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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

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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Overview | Comets Solar System Exploration: NASA Science

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], only 83 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

What Are Comets? – Time and Date

Comets are small celestial bodies that orbit the Sun. Primarily made of dust and ice, they are thought to be remnants of the formation of the Solar System.

Comet PanSTARRS was visible in early 2013.

Comet PanSTARRS C/2011 L4 was visible to observers on Earth in early 2013.

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Comets are thought to come from 2 places in the Solar System:

What are meteor showers?

One of the distinguishing features of a comet is that most of them develop a tail, known as a coma, when they come close to the Sun.

Away from the Sun, comets are frozen celestial bodies that are hard to detect. However, as a comet comes closer to the Sun, the Suns heat and radiation vaporize the ice and dust of the comet. These vaporized gasses collect dust and stream out of the center of the comet like a tail. This tail can be thousands of miles long.

While most comets passing by the Sun are hard to observe from Earth without specialized equipment, some comets are bright enough to be seen by the naked eye. The brightness of the comet is due to sunlight reflecting and refracting off the dust in the tail.

Comets usually have 2 tails, which point in different directions. The dust in the comet is responsible for one tail. This tail, also called the dust tail, tends to be broad and curved. The gasses in the comet make the second tail, called the plasma or the ion tail. This tail is thin and straight and tends to point directly away from the Sun.

What are asteroids?

A light year (light-year or lightyear) is a unit of distance and is defined by the International Astronomical Union as the distance traveled by light in a vacuum during a Julian year. It is approximately equal to 10 trillion kilometers (6 trillion miles).

Topics: Astronomy, Comets, Asteroids

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What Are Comets? – Time and Date

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.

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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

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.

As of July2018[update] there are 6,339 known comets,[5] 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.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] 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,[9] 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”.[10][11]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[12]

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.[13] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[14] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[15] 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.[16]

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.[17][18] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[19] 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.[20][21]

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,[22] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0%;[22] 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.[23]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[24] but ascertaining their exact size is difficult.[25] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[26] 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.[27] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[28] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[29]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[30] 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.[31][32] 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.[33][34] 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.[35][36][37]

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.[46]

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.[47] 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.[47] 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.[48]

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.[49] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[50] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[51] 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.[51] 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.[51] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[50]

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.[52] 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.[53] 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.[54]

In 1996, comets were found to emit X-rays.[55] 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.[56]

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,[57] 1P/Halley,[58] and 26P/GriggSkjellerup.[59] 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.[60]

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[61][62] but these detections have been questioned.[63][64] 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.[52] 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.[65] On occasions – such as when the Earth passes through a comet’s orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.[66]

The observation of antitails contributed significantly to the discovery of solar wind.[67] 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.[68]

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”.[68] 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.[69]

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.”[70][71]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[72] These streams of gas and dust can cause the nucleus to spin, and even split apart.[72] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[73] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[74]

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.[75] 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.[76] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[77] 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”.[78] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[79]

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).[80][81] 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).[82][83] As of 2018[update], only 82 HTCs have been observed,[84] compared with 659 identified JFCs.[85]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[86]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[87] 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.[81] 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.[88][89]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[90] 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).[91] 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.[92]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[93] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[94] 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[93] 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.[95] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[96] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[97] 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.[98] 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.[99] 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),[100] whereas others use it to mean exclusively short-period comets.[93] 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.[101]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[103] to as far as 50,000AU (0.79ly)[82] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[103] 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).[104] 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.[82] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[105] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[105][106][107] 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.[108]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[109] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[110][111] 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.[109][110]

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.[112] 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.[113] Halley’s Comet is the source of the Orionid shower in October.[113]

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.[114] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[16] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[115] 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.[116] 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.[117]

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.[118] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[119]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[citation needed] 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.[120] 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.[121] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley’s Comet,[122] causing panicked buying of gas masks and quack “anti-comet pills” and “anti-comet umbrellas” by the public.[123]

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.[124] 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.[125]

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.[30] 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.[126] Some asteroids in elliptical orbits are now identified as extinct comets.[127] [128] [129] [130] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[30]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[131] 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.[132][133] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[134] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[135] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[136]

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.[137]

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.[138]

Some comets meet a more spectacular end either falling into the Sun[139] 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.[140]

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.[142] Similarly, the second and third known periodic comets, Encke’s Comet[143] and Biela’s Comet,[144] 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.[145]

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.[145]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[146] 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.[147][148]

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.[149] Pliny the Elder believed that comets were connected with political unrest and death.[150]

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.[151]

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.[152][153]

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.[154]

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.[155] 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.[156][157] 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.[158]

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)[159]

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.[160]

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.[161] 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.[162]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[163] 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.[164]

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.[165] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[166] 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.”[166] 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).[167][168]

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.[135] 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.[177] 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.[178] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[179]

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.[180]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[181] 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.[182]

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.[183] 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.[184]

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.[185] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[186] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[187] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[188] 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,[189] 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.[190]

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.[191] SOHO’s 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[192] 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.[193] There are at least 18 comets that fit this category.[194]

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.[195] 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)[195] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson”.[195]

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,[196] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[197]

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).[195] 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.[198]

NASA is developing a comet harpoon for returning samples to Earth

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Comet – Wikipedia

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 | Definition of Comet by Merriam-Webster

Recent Examples on the Web. Near-Earth objects are comets (cosmic snowballs of frozen gases, rock and dust the size of a small town) and asteroids (basically, space rocks smaller than planets) that pass within 28 miles of Earths orbit. Ashley May, USA TODAY, “NASA: Here’s the big plan to protect the planet from ‘near-Earth objects’,” 21 June 2018 Organic molecules are being found in …

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Comet | Definition of Comet by Merriam-Webster

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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Overview | Comets Solar System Exploration: NASA Science

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.

As of November2014[update] there are 5,253 known comets,[5] 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.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] 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,[9] 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”.[10][11]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[12]

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.[13] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[14] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[15] 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.[16]

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.[17][18] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[19] 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.[20][21]

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,[22] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0%;[22] 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.[23]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[24] but ascertaining their exact size is difficult.[25] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[26] 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.[27] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[28] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[29]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[30] 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.[31][32] 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.[33][34] 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.[35][36][37]

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.[46]

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.[47] 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.[47] 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.[48]

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.[49] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[50] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[51] 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.[51] 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.[51] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[50]

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.[52] 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.[53] 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.[54]

In 1996, comets were found to emit X-rays.[55] 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.[56]

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,[57] 1P/Halley,[58] and 26P/GriggSkjellerup.[59] 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.[60]

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[61][62] but these detections have been questioned.[63][64] 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.[52] 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.[65] On occasions – such as when the Earth passes through a comet’s orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.[66]

The observation of antitails contributed significantly to the discovery of solar wind.[67] 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.[68]

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”.[68] 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.[69]

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.”[70][71]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[72] These streams of gas and dust can cause the nucleus to spin, and even split apart.[72] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[73] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[74]

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.[75] 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.[76] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[77] 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”.[78] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[79]

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).[80][81] 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).[82][83] As of 2018[update], only 82 HTCs have been observed,[84] compared with 659 identified JFCs.[85]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[86]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[87] 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.[81] 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.[88][89]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[90] 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).[91] 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.[92]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[93] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[94] 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[93] 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.[95] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[96] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[97] 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.[98] 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.[99] 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),[100] whereas others use it to mean exclusively short-period comets.[93] 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.[101]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[103] to as far as 50,000AU (0.79ly)[82] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[103] 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).[104] 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.[82] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[105] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[105][106][107] 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.[108]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[109] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[110][111] 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.[109][110]

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.[112] 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.[113] Halley’s Comet is the source of the Orionid shower in October.[113]

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.[114] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[16] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[115] 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.[116] 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.[117]

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.[118] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[119]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[citation needed] 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.[120] 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.[121] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley’s Comet,[122] causing panicked buying of gas masks and quack “anti-comet pills” and “anti-comet umbrellas” by the public.[123]

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.[124] 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.[125]

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.[30] 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.[126] Some asteroids in elliptical orbits are now identified as extinct comets.[127] [128] [129] [130] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[30]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[131] 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.[132][133] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[134] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[135] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[136]

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.[137]

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.[138]

Some comets meet a more spectacular end either falling into the Sun[139] 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.[140]

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.[142] Similarly, the second and third known periodic comets, Encke’s Comet[143] and Biela’s Comet,[144] 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.[145]

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.[145]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[146] 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.[147][148]

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.[149] Pliny the Elder believed that comets were connected with political unrest and death.[150]

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.[151]

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.[152][153]

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.[154]

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.[155] 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.[156][157] 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.[158]

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)[159]

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.[160]

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.[161] 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.[162]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[163] 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.[164]

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.[165] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[166] 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.”[166] 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).[167][168]

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.[135] 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.[177] 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.[178] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[179]

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.[180]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[181] 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.[182]

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.[183] 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.[184]

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.[185] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[186] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[187] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[188] 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,[189] 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.[190]

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.[191] SOHO’s 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[192] 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.[193] There are at least 18 comets that fit this category.[194]

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.[195] 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)[195] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson”.[195]

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,[196] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[197]

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).[195] 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.[198]

NASA is developing a comet harpoon for returning samples to Earth

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Comet – Wikipedia

Comet | Definition of Comet by Merriam-Webster

Recent Examples on the Web. Near-Earth objects are comets (cosmic snowballs of frozen gases, rock and dust the size of a small town) and asteroids (basically, space rocks smaller than planets) that pass within 28 miles of Earths orbit. Ashley May, USA TODAY, “NASA: Here’s the big plan to protect the planet from ‘near-Earth objects’,” 21 June 2018 Organic molecules are being found in …

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Comet | Definition of Comet by Merriam-Webster

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:

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Overview | Comets Solar System Exploration: NASA Science

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.

As of November2014[update] there are 5,253 known comets,[5] 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.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] 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,[9] 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”.[10][11]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[12]

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.[13] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[14] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[15] 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.[16]

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.[17][18] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[19] 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.[20][21]

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,[22] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0%;[22] 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.[23]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[24] but ascertaining their exact size is difficult.[25] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[26] 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.[27] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[28] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[29]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[30] 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.[31][32] 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.[33][34] 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.[35][36][37]

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.[46]

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.[47] 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.[47] 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.[48]

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.[49] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[50] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[51] 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.[51] 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.[51] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[50]

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.[52] 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.[53] 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.[54]

In 1996, comets were found to emit X-rays.[55] 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.[56]

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[57][58] but these detections have been questioned.[59][60] 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.[52] 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.[61] On occasions – such as when the Earth passes through a comet’s orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.[62]

The observation of antitails contributed significantly to the discovery of solar wind.[63] 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.[64]

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”.[64] 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.[65]

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.”[66][67]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[68] These streams of gas and dust can cause the nucleus to spin, and even split apart.[68] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[69] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[70]

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.[71] 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.[72] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[73] 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”.[74] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[75]

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).[76][77] 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).[78][79] As of 2018[update], only 82 HTCs have been observed,[80] compared with 659 identified JFCs.[81]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[82]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[83] 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.[77] 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.[84][85]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[86] 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).[87] 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.[88]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[89] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[90] 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[89] 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.[91] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[92] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[93] 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.[94] 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.[95] 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),[96] whereas others use it to mean exclusively short-period comets.[89] 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.[97]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[99] to as far as 50,000AU (0.79ly)[78] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[99] 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).[100] 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.[78] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[101] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[101][102][103] 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.[104]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[105] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[106][107] 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.[105][106]

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.[108] 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.[109] Halley’s Comet is the source of the Orionid shower in October.[109]

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.[110] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[16] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[111] 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.[112] 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.[113]

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.[114] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[115]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[citation needed] 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.[116] 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.[117] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley’s Comet,[118] causing panicked buying of gas masks and quack “anti-comet pills” and “anti-comet umbrellas” by the public.[119]

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.[120] 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.[121]

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.[30] 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.[122] Some asteroids in elliptical orbits are now identified as extinct comets.[123] [124] [125] [126] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[30]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[127] 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.[128][129] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[130] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[131] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[132]

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.[133]

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.[134]

Some comets meet a more spectacular end either falling into the Sun[135] 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.[136]

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.[138] Similarly, the second and third known periodic comets, Encke’s Comet[139] and Biela’s Comet,[140] 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.[141]

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.[141]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[142] 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.[143][144]

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.[145] Pliny the Elder believed that comets were connected with political unrest and death.[146]

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.[147]

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.[148][149]

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.[150]

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.[151] 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.[152][153] 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.[154]

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)[155]

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.[156]

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.[157] 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.[158]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[159] 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.[160]

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.[161] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[162] 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.”[162] 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).[163][164]

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.[131] 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.[173] 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.[174] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[175]

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.[176]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[177] 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.[178]

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.[179] 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.[180]

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.[181] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[182] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[183] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[184] 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,[185] 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.[186]

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.[187] SOHO’s 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[188] 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.[189] There are at least 18 comets that fit this category.[190]

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.[191] 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)[191] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson”.[191]

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,[192] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[193]

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).[191] 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.[194]

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.

As of November2014[update] there are 5,253 known comets,[5] 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.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] 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,[9] 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”.[10][11]

The astronomical symbol for comets is (in Unicode U+2604), consisting of a small disc with three hairlike extensions.[12]

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.[13] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[14] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[15] 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.[16]

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.[17][18] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[19] 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.[20][21]

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,[22] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0%;[22] 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.[23]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[24] but ascertaining their exact size is difficult.[25] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[26] 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.[27] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[28] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[29]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[30] 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.[31][32] 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.[33][34] 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.[35][36][37]

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.[46]

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.[47] 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.[47] 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.[48]

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.[49] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[50] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[51] 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.[51] 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.[51] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[50]

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.[52] 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.[53] 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.[54]

In 1996, comets were found to emit X-rays.[55] 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.[56]

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[57][58] but these detections have been questioned.[59][60] 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.[52] 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.[61] On occasions – such as when the Earth passes through a comet’s orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.[62]

The observation of antitails contributed significantly to the discovery of solar wind.[63] 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.[64]

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”.[64] 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.[65]

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.”[66][67]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[68] These streams of gas and dust can cause the nucleus to spin, and even split apart.[68] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[69] Infrared imaging of Hartley2 shows such jets exiting and carrying with it dust grains into the coma.[70]

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.[71] 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.[72] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[73] 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”.[74] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[75]

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).[76][77] 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).[78][79] As of 2018[update], only 82 HTCs have been observed,[80] compared with 659 identified JFCs.[81]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[82]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[83] 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.[77] 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.[84][85]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[86] 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).[87] 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.[88]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[89] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[90] 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[89] 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.[91] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[92] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[93] 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.[94] 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.[95] 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),[96] whereas others use it to mean exclusively short-period comets.[89] 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.[97]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[99] to as far as 50,000AU (0.79ly)[78] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[99] 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).[100] 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.[78] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[101] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[101][102][103] 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.[104]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[105] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[106][107] 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.[105][106]

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.[108] 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.[109] Halley’s Comet is the source of the Orionid shower in October.[109]

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.[110] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[16] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[111] 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.[112] 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.[113]

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.[114] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[115]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[citation needed] 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.[116] 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.[117] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley’s Comet,[118] causing panicked buying of gas masks and quack “anti-comet pills” and “anti-comet umbrellas” by the public.[119]

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.[120] 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.[121]

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.[30] 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.[122] Some asteroids in elliptical orbits are now identified as extinct comets.[123] [124] [125] [126] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[30]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[127] 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.[128][129] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[130] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[131] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[132]

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.[133]

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.[134]

Some comets meet a more spectacular end either falling into the Sun[135] 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.[136]

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.[138] Similarly, the second and third known periodic comets, Encke’s Comet[139] and Biela’s Comet,[140] 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.[141]

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.[141]

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[142] 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.[143][144]

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.[145] Pliny the Elder believed that comets were connected with political unrest and death.[146]

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.[147]

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.[148][149]

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.[150]

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.[151] 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.[152][153] 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.[154]

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)[155]

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.[156]

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.[157] 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.[158]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[159] 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.[160]

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.[161] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[162] 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.”[162] 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).[163][164]

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.[131] 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.[173] 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.[174] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[175]

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.[176]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[177] 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.[178]

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.[179] 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.[180]

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.[181] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[182] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[183] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[184] 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,[185] 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.[186]

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.[187] SOHO’s 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[188] 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.[189] There are at least 18 comets that fit this category.[190]

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.[191] 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)[191] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson”.[191]

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,[192] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[193]

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).[191] 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.[194]

NASA is developing a comet harpoon for returning samples to Earth

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