and their gravity too weak to affect the trajectory of the flyby spacecraft. The same was true for Tempel 1, but observations of the expanding dust cloud from the impact could be modeled so as to solve for the nucleus gravity. When combined with the volume of the nucleus as obtained from the camera images, it was shown that the Tempel 1 nucleus had a bulk density between 0.2 and 1.0 gram per cubic centimetre with a preferred value of 0.4 gram per cubic centimetre, less than half that of water ice. The measurement clearly confirmed ideas from telescopic research that comets were not very dense.

After the great success of Stardust and Deep Impact, NASA had additional plans for the spacecraft. Stardust was retargeted to go to Tempel 1 and image the crater from the Deep Impact explosion as well as more of the nucleus surface not seen on the first flyby. Deep Impact was retargeted to fly past 103P/Hartley 2, a small but very active comet.

Deep Impact, in its postimpact EPOXI mission, flew past Comet Hartley 2 on November 4, 2010. It imaged a small nucleus about 2.3 km (1.4 miles) in length and 0.9 km (0.6 mile) wide. As with Halley and Borrelly, the nucleus appeared to be two bodies stuck together, each having rough terrain but covered with very fine, smooth material at the neck where they came together. The most amazing result was that the smaller of the two bodies making up the nucleus was far more active than the larger one. The activity on the smaller body appeared to be driven by CO2 sublimationan unexpected result, given that short-period comets are expected to lose their near-surface CO2 early during their many passages close to the Sun. The other half of the nucleus was far less active and only showed evidence of water ice sublimation. The active half of the comet also appeared to be flinging baseball- to basketball-sized chunks of water ice into the coma, further enhancing the gas production from the comet as they sublimated away.

The EPOXI images also showed that the nucleus was not rotating smoothly but was in complex rotationa state where the comet nucleus rotates but the direction of the rotation pole precesses rapidly, drawing a large circle on the sky. Hartley 2 was the first encountered comet to exhibit complex rotation. It was likely driven by the very high activity from the smaller half of the nucleus, putting large torques on the nucleus rotation.

Stardust/NExT (New Exploration of Tempel 1) flew past Tempel 1 on February 14, 2011, and it imaged the spot where the Deep Impact daughter spacecraft had struck the nucleus. Some scientists believed that they saw evidence of a crater about 150 metres (500 feet) in diameter, but other scientists looked at the same images and saw no clear evidence of a crater. Some of the ambiguity was due to the fact that the Stardust camera was not as sharp as the Deep Impact cameras, and some of it was also due to the fact that sunlight was illuminating the nucleus from a different direction. The debate over whether there was a recognizable crater lingers on.

Among the new areas observed by Stardust-NeXT there was further evidence of geologic processes, including layered terrains. Using stereographic imaging, the scientists traced dust jets observed in the coma back to the nucleus surface, and they appeared to originate from some of the layered terrain. Again, the resolution of the images was not good enough to understand why the jets were coming from that area.

In 2004 ESA launched Rosetta (named after the Rosetta Stone, which had unlocked the secret of Egyptian hieroglyphics) on a trajectory to Comet 67P/Churyumov-Gerasimenko (67P). Rendezvous with 67P took place on August 6, 2014. Along the way, Rosetta successfully flew by the asteroids 2849 Steins and 21 Lutetia and obtained considerable scientific data. Rosetta uses 11 scientific instruments to study the nucleus, coma, and solar wind interaction. Unlike previous comet missions, Rosetta will orbit the nucleus until December 2015, providing a complete view of the comet as activity begins, reaches a maximum at perihelion, and then wanes. Rosetta carried a spacecraft called Philae that landed on the nucleus surface on November 12, 2014. Philae drilled into the nucleus surface to collect samples of the nucleus and analyze them in situ. As the first mission to orbit and land on a cometary nucleus, Rosetta is expected to answer many questions about the sources of cometary activity.

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comet | Definition, Composition, & Facts | Britannica.com

Comet Facts – Comets – Astronomy for Kids

Temperature is very irregular in outer space. The parts that are near stars are extremely hot! Think about Venus, the second closest planet to the Sun. It goes up to 462C. But the background temperature in space is about -270C super cold! Things can change states if temperatures change so much. They can go from solid, to liquid, to gas! This is actually the reason why comets have their tails!

The tail is one of the most distinctive features of a comet!

Comets may look small from a distance, but theyre actually gigantic!

See how the Kuiper belt is disc-shaped? The Oort Cloud is farther away, so gravity from the planets dont affect it as much. Thats why it envelopes the Solar System like a sphere or a cloud!

Where the comet moves in space is important for its shape! When comets are still in the far reaches of the Oort Cloud or the Kuiper Belt, theyre made up only of their nuclei. But everything changes once they move closer to the Sun! Remember a comet is mostly made out of ice.

And what happens to ice as it gets close to heat? It melts! In the case of comets, their nuclei start to sublimate, changing from ice to gas immediately. This is when the comet starts developing its other parts!

As the ice melts, the comet gains a coma. The coma is basically a giant cloud of dust and different gases that surrounds the nucleus. Comas are extremely big up to 600,000 miles across! The coma and the nucleus make up the head of the comet. A hydrogen cloud also develops around the comets head, but we cant see it with our eyes. Hydrogen clouds are even bigger than comas they can get as big as 10 Suns!

Heres an easyway to remember what a comets head is called. The coma looks kind of like the head of a comma without its tail!

The comets tail appears when it gets close to the Sun. The tail is probably the most special feature of comets!

Asteroids are not icy like comets. Instead, theyre made out of rock and metals

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Comet Facts - Comets - Astronomy for Kids


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