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Comet – Wikipedia

Icy small Solar System body

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

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

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

As of July2019[update] there are 6,619 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 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 latinization 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, many comets have a higher dust content, leading them to be called "icy dirtballs".[17] The term "icy dirtballs" arose after observation of Comet 9P/Tempel 1 collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. 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, 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 kilometers (19mi) have been observed,[26] but ascertaining their exact size is difficult.[27] The nucleus of 322P/SOHO is probably only 100200 meters (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 meters (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 water 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 kilometers (37mi) across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun.[51] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[52] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[53] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[53] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[53] Ion tails have been observed to extend one astronomical unit (150million km) or more.[52]

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

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

Bow shocks form 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 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 ionization 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 2020[update], 91 HTCs have been observed,[86] compared with 685 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 hypothesized 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] When flung into the orbit of the sun, and being continuously dragged towards it, tons of matter are stripped from the comets which greatly influence their lifetime; the more stripped, the shorter they live and vice versa.[95]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[96] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[97] 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 70,000AU (0.34pc; 1.1ly) 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[96] 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.[98] The Sun's Hill sphere has an unstable maximum boundary of 230,000AU (1.1pc; 3.6ly).[99] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[100] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

As of 2019[update], only two objects have been discovered with an eccentricity significantly greater than one: 1I/Oumuamua and 2I/Borisov, indicating an origin outside the Solar System. While Oumuamua, with an eccentricity of about 1.2, 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 probably a comet.[101] On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet.[102][103] Comet C/1980 E1 had an orbital period of roughly 7.1million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[104] 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),[105] whereas others use it to mean exclusively short-period comets.[96] 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.[106]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[108] to as far as 50,000AU (0.79ly)[84] from the Sun. This cloud encases the celestial bodies that start at the middle of our solar systemthe sun, all the way to outer limits of the Kuiper Belt. The Oort cloud consists of viable materials necessary for the creation of celestial bodies. The planets we have today, exist only because of the planetesimals (chunks of leftover space that assisted in the creation of planets) that were condensed and formed by the gravity of the sun. The eccentric made from these trapped planetesimals is why the Oort Cloud even exists.[109] Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[108] 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).[110] 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.[111] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[111][112][113] 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.[114]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[115] The first exocomet system detected was around a star named Beta Pictoris, a very young sitting at around 20 million years old A-type main-sequence star, in 1987.[116][117] A total of 11 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.[115][116] For ten years the Kepler Space Telescope was responsible for searching for planets and other forms outside of the solar system. The first transiting exocomets were found in February 2018 by a group consisting of professional astronomers and citizen scientists in light curves recorded by the Kepler Space Telescope.[118][119] After Kepler Space Telescope retired in October 2018, a new telescope called TESS Telescope has taken over Kepler's mission. Since the launch of TESS, astronomers have discovered the transits of comets around the star Beta Pictoris using a light curve from TESS.[120][121] Since TESS has taken over, astronomers have since been able to better distinguish exocomets with the spectroscopic method. New planets are detected by the white light curve method which is viewed as a symmetrical dip in the charts readings when a planet overshadows its parent star. However, after further evaluation of these light curves, it has been discovered that the asymmetrical patterns of the dips presented are caused by the tail of a comet or of hundreds of comets.[122]

As a comet is heated during close passes to the Sun, outgassing of its icy components also releases solid debris too large to be swept away by radiation pressure and the solar wind.[123] If Earth's orbit sends it through that trail of debris, which is composed mostly of fine grains of rocky material, there is likely to be a meteor shower as Earth passes through. Denser trails of debris produce quick but intense meteor showers and less dense trails create longer but less intense showers. Typically, the density of the debris trail is related to how long ago the parent comet released the material.[124][125] The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle. Halley's Comet is the source of the Orionid shower in October.[126][127]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4billion years ago brought the vast quantities of water that now fill Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea.[128] 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.[129] 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.[130] The speed at which the comets entered the atmosphere, combined with the magnitude of energy created after initial contact, allowed smaller molecules to condense into the larger macro-molecules that served as the foundation for life.[131] 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.[132]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to Earth's Moon, some of which may have survived as lunar ice.[133] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[134]

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650.[135] 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.[136] 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 Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters.[135] Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet,[137] causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[138]

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.[139] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[140]

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.[141] Some asteroids in elliptical orbits are now identified as extinct comets.[142][143][144][145] 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.[146] 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.[147][148] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[149] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[150] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[151]

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.[152]

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 Earth crosses the orbit of Biela's Comet.[153]

Some comets meet a more spectacular end either falling into the Sun[154] 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.[155]

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.[157] Similarly, the second and third known periodic comets, Encke's Comet[158] and Biela's Comet,[159] 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.[160]

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.[160] For example, in 2019, astronomer Gennady Borisov observed a comet that appeared to have originated outside of the solar system, the comet was named C/2019 Q4 (Borisov) after him.

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia.[161] 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.[162][163] In the 11th century Bayeux Tapestry, Halley's Comet is depicted portending the death of Harold and the triumph of the Normans at the Battle of Hastings.[164]

According to Norse Mythology, comets were actually a part of the Giant Ymir's skull. According to the tale, Odin and his brothers slew Ymir and set about constructing the world (Earth) from his corpse. They fashioned the oceans from his blood, the soil from his skin and muscles, vegetation from his hair, clouds from his brains, and the sky from his skull. Four dwarves, corresponding to the four cardinal points, held Ymir's skull aloft above the earth. Following this tale, comets in the sky, as believed by the Norse, were flakes of Ymir's skull falling from the sky and then disintegrating.[165]

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.[166]

In 1301, the Italian painter Giotto was the first person to accurately and anatomically portray a comet. In his work Adoration of the Magi, Giotto's depiction of Halley's Comet in the place of the Star of Bethlehem would go unmatched in accuracy until the 19th century and be bested only with the invention of photography.[164]

Aristotle was the first known scientist to utilize various theories and observational facts to employ a consistent, structured cosmological theory of comets. He 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. Aristotle's cometary theory arose from his observations and cosmological theory that everything in the cosmos is arranged in a distinct configuration.[167] Part of this configuration was a clear separation between the celestial and terrestrial, believing comets to be strictly associated with the latter. According to Aristotle comets must be within the sphere of the moon and clearly separated from the heavens. His theory on comets was widely accepted throughout the Middle Ages, despite several discoveries from various individuals challenging aspects of his work.[168] One notable challenger was Seneca, who questioned the logic of his predecessors sparking much debate among Aristotle's critics in the 16th and 17th centuries. Seneca thought comets to be more permanent than suggested by their brief flashes across the sky and provided thought provoking evidence of the celestial nature of comets .[168] He posed many questions regarding the validity of contemporary theories on comets, however, he did not author a substantial theory of his own.[169] Pliny the Elder believed that comets were connected with political unrest and death.[170] Pliny observed comets as "human like", often describing their tails with "long hair" or "long beard".[171] His system for classifying comets according to their color and shape was used for centuries.[172]

Astrological interpretations of comets proceeded to take precedence clear into the 15th century, despite the presence of modern scientific astronomy beginning to take root. In the 1400s, comets continue to forewarn disaster as seen in the Luzerner Schilling chronicles and in the warnings of Pope Callixtus III.[164] Regiomontanus was the first to attempt to calculate Diurnal parallax by observing the great comet of 1472. His predictions were not very accurate, but they were conducted in the hopes of estimating the distance of a comet from the Earth.[172]

In the 16th century, Tycho Brahe and Michael Maestlin demonstrated that comets must exist outside Earth's atmosphere by measuring the parallax of the Great Comet of 1577.[173] Within the precision of the measurements, this implied the comet must be at least four times more distant than from Earth to the Moon.[174][175] Based on observations in 1664, Giovanni Borelli recorded the longitudes and latitudes of comets that he observed, and suggested that cometary orbits may be parabolic.[176] Galileo Galilei one of the most renowned astronomers to date, even attempted writings on comets in The Assayer. He rejected Tycho Brahe's theories on the parallax of comets and claimed that they may be a mere optical illusion. Intrigued as early scientists were about the nature of comets, Galileo could not help but throw about his own theories despite little personal observation.[172] Kepler responded to these unjust criticisms in his work Hyperaspistes.

Also occurring in the Early modern period was the study of comets and their astrological significance in medical disciplines. Many healers of this time considered medicine and astronomy to be inter-disciplinary, and employed their knowledge of comets and other astrological signs for diagnosing and treating patients.[177]

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.[178] Newton was one of the first to contribute to the physical understanding of the nature of comets.

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.[179] 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.[180][181] 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.[182]

In the 19th century, the Astronomical Observatory of Padova, was an epicenter in the observational study of comets. Led by Giovanni Santini (1787-1877) and followed by Giuseppe Lorenzoni (1843-1914), this observatory was devoted to classical astronomy, mainly to the new comets and planets orbit calculation, with the goal of compiling of a catalog of almost ten thousand stars. Situated in the Northern portion of Italy, observations from this observatory were key in establishing important geodetic, geographic, and astronomical calculations, such as the difference of longitude between Milan and Padua as well as Padua to Fiume.[183] In addition to these geographic observations, correspondence within the observatory, particularly between Santini and another astronomer Giuseppe Toaldo, about the importance of comet and planetary orbital observations.[184]

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.[185]

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)[186]

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.[187] 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.[188]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[189] 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.[190]

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.[191] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[192] 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."[192] 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).[193][194]

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.[150] 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.[203] 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 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.[204] Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[205]

The Great Comet of 1577 is a well known example of a great comet. The Great Comet of 1577 passed near Earth as a non-periodic comet and was seen by many, including well-known astronomers Tycho Brahe and Taqi ad-Din. Observations of this comet led to several significant findings regarding cometary science, especially for Brahe.

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.[206]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometers.[207] 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.[208]

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.[209] 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.[210]

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.[211] 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[212] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[213] (See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[214] 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,[215] 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.[216]

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.[217] SOHO's 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[218] and both discoverers of HaleBopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[219] There are at least 18 comets that fit this category.[220]

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.[221] 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)[221] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".[221]

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, Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[222] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven's Gate cult.[223]

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).[221] 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.[224]

NASA is developing a comet harpoon for returning samples to Earth

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Comet - Wikipedia

Overview | Comets NASA Solar System Exploration

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.

Visit NASA Space Place for more kid-friendly facts.

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Overview | Comets NASA Solar System Exploration

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.

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In May 2020, many Southern Hemisphere residents and some Northern Hemisphere dwellers will be lucky enough to see Comet SWAN (C/2020 F8). Discovered in March 2020 by the Solar and Heliospheric Observatory spacecraft (SOHO), the comet will make its closest approach to Earth on May 13, 2020, and to the Sun around May 27.

Comets are notoriously unpredictable and can disintegrate at any time. As it stands now, however, Comet SWAN is promising to put on quite a good show for both Northern and Southern Hemisphere viewers.

If you live in the Southern latitudes, you'll be able to spot it until mid-May. With its current magnitude (5.4 on May 7, 2020), it is not easily seen by the naked eye. Experts suggest that you first spot the comet using binoculars. Look towards the constellation Cetus in the early mornings until May 8 and then Pisces until May 15.

Use our Night Sky Map to find constellations in the sky

If all goes well with the comet, those in lower Northern latitudes will be able to spot the comet starting early May during the early mornings. Look towards the constellation of Cetus or Pisces. It will then move through Triangulum (around May 17), Perseus (around May 20), and Auriga (in early June). After this, the comet will no longer be visible with the naked eye. Mid-northern latitude viewers should be able to see the comet, weather permitting of course, in the evenings around May 21 onwards.

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, just 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

comet | Definition, Composition, & Facts | Britannica

Comet, a small body orbiting the Sun with a substantial fraction of its composition made up of volatile ices. When a comet comes close to the Sun, the ices sublimate (go directly from the solid to the gas phase) and form, along with entrained dust particles, a bright outflowing atmosphere around the comet nucleus known as a coma. As dust and gas in the coma flow freely into space, the comet forms two tails, one composed of ionized molecules and radicals and one of dust. The word comet comes from the Greek (kometes), which means long-haired. Indeed, it is the appearance of the bright coma that is the standard observational test for whether a newly discovered object is a comet or an asteroid.

Comets are among the most-spectacular objects in the sky, with their bright glowing comae and their long dust tails and ion tails. Comets can appear at random from any direction and provide a fabulous and ever-changing display for many months as they move in highly eccentric orbits around the Sun.

Comets are important to scientists because they are primitive bodies left over from the formation of the solar system. They were among the first solid bodies to form in the solar nebula, the collapsing interstellar cloud of dust and gas out of which the Sun and planets formed. Comets formed in the outer regions of the solar nebula where it was cold enough for volatile ices to condense. This is generally taken to be beyond 5 astronomical units (AU; 748 million km, or 465 million miles), or beyond the orbit of Jupiter. Because comets have been stored in distant orbits beyond the planets, they have undergone few of the modifying processes that have melted or changed the larger bodies in the solar system. Thus, they retain a physical and chemical record of the primordial solar nebula and of the processes involved in the formation of planetary systems.

A comet is made up of four visible parts: the nucleus, the coma, the ion tail, and the dust tail. The nucleus is a solid body typically a few kilometres in diameter and made up of a mixture of volatile ices (predominantly water ice) and silicate and organic dust particles. The coma is the freely escaping atmosphere around the nucleus that forms when the comet comes close to the Sun and the volatile ices sublimate, carrying with them dust particles that are intimately mixed with the frozen ices in the nucleus. The dust tail forms from those dust particles and is blown back by solar radiation pressure to form a long curving tail that is typically white or yellow in colour. The ion tail forms from the volatile gases in the coma when they are ionized by ultraviolet photons from the Sun and blown away by the solar wind. Ion tails point almost exactly away from the Sun and glow bluish in colour because of the presence of CO+ ions.

Comets differ from other bodies in the solar system in that they are generally in orbits that are far more eccentric than those of the planets and most asteroids and far more inclined to the ecliptic (the plane of Earths orbit). Some comets appear to come from distances of over 50,000 AU, a substantial fraction of the distance to the nearest stars. Their orbital periods can be millions of years in length. Other comets have shorter periods and smaller orbits that carry them from the orbits of Jupiter and Saturn inward to the orbits of the terrestrial planets. Some comets even appear to come from interstellar space, passing around the Sun on open, hyperbolic orbits, but in fact are members of the solar system.

Comets are typically named for their discoverers, though some comets (e.g., Halley and Encke) are named for the scientists who first recognized that their orbits were periodic. The International Astronomical Union (IAU) prefers a maximum of two discoverers to be in a comets name. In some cases where a comet has been lost (its orbit was not determined well enough to predict its return), the comet is named for the original discoverer and also the observer(s) who found it again. A designation of C/ before a comets name denotes that it is a long-period comet (period greater than 200 years), while P/ denotes that the comet is periodic; i.e., it returns at regular, predictable intervals of fewer than 200 years. A designation of D/ denotes that the comet is deceased or destroyed, such as D/Shoemaker-Levy 9, the comet whose components struck Jupiter in July 1994. Numbers appearing before the name of a comet denote that it is periodic; the comets are numbered in the order that they are confirmed to be periodic. Comet 1P/Halley is the first comet to be recognized as periodic and is named after English astronomer Edmond Halley, who determined that it was periodic.

In 1995 the IAU implemented a new identification system for each appearance of a comet, whether it is periodic or long-period. The system uses the year of the comets discovery, the half-month in the year denoted by a letter A through Y (with I omitted to avoid confusion), and a number signifying the order in which the comet was found within that half-month. Thus, Halleys Comet is designated 1P/1682 Q1 when Halley saw it in August 1682, but 1P/1982 U1 when it was first spotted by astronomers before its predicted perihelion (point when closest to the Sun) passage in 1986. This identification system is similar to that now used for asteroid discoveries, though the asteroids are so designated only when they are first discovered. (The asteroids are later given official catalog numbers and names.) Formerly, a number after the name of a periodic comet denoted its order among comets discovered by that individual or group, but for new comets there would be no such distinguishing number.

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comet | Definition, Composition, & Facts | Britannica

Comets Facts | Types, Composition, Size, Information …

Unlike the other small bodies in the solar system, comets have been known since antiquity. There are Chinese records ofComet Halleygoing back to at least 240 BC. The famousBayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of Comet Halley.

As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated. Of these 184 areperiodiccomets (orbital periods less than 200 years); some of the remainder are no doubt periodic as well, but their orbits have not been determined with sufficient accuracy to tell for sure.

Comets are sometimes calleddirty snowballsor icy mudballs. They are a mixture of ices (both water and frozen gases) and dust that for some reason didnt get incorporated into planets when the solar system was formed. This makes them very interesting as samples of the early history of the solar system.

When they are near theSunand active, comets have several distinct parts:

Comets are invisible except when they are near the Sun. Most comets have highly eccentric orbits which take them far beyond the orbit ofPluto; these are seen once and then disappear for millennia. Only the short- and intermediate-period comets (like Comet Halley), stay within the orbit of Pluto for a significant fraction of their orbits.

After 500 or so passes near the Sun off most of a comets ice and gasis lost leaving a rocky object very much like anasteroidin appearance. (Perhaps half of the near-Earth asteroids may be dead comets.) A comet whose orbit takes it near the Sun is also likely to either impact one of the planets or the Sun or to be ejected out of the solar system by a close encounter (esp. with Jupiter).

By far the most famous comet isComet HalleybutSL 9was a big hit for a week in the summer of 1994.

Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity: thePerseidmeteor shower occurs every year between August 9 and 13 when the Earth passes thru the orbit of CometSwift-Tuttle. Comet Halley is the source of theOrionidshower in October.

Many comets are first discovered by amateur astronomers. Since comets are brightest when near the Sun, they are usually visible only at sunrise or sunset. Charts showing the positions in the sky of some comets can be created with aplanetarium program.

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Comets Facts | Types, Composition, Size, Information ...

Comets | Facts and Information on Visible Comets – Sky …

Comets are a wondrous sight for amateur astronomers. As one approaches it often reveals a tail, slowly unfurling a long ghostly banner of light. Often marked by a distinct greenish-blue haze, they are not to be missed.Hale-Bopp was a splendid sight during 1996, when multiple jets spewed dust and gas from its nucleus. The clarity of features was outstanding even by historic standards. Charles Messier, of the Messier Catalog, actually created his listto help him in his hunt for the ghostly apparitions.Messier objects were things that might trip up a seasoned comet hunter and make him lose precious time in his nightly quest for the elusive cosmic snowballs.

Check here for guides and fun tips that you can use to observe comets. While Hale-Bopp wont return for another 2,500 years, there will certainly be another Comet ISON in the not-so-distant future. You can also check our news section for updates on the latest comets.

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Comets | Facts and Information on Visible Comets - Sky ...

Comets: Facts About The ‘Dirty Snowballs’ of Space | Space

Comets are icy bodies in space that release gas or dust. They are often compared to dirty snowballs, though recent research has led some scientists to call them snowy dirtballs. Comets contain dust, ice, carbon dioxide, ammonia, methane and more. Astronomers think comets are leftovers from the material that initially formed the solar system about 4.6 billion years ago.

Some researchers think comets might have originally brought some of the water and organic molecules to Earth that now make up life here. To research this hypothesis, the Rosetta mission, which landed a probe on a comet on Nov. 12, 2014, is studying its nucleus and environment, observing how it changes as it approaches the sun.

Comets orbit the sun, but most are believed to inhabit in an area known as the Oort Cloud, far beyond the orbit of Pluto. Occasionally a comet streaks through the inner solar system; some do so regularly, some only once every few centuries. Many people have never seen a comet, but those who have won't easily forget the celestial show.

The solid nucleus or core of a comet consists mostly of ice and dust coated with dark organic material, according to NASA, with the ice composed mainly of frozen water but perhaps other frozen substances as well, such as ammonia, carbon dioxide, carbon monoxide and methane. The nucleus may have a small rocky core.

As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into gas, forming a cloud known as the coma. Radiation from the sun pushes dust particles away from the coma, forming a dust tail, while charged particles from the sun convert some of the comet's gases into ions, forming an ion tail. Since comet tails are shaped by sunlight and the solar wind, they always point away from the sun. Comet tails may spray planets, as was the case in 2013 with Comet Siding Spring and Mars.

At first glance, comets and asteroids may appear very similar. The difference lies in the presence of the coma and tail. Sometimes, a comet may be misidentified as an asteroid before follow-up observations reveal the presence of either or both of these features.

The nuclei of most comets are thought to measure 10 miles (16 kilometers) or less. Some comets have comas that can reach nearly 1 million miles (1.6 million km) wide, and some have tails reaching 100 million miles (160 million km) long.

We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun. However, most comets are too small or too faint to be seen without a telescope.

Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor shower occurs every year between August 9 and 13 when Earth passes through the orbit of the Swift-Tuttle comet.

Astronomers classify comets based on the durations of their orbits around the sun. Short-period comets need roughly 200 years or less to complete one orbit, long-period comets take more than 200 years, and single-apparition comets are not bound to the sun, on orbits that take them out of the solar system, according to NASA. Recently, scientists have also discovered comets in the main asteroid belt these main-belt comets might be a key source of water for the inner terrestrial planets.

Scientists think short-period comets, also known as periodic comets, originate from a disk-shaped band of icy objects known as the Kuiper Belt beyond Neptune's orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets. Long-period comets are thought to come from the nearly spherical Oort Cloud even further out, which get slung inward by the gravitational pull of passing stars. In 2017, scientists found there may be seven times more big long-period comets than previously thought.

Some comets, called sun-grazers, smash right into the sun or get so close that they break up and evaporate. Some researchers are also concerned that comets may pose a threat to Earth as well.

In general, comets are named after their discoverer. For example, comet Shoemaker-Levy 9 got its name because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Spacecraft have proven very effective at spotting comets as well, so the names of many comets incorporate the names of missions such as SOHO or WISE.

In antiquity, comets inspired both awe and alarm, "hairy stars" resembling fiery swords that appeared unpredictably in the sky. Often, comets seemed to be omens of doom the most ancient known mythology, the Babylonian "Epic of Gilgamesh," described fire, brimstone, and flood with the arrival of a comet, and the Roman emperor Nero saved himself from the "curse of the comet" by having all possible successors to his throne executed. This fear was not just limited to the distant past in 1910, people in Chicago sealed their windows to protect themselves from what they thought was the comet's poisonous tail.

For centuries, scientists thought comets traveled in the Earth's atmosphere, but in 1577, observations made by Danish astronomer Tycho Brahe revealed they actually traveled far beyond the moon. Isaac Newton later discovered that comets move in elliptical, oval-shaped orbits around the sun, and correctly predicted that they could return again and again.

Chinese astronomers kept extensive records on comets for centuries, including observations of Halley's Comet going back to at least 240 B.C., historic annals that have proven valuable resources for later astronomers.

A number of recent missions have ventured to comets. NASA's Deep Impact collided an impactor into Comet Tempel 1 in 2005 and recorded the dramatic explosion that revealed the interior composition and structure of the nucleus. In 2009, NASA announced samples the Stardust mission returned from Comet Wild 2 revealed a building block of life.

In 2014, the European Space Agency's Rosetta spacecraft entered orbit around Comet 67P/Churyumov-Gerasimenko. The Philae lander touched down on Nov 12, 2014. Among the Rosetta mission's many discoveries was the first detection of organic molecules on the surface of a comet; a strange song from Comet 67P/Churyumov-Gerasimenko; the possibilities that the comet's odd shape may be due to it spinning apart, or resulting from two comets fusing together; and the fact that comets may possess hard, crispy outsides and cold but soft insides, just like fried ice cream. On Sept. 30, 2016, Rosetta intentionally crash-landed on the comet, ending its mission.

Halley's Comet is likely the most famous comet in the world, even depicted in the Bayeux Tapestry that chronicled the Battle of Hastings of 1066. It becomes visible to the naked eye every 76 years when it nears the sun. When Halley's Comet zoomed near Earth in 1986, five spacecraft flew past it and gathered unprecedented details, coming close enough to study its nucleus, which is normally concealed by the comet's coma. The roughly potato-shaped, 9-mile-long (15 km) comet contains equal parts ice and dust, with some 80 percent of the ice made of water and about 15 percent of it consisting of frozen carbon monoxide. Researchers believe other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet was unexpectedly extremely dark black its surface, and perhaps those of most others, is apparently covered with a black crust of dust over most of the ice, and it only releases gas when holes in this crust expose ice to the sun.

The comet Shoemaker-Levy 9 collided spectacularly with Jupiter in 1994, with the giant planet's gravitational pull ripping the comet apart for at least 21 visible impacts. The largest collision created a fireball that rose about 1,800 miles (3,000 km) above the Jovian cloud tops as well as a giant dark spot more than 7,460 miles (12,000 km) across about the size of the Earth and was estimated to have exploded with the force of 6,000 gigatons of TNT.

A recent, highly visible comet was Hale-Bopp, which came within 122 million miles (197 million km) of Earth in 1997. Its unusually large nucleus gave off a great deal of dust and gas estimated at roughly 18 to 25 miles (30 to 40 km) across appeared bright to the naked eye.

Comet ISON was expected to give a spectacular show in 2013. However, the sun-grazer did not survive its close encounter with the sun and was destroyed in December.

Additional reporting by Nola Taylor Redd, Space.com contributor

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Comet Facts – Interesting Facts about Comets

Comet ISON stardustobservatory.org/images.php?page=details&id=363What is a Comet?

A comet is a very small solar system body made mostly of ices mixed with smaller amounts of dust and rock. Most comets are no larger than a few kilometres across. The main body of the comet is called the nucleus, and it can contain water, methane, nitrogen and other ices.

When a comet is heated by the Sun, its ices begin to sublimate (similar to the way dry ice fizzes when you leave it in sunlight). The mixture of ice crystals and dust blows away from the comet nucleus in the solar wind, creating a pair of tails. The dust tail is what we normally see when we view comets from Earth.

A plasma tail also forms when molecules of gas are excited by interaction with the solar wind. The plasma tail is not normally seen with the naked eye, but can be imaged. Comets normally orbit the Sun, and have their origins in the Oort Cloud and Kuiper Belt regions of the outer solar system.

There are many misconceptions about comets, which are simply pieces of solar system ices travelling in orbit around the Sun. Here are some fascinating and true facts about comets.

Comets come in several categories. The most common are periodic and non-periodic.

In the past, comets were named for their discoverers, such as Comet Halley for Sir Edmond Halley. In modern times, comet names are governed by rules set forth by the International Astronomical Union (IAU). A comet is given an official designation, and can also be identified by the last names of up to three independent discoverers.

Heres how it works. Once a comet has been confirmed, the following naming rules are followed. First, if the comet is a periodic comet, then it is indicated with a P/ followed by the year of its discovery, a letter indicating the half-month in which it was discovered, followed by a number indicating its order of discovery. So, for example, the second periodic comet found in the first half of January, 2015 would be called P/2015 A2.

A non-periodic comet would be indicated with a C/ followed by the year of its discovery, a letter indicating the half-month in which it was discovered, followed by a number indicating its order of discovery.

If a comet is independently discovered by three people named Smith, Jones, and Petersen, it could also be called Comet Smith-Jones-Petersen, in addition to its formal designation. Today, many comets are found through automated instrument searches, and so the formal designations are more commonly used.

Well-known comets include the non-periodic comets Hale-Bopp (C/1995 O1), Hyakutake (C/1996 B2), McNaught (C2006 P1), and Lovejoy (C/2011 W3). These flared brightly in our skies and then faded into obscurity.

In addition, Comet Shoemaker-Levy 9 (D/1993 F2) was spotted after it had broken up after a close call with Jupiter. (The D in its proper designation means it has disappeared or is determined to no longer exist). More than a year later, the pieces of the comet crashed into Jupiter.

The periodic Comet Halley (1P/Halley) is the most famous in history. It returns to the inner solar system once every 76 years. Other well-known periodic comets include 2P/Encke, which appears ever 3.3 years and 9P/Tempel (Tempel 2), which was visited by the Deep Impact and Stardust probes, and makes perihelion around the Sun every 5.5 years.

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Comet Neowise is the latest to tease with the potential of a big show – CNET

Comet Neowise seen from Australia.

I don't expect you to believe me at this point, but there is another newly discovered comet that has the potential to put on quite a show in the next few weeks.

From the lab to your inbox. Get the latest science stories from CNET every week.

Comet C/2020 F3 NEOWISE (or just Neowise, for short) is the third comet this year to be discovered by astronomers, who say it could become visible to the naked eye and perhaps be even brighter. Comet Atlas and Comet Swan also held the same potential, but they broke up or fizzled out before ever really becoming so bright as to cause a mainstream stir.

"It might not look like much now, but this comet could blossom in the weeks after perihelion (closest approach to the sun)," writes astronomer Dr. Tony Phillips on Spaceweather.com. "Northern hemisphere observers would be able to easily see it in the evening sky in mid-July."

If that pans out, Comet Neowise could be as bright as the most brilliant stars in the sky. But if Comets Atlas and Swan are any indication, sky watchers shouldn't get too excited. Both failed to hold up as they raced toward the sun, headlong into its barrage of radiation.

Comets are notoriously fickle in this way: It's just very difficult to predict how they'll fare during their close encounters with the sun. Some fail to resist its gravitational pull and dive straight into our star, extinguishing themselves forever.

But veteran comet spotter Michael Mattiazzo, who took the photo at the top of this story, thinks there's a decent chance Neowise will have more staying power than Swan or Atlas.

"I'd say there's a 70 percent chance this comet will survive perihelion," he told Phillips. "Comet Neowise could be a case of third time lucky."

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Comet Neowise is the latest to tease with the potential of a big show - CNET

Special Topic: Orbits and Future Returns – RocketSTEM

Artists conception of ESAs Gaia spacecraft. Courtesy ESA.

Ice and Stone 2020 participants have undoubtedly noticed that I have often discussed how this-or-that comet or asteroid will be returning to the inner solar system or passing by Earth at some point in the future, and perhaps have wondered how such things are determined. In principle, the processes by which such events are calculated are relatively straightforward, although as is usually true in many other scientific disciplines and, indeed, life as a whole the reality can be considerably more complex. With modern computer technology, this can nevertheless be performed with relative ease, and the results are considerably more accurate than they were in earlier times although a small bit of uncertainty is always present.

Once a new object is discovered, the first priority is the measurement of its position i.e., its celestial coordinates of right ascension and declination with respect to background stars, a practice called astrometry. In theory, this can be performed with the unaided eye, and indeed this was the case prior to the invention of the telescope; the 16th-Century Danish astronomer Tycho Brahe could do so with an accuracy of an arcminute, and indeed it was from his astrometric measurements of the planets, Mars in particular, that his protg Johannes Kepler derived his Three Laws of Planetary Motion. Before the development of astrophotography astrometry was often performed by the usage of a device called a filar micrometer that was inserted within the eyepiece of a telescope, but once astrophotography came into its own during the latter part of the 19th Century astrometric measurements could be performed from photographs. With modern electronic devices like charge-coupled devices, i.e., CCDs, and specially-designed software it is now possible to perform astrometric measurements to a high degree of precision and accuracy, to well within an arcsecond.

An astrometric measurement can only be as good as the stars positions from which it is measured. The development of accurate star catalogs is thus an important part of this overall process, and this has steadily improved over the years. Until fairly recently stellar positions measured by ESAs HIgh Precision PARallax COllecting Satellite (Hipparcos) mission an acronym that references the 2nd Century B.C. Greek astronomer Hipparchus of Nicaea, who performed pioneering work in the measurements of stars that was launched in 1989 provided the foundation for the best catalogs, but these are now being superseded by measurements from ESAs Gaia mission (launched in 2013) that ultimately will provide high-accuracy positional determinations for approximately one billion stars. Because of the wobbling phenomenon called precession discovered, incidentally, by Hipparchus and also the fact that the sun and all the other stars are in constant motion with respect to each other, star catalogs need to be referenced to a specific date in time. At present this is the beginning of the year 2000, although presumably within a couple of decades this will shift to 2050.

One other significant issue that arises in astrometric measurements is parallax. Measurements are, for obvious reasons, not made from the center of the earth, but rather from various locations on Earths surface, and this can affect an objects measured position, especially in the case of an object near Earth. Observatories and institutions that have demonstrated the successful ability to perform astrometric measurements are assigned an official Observatory Code by the IAUs Minor Planet Center, which lists each sites parallax factors based upon its latitude, longitude, and altitude above sea level. Although I no longer perform astrometric measurements from my home site, when I was doing so during the early 2000s my Observatory Code was 921.

Once astrometric measurements are obtained, an orbit can be calculated from these. Then, after an orbit is determined, it is possible to compute an ephemeris (plural ephemerides), i.e., a list of appropriate celestial coordinates that the object will occupy at various points in time. In this procedure, the objects location in its orbit at the time in question is determined, and then the earths location in its orbit is determined for the same time, and via a coordinate transformation the objects sun-centered location is transferred to an Earth-centered location. With the application of a sites parallax factors it is possible to calculate an ephemeris for a specific geographical location on Earth (or in space, for that matter).

An orbit is defined by various terms called elements that describe an orbits size, shape, and orientation, and there is also a time element involved. One of the orbital elements is the inclination, i.e., how steeply the orbit is inclined with respect to the plane of the earths orbit (otherwise known as the ecliptic). An orbital inclination of 0 degrees is in the same plane as the ecliptic, whereas an inclination of 90 degrees is exactly perpendicular to the ecliptic. Inclinations greater than 90 degrees (up to 180 degrees) are retrograde, i.e., an object in such an orbit travels around the sun in the direction opposite that of Earth.

Another important orbital element is the eccentricity (usually written as e) which in general terms describes the shape of the orbit. An orbit with an eccentricity of 0 is a circle, whereas eccentricity values between 0 and 1 are ellipses, with the higher the eccentricity indicating a more elongated orbit. An eccentricity of exactly 1 is a parabola, and an eccentricity greater than 1 is a hyperbola. Objects in parabolic and hyperbolic orbits are unbounded, i.e., they will never return to the inner solar system, whereas objects in elliptical and circular orbits are bounded and will return after a period of time. (Obviously, objects in circular orbits remain the same distance from the sun all the time.) The highest eccentricity ever observed in a natural object is the recent interstellar Comet 2I/Borisov I/2019 Q4 a future Comet of the Week which has an orbital eccentricity of 3.4.

The calculation of an orbit follows directly from Newtons Law of Universal Gravitation, although in the pre-computer era this was mathematically laborious. In principle, an orbit can be calculated from three positions, however in practice, since each position has some error associated with it the more positions that are available, the better-determined the orbit. Orbits based on only a few positions and/or over a short arc can be indeterminate, i.e., any number of widely disparate orbits can be fit through the available measurements. As more and more astrometric measurements become available and as the observation arc becomes longer, the true orbit begins to emerge, although this is always subject to refinement as more data is collected. It sometimes happens that, once a reasonably valid orbit is determined, pre-discovery images of the object in question may be identified weeks or months after the fact, thus allowing for a much more accurate orbit to be calculated. An example of this is Comet Hale-Bopp C/1995 O1 (a future Comet of the Week); once the first reasonably good orbits were calculated, a pre-discovery image on a photograph taken over two years earlier allowed the determination of a very solid orbit.

If the sun and the orbiting object were the only objects in the universe, the object would remain on that same orbit indefinitely. Of course, there are many other objects around, primarily the various planets, including especially Jupiter, and each of these objects exerts a gravitational pull that perturbs the object and affects its orbit accordingly. (Indeed, numerous comets have approached closely to Jupiter and have had their orbits dramatically affected, some of these even ejected from the solar system altogether on hyperbolic orbits.) While the solution of the two-body problem is relatively straightforward, it turns out that there is no analytical solution to the three-body or general n-body problem; the calculation of orbits that properly involves these perturbing effects can only be performed numerically. Again, back in the pre-computer days this was an extremely laborious process mathematically but is now accomplished via computers with relative ease. Such orbits are called osculating orbits and are referenced to a specific date called the osculation epoch; in real terms, such an orbit is the one that the object in question is traveling in at that specific point in time.

Other effects can appear as well. Comets eject material from their nuclei in jet-like geysers that act as small rocket engines that push the nuclei in the opposite direction; this effect is described under the term non-gravitational forces and these were first detected in Comet 2P/Encke, this weeks Comet of the Week. Each comet is different, and sometimes the same comet will exhibit different non-gravitational forces at different times, and thus these can only be determined empirically. Small asteroids, in particular, can experience something called the Yarkovsky-OKeefe-Radzievskii-Paddack, or YORP, effect, wherein sunlight striking different sides of the asteroid and its own resulting thermal emission can affect its rotation and thus introduce small changes in its orbit. Astrometric measurements of objects near the sun, and the orbits of the objects themselves, can also be affected by General Relativity.

Once all the various effects are allowed for inasmuch as the available data will permit, it is now possible to make predictions for where an object will be in the future. For main-belt asteroids, which generally travel in low-inclination nearly-circular orbits, this is a relatively straightforward process, and once an asteroid has been observed at a few successive oppositions its orbit can be considered safe and it can be assigned a permanent number. (The designation and numbering processes are described in a previous Special Topics presentation.)

The first predicted return of a periodic comet is somewhat more uncertain, in part because of unknown non-gravitational forces, and it is not unusual for a predicted time of perihelion passage to be off by up to a day or so. (In the pre-computer era, predicted perihelion times could be off by up to several weeks.) Once a comet has been observed on a second return it can then receive a permanent number.

The situation is similar with respect to near-Earth asteroids. These tend to be relatively small objects and are often only detectable when they are relatively close to Earth, and thus several returns may elapse before they are recovered; it is not unusual for first-time recoveries to be off by a few days or more. As with the other objects, once a near-Earth asteroid has been well observed enough such that its orbit can be considered safe, it can be assigned a permanent number.

Even the orbits of objects especially periodic comets and near-Earth asteroids that are considered safe and that are numbered can only be considered safe for a few centuries or, at most, a few millennia. The uncertainties in even the best-determined orbits propagate and grow larger over time, and objects can drift into and out of resonances with planets such as Jupiter (i.e., an object in 3:2 resonance with Jupiter will orbit the sun three times for every two orbits that Jupiter makes). The orbits of the centaurs discussed in a previous Special Topics presentation are unstable over a timescale of millennia, and over timescales of tens to hundreds of millions of years, the orbits of all the planets are unstable. For example, numerical simulations have shown a tiny but nevertheless real chance that Mercury could be ejected from the solar system, or could strike the sun, or Venus or even Earth sometime with the next few billion years. The upshot of all this is that the solar system we see now, including all the various small bodies that are the focus of Ice and Stone 2020, is a transient thing, just like everything else in life.

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Special Topic: Orbits and Future Returns - RocketSTEM

Comet finder and ‘national anthem’ composer to be remembered 100 years after death – Stuff.co.nz

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Thames astronomer John Grigg discovered his first comet in July 1902.

Behind a piano shop on Pollen St in Thames, John Grigg built an observatory that would later be used to locate his first comet.

Two more comet discoveries followed, and after more than 100 years, Grigg's astronomical achievements with the stars are still being acknowledged on Earth.

Grigg was born in London on June 4, 1838 and moved to Thames in 1867, at the height of the gold rush.

He relocated his observatory - equipped with a resolving roof - to his home in Queen St, where he discovered his first comet in July, 1902.

READ MORE:* 'Ball of fire' seen travelling across Auckland and Bay of Plenty skies* 'Our 45-year marriage was written in the stars': The clever Kiwi couple brought together by astronomy* Unlocking New Zealand's meteor showers to discover whether life really is out there

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A centenary making the death of John Grigg will be held in Thames this weekend.

At that time, very few explorers of comets had performed the triple task of making the original discovery, securing good observations of position, and computing an orbit.

No one at that time saw Grigg's comet, and it was not until it returned in 1922 and was rediscovered by Frank Skjellerup in South Africa that the accuracy of his observing and computing enabled the two discoveries to be recognised as one-of-the-same.

It is officially known as Comet 1902 II P/GriggSkjellerup.

In 1903, Grigg discovered his second comet, known as Comet 1903 III Grigg, and in 1907, he discovered another comet, confirmed by U.S. astronomer John Mellish 5 days later. It is known as 1907 II GriggMellish.

Throughout the years, Grigg was awarded the Donahue Medal of the Astronomical Society of the Pacific, and was elected a Fellow of the Royal Astronomical Society of England.

He photographed Halleys Comet when Earth passed through its tail in 1910.

Waikato-Times

Former Congregational Church/Thames Baptist Church, Mary Street, Thames.

The keen musician also composed a piece of music which quickly became a "national anthem", called My Own New Zealand Home.

It was sung at a number of civic functions, including the opening of the Thames-Paeroa railway line.

Griggs' great-granddaughter Dorothy Finlay said that while Grigg would be remembered for many of his feats, his morals were held in high regard.

"He'll always be known more for his integrity. He was a leader," she said.

Grigg died in Thames on June 20, 1920, at the age of 82, and on the centenary of his death, his family are holding a memorial service in the Thames Baptist Church.

Commencing at 10am on Saturday, June 20, the gathering is open to all interested.

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Comet finder and 'national anthem' composer to be remembered 100 years after death - Stuff.co.nz

Head coach of Comet softball: we havent lost a step compared to last years team – KCHA News

A year ago, the Charles City Comet Softball team finished third in the Class 4A softball tournament. The team had state champion aspirations and finished the season having won more than 90% of their games. That team went on to lose an extra-inning game in the semifinal match. A year later, and a similar-but-different team is about to take the field post-pandemic. With the Comets eyeing their first game on June 15th, Head Coach Brian Bohlen expects some people to believe the team has regressed from last years dominance.

Bohlen reassures locals this team has not been negatively affected by the pandemic and is in no universe worse than last year.

As the pandemic toiled on, Bohlen says the team definitely was working on their own.

This year, both Comet baseball and softball will be able to play on the new sports complexes located at the Charles City high school.

The Comet softball team will be conducting their opening day versus Decorah at home on June 15th.

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Head coach of Comet softball: we havent lost a step compared to last years team - KCHA News

Comet trapped in a strange orbit near Jupiter! – Science 2.0

By Professor Mark R.... | June 14th 2020 09:06 AM | Print | E-mail

Jupiter has captured an icy comet from the outer solar system in a bizarre orbit that will bring it back to within 3 million kilometres of the giant planet in 2063. The only Sun orbiting objects known to come closer were the fragments of Comet Shoemaker Levy 9, which plunged into the Jovian cloud deck in July 1994.A CENTAUR, NOT A TROJAN:During 2019 NASA's asteroid hunting ATLAS project in Hawai'i discovered 2019 LD2, and further observations showed it was a comet. New observations confirmed it as a periodic comet and placed its orbit near Jupiter, leading (Denneau, L.) to announce that P/2019 LD2 was the first comet among the Trojans. This family of several thousand asteroids shares Jupiter's orbit but stays steady at approximately 60 ahead or behind of the planet. The discovery of a comet among Trojan asteroids was surprising because most of them are thought to have been captured in the solar systems early years, any ice should have evaporated long ago.However, when amateur astronomer Sam Deen used software on the Jet Propulsion Laboratory's (JPL) solar system dynamics website to calculate the objects orbit, he found P/2019 LD2 had a close encounter with Jupiter that left its orbit unstable. The model showed that the comet had likely been a Centaur, part of a family of outer solar system asteroids, with an orbit reaching out to Saturn. On February 17th, 2017, it passed approximately 14 million kilometres from Jupiter, an encounter that sent the comet on a wild ride and inserted it into an odd Jupiter like orbit.Although the swing past Jupiter put P/2019 LD2 into a Jupiter like orbit, it did not move it near to one of the two Lagrange points where the combination of gravitational forces from Jupiter and the Sun hold Trojan asteroids. Instead of being 60, one sixth of the giant planet's orbit, from Jupiter, P/2019 LD2 is only 21 ahead of Jupiter. The model predicts the comet will drift to no more than 30 ahead before the two begin converging again.FUTURE PASSES: The comet will pass within approximately 18 million km on May 13th, 2028. That will alter the orbit again (Deen, S.). That makes P/2019 LD2 a Jupiter family comet, but not a Jupiter Trojan, as the Hawaii group now acknowledges. Project Pluto's Find Orb model gives similar results to JPLs model.Also, amateur astronomer, Tony Dunn, found similar results. He illustrated the orbit using his own model to show the path of the object as seen from Jupiter.The 2028 encounter will shift the comet from an orbit close to a 1:1 resonance with Jupiter to one near a 2:3 resonance. But that orbit will not last, because it will put P/2019 LD2 on course for a much closer planetary encounter."From Jupiter's perspective the comet will appear to slowly move around the Sun before coming back," (Deen, S.). In January 2063, it will pass approximately 3 million kilometres from Jupiter, just outside the orbits of its Galilean satellites, close enough to cause a major redirection of the comet's orbit.2063 AND BEYOND: Where P/2019 LD2 will go from there is unclear. The orbital uncertainties are large enough to make the results of that encounter difficult to predict (Gray, B.). "It could get tossed almost anywhere," (Gray, B.) depending on how close it comes to Jupiter. The closer the encounter, the more dramatic the results could be.Another wild card, is that comets eject gas from various points on their surfaces, and it is difficult to model the impact of the resulting non gravitational effects on their orbits. Such effects could enhance or diminish the effect of Jupiters gravitational force and could send it veering off in an unexpected direction. Strong forces can also fragment a comet, as happened to Shoemaker Levy 9 before its impact (Gray, B.).The 2063 approach should yield new insight into a process that has helped shape the solar system: intense interactions between planetesimals and the gravitational giant of the solar system. The event should give "a detailed look at the dynamics that convert Centaurs and long period comets into short period comets," (Deen, S.)."Odds are that we will have some spacecraft orbiting Jupiter by then that will be able to visit the interplanetary visitor close up." That would give a much better view of the action than we had for Shoemaker Levy 9, when the collision itself happened on the side of Jupiter hidden from terrestrial observers and spacecraft.References:Deen, S. (Minor Planet Mailing List).Denneau, L. (University of Hawaii).Dunn, T.Gray, B. (Project Pluto).Heinze, A. (Asteroid Terrestrial impact Last Alert System/ Institute for Astronomy (ATLAS/ IfA)).

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Comet trapped in a strange orbit near Jupiter! - Science 2.0

Utica Comets GM Johnson proud of team – Utica Observer Dispatch

Ryan Johnson had a key takeaway from his wrap-up meetings with Utica Comets players.

"It was how much fun they said they had coming to the rink and how much they valued the staff and their teammates," said Johnson, the Comets General Manager since July 2017. "The feeling was how excited they were to make a run at something (in the playoffs).

The high-scoring Comets had been in the thick of the North Division, sitting in third place with a 34-22-3-2 record after 61 games and trying to make a push for the playoffs when the American Hockey Leagues season was ultimately canceled due to concerns with the coronavirus pandemic.

"I dont have a ton of complaints about our team," Johnson said. "Right when things stopped (this season), I felt we were starting to become a very defensive-aware team. Although we were still finding ways to score goals, we were doing it the right way and not being riverboat gamblers every time. Thats what was exciting to me about heading into playoffs. They were really starting to become a complete team."

It is a similar feeling that has been shared by players and head coach Trent Cull since the season was halted. The group featured one of the more skilled and talented rosters in the Comets seven seasons.

There was growth throughout the roster, Johnson said. Dynamic wing Reid Boucher and rookie defenseman Brogan Rafferty each were AHL All-Star selections. The speedy Justin Bailey was also a key part of the group and his frequent linemate Kole Lind took notable steps in his second pro season, Johnson noted. Goaltending prospect Mike DiPietro who turned 21 years old Tuesday also helped give the team confidence, Johnson said. They are among the players being considered to join the parent Vancouver Canucks should the NHL resume this summer.

"The message from Day 1 with players and coaches was we want you all to get to a point where youve had your best year. We do that, and youll elevate to the next level," Johnson said. "I was proud of our players, they put in the work and they took the resources."

While the AHL is known for its amount of turnover, Johnson said there is a hope to have a similar roster of skill and talent when the Comets hit the ice for the 2020-21 season. Hed like to add some players "who play with pace and are a little bit hard to play against." A few Canucks prospects Marc Michaelis, Will Lockwood and Jett Woo among them could also join the Comets.

The AHL is in the process of determining what next season looks like as it navigates through the pandemic with different options being considered with regard to the schedule. The season could be pushed back from its usual October start.

Johnson said hes in "a holding pattern" when it comes to decisions regarding AHL contracts because of the variables and unknowns. The Comets had 13 players signed to AHL deals for the 2019-20 season and could have a similar number going forward.

"Ive got a group of players that I certainly want back and would like to have back," Johnson said. "All I can do is be up front and honest with players and wait and see how things trickle down from the NHL to the AHL level and how we have the ability to operate."

There have been reports veteran Carter Camper could sign in Europe, leaving the Comets without a key center from the season.

"Carter and I have talked a few times and Ive said, Im in a holding pattern here. But if theres opportunity or something you dont want to pass up ..." Johnson said. "It is not just Carter. Many players say Im not going to sit (and wait)."

Johnson said hes challenged some players on both AHL and NHL contracts to help continue what the Comets have next season.

"They have to be torchbearers of this culture that weve built there," he said.

Contact reporter Ben Birnell at 315-792-5032 or follow him on Twitter @OD_Birnell.

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Utica Comets GM Johnson proud of team - Utica Observer Dispatch

Asteroid, comet, alien spaceship? Scientists have new theory on what mysterious interstellar object Oumuamua i – MEAWW

Oumuamua, the first known interstellar object to pass through the Earths solar system, has been called many things: an asteroid, a comet, a cigar-shaped spaceship. Now it has a new description: astronomers at Yale University and the University of Chicago say it could be a hydrogen iceberg. This is a new type of object and it is likely that a large population of similar objects exist, says the research team.

Oumuamua is about 900 feet long and traveled through space for millions of years before entering the solar system. It has now passed beyond Saturns orbit and will travel another 10,000 years before exiting the system.

We developed a theory that explains all of Oumuamuas weird properties. We show that it was likely composed of hydrogen ice. This is a new type of object, but it looks like there may be many more of them showing up, going forward, says co-author Gregory Laughlin, a professor of astronomy at Yales Faculty of Arts and Sciences, in the analysis that appears on the preprint website arXiv. The study has been accepted for publication by the Astrophysical Journal Letters.

Its a frozen iceberg of molecular hydrogen. This explains every mysterious property about it. And if its true, its likely that the galaxy is full of similar objects, says the first author of the study, Darryl Seligman, a former Yale graduate student now at the University of Chicago, who began the research at Yale. Researchers at the University of Hawaii first discovered Oumuamua in 2017, more than a month after it passed its closest point to the Sun. They named the object after the Hawaiian word for scout. It was spotted by the Panoramic Survey Telescope and Rapid Response System 1 (PanSTARRS1) telescope, located at the University of Hawaii's Haleakala Observatory and made headlines as the first object to visit from outside the solar system.

Telescopes did not pick it up until it had already swung past the sun and was on its way out, but its trajectory indicated it came from interstellar space. It was Oumuamuas odd and unique characteristics that had led to widespread interest and speculation the object could be an alien spacecraft, sent from a distant civilization to examine our star system. Astronomers formally named the object 1I/2017 U1 and appended the common name Oumuamua. A previous study had stated that Oumuamua likely has an elongated, cigar-like shape and an odd spin pattern, much like a soda bottle laying on the ground, spinning on its side.

As Oumuamua hurtled through the inner part of the solar system, astronomers noticed it had several unusual properties. It varied rapidly in brightness, suggesting it was either saucer-shaped or cigar-shaped. It also accelerated in a fashion similar to a comet yet it showed no evidence of emitting gas or the fine billows of dust normally associated with comets. The typical comets scientists have seen in our solar system have cometary tails, which can be seen when small dust particles in the outflow reflect sunlight, but they could not see any such sparkle of dust from Oumuamua. Last year, however, the research team showed that it could be a comet whose outflow was simply invisible to telescopes. Starting from that idea, the researchers worked their way backward to see what the substance could be in the outflow. They knew where Oumuamua was, how fast it was moving and how much energy it should be getting from the sun at any given time, so they checked the list of what materials would give the acceleration they saw when burned up.

According to scientists, Oumuamuas behavior can be explained if it is composed of hydrogen ice. The only kind of ice that really explains the acceleration is molecular hydrogen, says Seligman. Molecular hydrogen ice is a strange substance, only formed when the temperature is just a little above absolute zero. It does not reflect light or produce any light as it burns up, so telescopes would not be able to see it. While hydrogen is the most common element in the universe, it is rarely found in solid form, which requires extremely cold temperatures. Frozen hydrogen does, however, offer a compelling mechanism for acceleration, says the study.

Laughlin elaborates, that as Oumuamua passed close to the Sun and received its warmth, melting hydrogen would have rapidly boiled off the icy surface, providing the observed acceleration and also winnowing Oumuamua down to its weird, elongated shape much as a bar of soap becomes a thin sliver after many uses in the shower.Seligman argues that since they saw one at all implies that the galaxy must be filled with these dark hydrogen icebergs.

The study theorizes that iceberg-like objects made of hydrogen can potentially form in the dense cores of molecular clouds that pervade the Milky Way galaxy and give rise to new stars and planetary systems. Their presence would be an accurate probe of the conditions in the dark recesses of star-forming clouds and provide a critical new clue for understanding the earliest phases of the still-mysterious processes that generate the birth of stars and their accompanying planets, says Laughlin.

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Asteroid, comet, alien spaceship? Scientists have new theory on what mysterious interstellar object Oumuamua i - MEAWW

Comet of the Week: PANSTARRS C/2017 K2 – RocketSTEM

LEFT: Pre-discovery image of Comet PANSTARRS taken with the Canada-France-Hawaii Telescope at Mauna Kea on May 12, 2013, when its heliocentric distance was 23.7 AU. From the paper by Jewitt et al. (2017). RIGHT: Hubble Space Telescope image of Comet PANSTARRS taken on June 27, 2017, when its heliocentric distance was 15.9 AU. Courtesy NASA.Perihelion: 2022 December 19.67, q = 1.797 AU

Ive hinted in some of the earlier presentations of Ice and Stone 2020 that, for the past few years, we have been in a slow period when it comes to bright comets. Indeed, for Great Comets discussed in a previous Special Topics presentation the most recent one appeared in 2011, and the most recent one that was easily visible from the northern hemisphere appeared as long ago as 1997. Weve been teased a couple of times in the very recent past, but Comet ATLAS C/2019 Y4 (an earlier Comet of the Week) disintegrated as it approached perihelion, and while it was never expected to become Great, Comet SWAN C/2020 F8 (discussed in that same presentation) apparently did likewise. So far, Comet NEOWISE C/2020 F3 seems to be brightening normally, but well have to wait and see what happens next month.

Intrinsically, this weeks Comet of the Week is one of the brightest ever detected its intrinsic brightness being comparable to that of Comet Hale-Bopp C/1995 O1 and while it certainly possesses the potential to become a bright, perhaps even conspicuous, naked-eye object around the time of its perihelion passage, its somewhat large perihelion distance, combined with the fact that it remains far from Earth and will be visible only under mediocre viewing geometry, will likely keep it from becoming Great. Furthermore, since it will be accessible only from the southern hemisphere for almost a full year around the time of perihelion, as far as observers in the northern hemisphere are concerned any potential it might have for becoming conspicuous is quite limited.

At the time of its discovery by the Pan-STARRS program based in Hawaii on May 21, 2017, the comet was about magnitude 19.5, and despite being located at a heliocentric distance of 16.1 AU was already clearly active. Researchers later identified images of it on exposures taken with the Canada-France-Hawaii Telescope at Mauna Kea as far back as May 12, 2013, at which time it was located 23.7 AU from the sun the second-largest heliocentric distance at which any long-period comet has ever been detected; it was about magnitude 22.5 but was still showing activity even at that distance. Since this is far too large a distance for the sublimation of water ice, researchers have concluded that the activity was driven by sublimation of substances like carbon dioxide, carbon monoxide, and molecular oxygen and nitrogen.

Traveling in an orbit almost perpendicular to the ecliptic (inclination 87.5 degrees), Comet PANSTARRS has brightened steadily ever since its discovery as it has approached the inner solar system, and presently is around 15th to 16th magnitude. It is currently at opposition at a heliocentric distance of 8.9 AU, and located within the head of the constellation Draco. It travels slowly southward over the coming months, and will be in conjunction with the sun 59 degrees north of it in mid-December. A year from now it is again at opposition, when its heliocentric distance will be 6.1 AU and it will be located 10 degrees south of its current location; by then it may be close to 13th magnitude and thus amenable to visual observations. One year later, i.e., in mid-June 2022, it will once again be near opposition, at a heliocentric distance of 2.9 AU and located in northern Ophiuchus; by then it should be bright enough to detect with binoculars and perhaps may even be close to naked-eye visibility.

After this, Comet PANSTARRS travels southward more rapidly, becoming inaccessible from northern temperate latitudes by the end of September and entering southern circumpolar skies in mid-December. At the time of perihelion is in conjunction with the sun 38 degrees south of it and is located on the far side of the sun from Earth. It reaches a peak southerly declination of -71 degrees shortly before the end of January 2023, and is still in southern circumpolar skies when it is nearest Earth (2.23 AU) in mid-February. Although it continues traveling northward it remains at a relatively small elongation south of the sun for the next several months, and is again in conjunction with the sun 37 degrees south of it in mid-June. During August it emerges into the morning sky and once again becomes accessible from the northern hemisphere, and will be at opposition shortly after mid-December when it will be located a few degrees east of the Orion Nebula M42; during the first few days of January 2024 it travels northwestward parallel to the stars of Orions Belt about 1 degrees south of that prominent stellar pattern. Visual observations may be possible for a few more months as it travels northward through the western evening sky and fades as it recedes from the earth and sun.

As is always the case with inbound long-period comets, any forecasts as to its brightness should be taken cautiously. The comet appears to be a new one making its first visit to the inner solar system from the Oort Cloud, and as discussed in a previous Special Topics presentation such objects tend to under-perform compared to what their earlier brightnesses might suggest. The fact that Comet PANSTARRS appears to be quite bright intrinsically and was active at such large heliocentric distances works in its favor, but its rather large perihelion distance, its remaining far from Earth, and its poor geometric viewing conditions around the time of perihelion work against it. The comet should certainly reach naked-eye brightness, perhaps 5th magnitude, and could become somewhat brighter than that; 3rd magnitude perhaps is reasonable, and while I would consider a peak brightness of 1st magnitude as unlikely, it is not out of the question. As I stated at the outset, though, I doubt that this will become a Great Comet, although I suppose one never knows. We will just have to wait and see what the comet does.

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Comet of the Week: PANSTARRS C/2017 K2 - RocketSTEM

Comet me, bro: A giant asteroid will pass relatively close to Earth – The New Daily

An asteroid the size of a stadium is set to pass relatively close to Earth on Saturday.

If there was any catastrophic event we were missing in 2020, its a giant asteroid hurtling towards us.

NASA has advised that while the 335-metre mass is near enough to make their close approach list, it will still miss us by a landslide about 5.1 million kilometres to be exact.

The asteroid, named 2002 NN4 (the runner-up in Elon Musk and Grimes baby name list), is said to be bigger than 90 per cent of asteroids and could have devastating effects if it entered Earths orbit.

But we can breathe a sigh of relief, because despite travelling at more than 32,000 kilometres per hour, 2002 NN4 will still be more than 13 times further away from us than the Moon.

Ian J. ONeill, of NASAs Jet Propulsion Laboratory, said the asteroid was too far away to worry.

In short, 2002 NN4 is a very well-known asteroid with a known orbit that will pass Earth at a (very) safe distance Dr ONeill said.

Along with 2002 NN4, there are another four asteroids on their way over, but they are significantly smaller.

One is roughly the size of a house while the remaining three are the size of an aeroplane.

NASA has also blessed star-gazers with an Asteroid Watch Widget, which tracks asteroids and comets as they approach Earth.

Users will be able to find out the date of the closest approach, name and size of the asteroid and its distance from Earth.

Despite fitting in well with the chaos this year has brought, 2002 NN4 will be so far away sky-gazers will still need a telescope to see it.

But what would happen if it came any closer?

Scientists from all over the world met in 2019 to discuss how to respond if a disastrously large asteroid actually was headed for Earth.

Dr Lori Glaze, director of planetary science at NASA, said that literally saving the world wouldnt be too hard.

All we have to do is change its speed a little faster or a little slower so that when it crosses Earths orbit, it crosses either in front of us or behind us, Dr Glaze said.

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Carol Schram – Utica Comets year-end award winners will help fill out Canucks’ Black Aces – Hockeybuzz.com

Last week, the Vancouver Canucks' AHL farm team rolled out its team awards for 2019-20.

Leading scorer Reid Boucher picked up three awards named MVP by both the team and the local media, as well as winning the Three Stars Award.

Goaltender Mikey DiPietro was named 'Class Guy' and 'Guy With The Most Heart':

Guillaume Brisebois earned the award for best defenseman:

Sophomore Kole Lind was named the team's most improved player.

And team captain Carter Bancks was named Best in the Community.

Some players from that group will be joining the Canucks when their training camp gets underway in Phase 3 of the Return To Play Plan, likely in July as things look now.

Back in May, we started discussing which Utica Comets players could earn the call to be Black Aces when the Canucks get back in action.

Iain MacIntyre of Sportsnet took on that subject on Monday.

He emphasizes once again that we don't know if Micheal Ferland or Josh Leivo will be ready to play when camp gets underway but regardless, the big club already has a surplus of forwards.

We have five lines of forwards (at the NHL level), so we dont have to bring in as many forwards," Canucks general manager Jim Benning told MacIntyre. "But weve only got seven D, so if you have two guys go down, the second injury on defence, one of those guys called up is going to play."

I've read that total roster size is still something that needs to be finalized in the Phase 3 protocol. Using the parameter of 28 skaters that has been widely circulated, MacIntyre figures probably two forwards and three defensemen, plus DiPietro, will get the nod from the group of "10 or 11" players who were told to stay ready after the AHL season was officially cancelled last month.

For his part, Travis Green left little doubt that he's looking for a roster that'll give his team the best chance to win and advance.

People say, Youll finally play meaningful games, Green told MacIntyre. Thats not what this is about to me. Its not about getting a few games under our belt under hard and exciting circumstances, in pressure-filled games. This is about, how do we win the first series, and then how do we win the second series? And I can tell you, were not going into this thinking were only going to be playing one or two rounds.

As MacIntyre points out, that mindset probably gives the edge to players with experience.

After factoring in team needs, he namechecks Brogan Rafferty, Guillaume Brisebois and Olli Juolevi as the likely candidates on defense, along with Tyler Graovac and Reid Boucher up front.

Based on the experience theory, I'd probably put Ashton Sautner ahead of Juolevi as a left-side blueliner. And despite another stellar AHL season from Boucher, I wonder if the Canucks would be more tempted to tap Sven Baertschi, who stayed healthy all season and whose skating is more NHL-ready?

Baertschi's 27, has 291 games of NHL experience, and has 138 points 0.47 points per game.

Boucher's a year younger at 27, has 133 games of NHL experience, and has 42 points 0.32 points per game.

Also even if the first priority is to try to win some hockey games, Benning might get an additional benefit from putting Baertschi into the lineup. If he does well especially on such a big stage perhaps that opens the door for Benning to deal the last year of Baertschi's contract, something he wasn't able to do at the trade deadline in February.

It'll be interesting to see how these decisions play out once an official date for the start of training camp is set and roster sizes are finalized.

To close out today this one's for all you foodies. The NHL is launching a new cooking show this week.

'Skates & Plates' debuts this Wednesday, with one-time Vancouver Canucks centre Nick Bonino learning how to make the classic French dish Steak Diane and pommes frites from celebrity chef Ludo Lefebvre.

I've seen the episode it's a lot of fun. Bonino's great, and the meal looks amazing.

I wrote about it for Forbes. Read the story and check out the trailer here. Thanks!

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Comet of the Week: IRAS-Araki-Alcock 1983d – RocketSTEM

Comet IRAS-Araki-Alcock on May 11, 1983, during the time of its closest approach to Earth. Photograph courtesy Alan Gorski.Perihelion: 1983 May 21.25, q= 0.991 AU

On January 25, 1983, the InfraRed Astronomical Satellite (IRAS) spacecraft was launched from Vandenberg Air Force Base in California. For the next ten months, until its supply of superfluid liquid helium coolant ran out, IRAS surveyed the entire sky in near- to far-infrared wavelengths, and its findings completely revolutionized much of our knowledge about the solar system, the Galaxy, and the entire universe.

On April 25 three months after its launch the infrared sensors aboard IRAS detected a fast-moving object. Due to some breakdowns in communication and some initial uncertainty as to just what this object was, it wasnt until early May that astronomers finally determined that the new IRAS object was a previously-unknown comet. By that time the comet had been independently discovered on May 3 by two amateur astronomers: Genichi Araki in Japan and George Alcock in England. (Alcock was a very well-known amateur astronomer who had discovered four comets from the late 1950s through mid-1960s as well as several novae after that, and at the time of his discovery of this comet was searching for novae with binoculars from indoors through a closed window!) Visual observations at the time indicated that the comet was as bright as 6th magnitude and exhibited a large coma 15 to 20 arcminutes in diameter.

Orbital calculations soon indicated that Comet IRAS-Araki-Alcock was rapidly approaching Earth, and would pass just 0.031 AU from Earth on May 11 the closest confirmed cometary approach to Earth in over two centuries. It brightened rapidly as it approached Earth, and a couple of days before its closest approach it was as bright as 3rd magnitude with a coma approximately one degree in diameter; it never exhibited much in the way of a tail.

On the night of closest approach, May 10-11, the comet was as bright as 2nd magnitude with a coma between 2 and 3 degrees across. At the beginning of the night in was located a few degrees west of the bowl of the Big Dipper, and was moving towards the southwest at two degrees per hour. To the unaided eye it appeared as a diffuse cloud, but telescopically it exhibited a number of fanlike features, streamers, and pillar-like structures throughout the inner coma. The sight of the comets central condensation traveling against the background stars in real time remains among the most dramatic sights I have seen in all my years of comet observing.

During its passage by Earth the Arecibo radio telescope in Puerto Rico and the Deep Space Networks tracking antenna in Goldstone, California both successfully detected the comets nucleus via radar the first radar detections of a long-period comet with the data indicating that the nucleus is a non-spherical object some five to eight km in diameter, and accompanied by a dense swarm of particles (centimeter-sized and larger) out to a distance of approximately 800 km or more. The International Ultraviolet Explorer (IUE) satellite detected the presence of diatomic sulfur (S2) in the inner coma; since this molecule can only be formed and maintained in very cold conditions, this tells us much about the environments within which comets are formed.

Following its passage by Earth Comet IRAS-Araki-Alcock rapidly headed southward and within a couple of days was accessible only from the southern hemisphere. After maintaining its brightness for another day or so it faded rapidly, dropping below naked-eye visibility during the third week of May and to 7th magnitude by the end of the month. It was followed visually until mid-July, and the final observations were obtained in early October.

Comet IRAS-Araki-Alcocks approach to Earth was the closest confirmed cometary approach during the entire 20th Century, and at this writing is the fifth-closest confirmed approach in all of recorded history. Close cometary approaches to Earth, both past and future, are discussed in this weeks Special Topics presentation.

By a most remarkable coincidence, the comet that accounted for the 20th Centurys fourth-closest confirmed approach to Earth was discovered while all the excitement was taking place with Comet IRAS-Araki-Alcock. Comet Sugano-SaigusaFujikawa 1983e was independently discovered by three Japanese amateur astronomers on May 8, having already passed through perihelion on May 1 at a heliocentric distance of 0.471 AU. This newer comet was apparently much smaller than its predecessor, with radar bounce observations indicating that its nucleus was no more than a few hundred meters in diameter, and while it did reach 6th magnitude at the time of its closest approach of 0.063 AU on June 12 it appeared as little more than a vague diffuse cloud one degree in diameter. It faded rapidly after that and disappeared from view within a week.

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Comet of the Week: IRAS-Araki-Alcock 1983d - RocketSTEM

Halifax County High School baseball seniors reflect on what it meant to wear the Comets jersey – YourGV.com

Halifax County High School baseball is known throughout Virginia as being a perennial contender for the state championship.

Teams near and far know the reputation that Halifax baseball has and for those that grow up in Halifax, playing for the varsity baseball team means more than many realize.

The seniors on this years team will not get to play their final season on the diamond, but for most being a part of the team in the first place was something they have dreamed about since they were young.

My dad took me to the high school games when I was in elementary school, and I would watch them and they were the best players ever and then to be able to come up and make the team was the most exciting thing ever, Luke Fulcher said.

Christian Worley, who will be continuing his baseball career at Virginia Tech next year recalls sitting at those games as a kid and dreaming of one day being a part of the team. We would all go to the high school games as kids and look up to the older players, and we would just be sitting there watching the game and thinking this is where I want to be one day and who I want to be, Worley said. Getting to start and play was amazing and looking back when I was younger seeing that I made it, he added.

Jackson Dunavant grew up wanting to be like those players on the high school team as well, and he said that they look up to those players and try their best so that they can be a part of it.

Baseball in Halifax County has been special for many decades, and many of the seniors on this years team had fathers and other family members who also played for the Comets when they were growing up, and being able to follow in those footsteps meant a lot to them.

One of those players is Blake Duffer. It meant a lot to me because my dad did it, and it was a bucket list item to have the Halifax jersey on, he said.

To be able to go out and play and have that blue and white jersey on that says Halifax, I was always really proud of that, Dylan Newton said.

Shabazz Buster didnt start playing baseball until middle school, but he knows the pride that being a part of Halifax baseball brings. Buster didnt think he would ever be able to be a part of the team, and once he made it on the team it was a big blessing.

Head coach Kenneth Day was ready for the season to begin and looking forward to the challenges ahead. This season will definitely be missed, Day said. I was looking forward to having a great season with this group of kids, and we were definitely looking forward to a very tough schedule and competing against some top schools, he added.

The seniors on this years team echoed coach Days thoughts on the upcoming season. While the Comets lost a lot of production on last years team due to graduation, the members of this years team were optimistic about their chances to continue the dominance that teams the last few years have had.

Worley thought that this years team had just as good of a shot as last years team to go back to the state tournament. He praised the teams chemistry and felt that they could have pulled it together and had a good run at it.

This team was great and could have done something special, Thomas Lee said.

All the guys and especially the seniors had real high expectations, and I felt like we had a good chance of going far again, Newton said.

Duffer wasnt sure what to expect from this years team coming into the season, but said once they started practicing together that everyone developed well quickly, and he thought they were going to have a chance to do something special this season.

Dunavant wanted the chance to keep the successes of Comets baseball going as his class finished their high school careers. We have been going to states and been successful for the last three of four years, and we wanted to keep it going, he said.

While they will never know how those expectations would have played out one thing is for sure, missing the chance to play their senior season is hard on them.

I couldnt get my senior year in baseball and that is what you look forward to, Jared Dawson said. It is really heartbreaking, I was really looking forward to playing my last year and going out there on senior night, he added.

You play so long and worked up to your senior year, and it got taken away from us, Fulcher said. Fulcher went on to say that for those that are going to play baseball in college, losing the season may not hurt as much, but he will probably not be able to play baseball ever again. This year also meant a lot to him because he was going to be a starter after spending last year behind several good players, and this was his season to showcase his skills.

Senior year got ripped away from us, Worley said. Lee added that it was a bad feeling and tough to swallow that his senior year of baseball would not happen.

As the Virginia High School League announced late last week that the spring sports season was officially canceled, coach Day is hoping to have a camp or a showcase for his players this summer if they are able to, and Dawson echoed what every senior on this years Halifax baseball feels, I would love to play a game with my team one last time.

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