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Comet – Wikipedia

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

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

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun’s light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets.

As of November 2014[update] there are 5,253 known comets,[5] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] Particularly bright examples are called “Great Comets”. Comets have been visited by unmanned probes such as the European Space Agency’s Rosetta, which became the first ever to land a robotic spacecraft on a comet,[9] and NASA’s Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek (“wearing long hair”), and the Oxford English Dictionary notes that the term () already meant “long-haired star, comet” in Greek. was derived from (“to wear the hair long”), which was itself derived from (“the hair of the head”) and was used to mean “the tail of a comet”.[10][11]

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

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[13] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[14] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[15] Research conducted in 2014 suggests that comets are like “deep fried ice cream”, in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[16]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[17][18] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[19] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[20][21]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley’s Comet reflects about four percent of the light that falls on it,[22] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0% of the light that falls on it;[22] by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.[23]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[24] but ascertaining their exact size is difficult.[25] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[26] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[27] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[28] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[29]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[30] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/ChuryumovGerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[31][32] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1km (0.62mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[33][34] Instruments on the Philae lander found at least sixteen organic compounds at the comet’s surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[35][36][37]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the “coma”. The force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous “tail” to form pointing away from the Sun.[46]

The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000km; 280,000,000 to 370,000,000mi) of the Sun.[47] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[47] Larger dust particles are left along the comet’s orbital path whereas smaller particles are pushed away from the Sun into the comet’s tail by light pressure.[48]

Although the solid nucleus of comets is generally less than 60 kilometres (37mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[49] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[50] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[51] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[51] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[51] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[50]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects Sunlight directly while the gases glow from ionisation.[52] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[53] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[54]

In 1996, comets were found to emit X-rays.[55] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, “stealing” one or more electrons from the atom in a process called “charge exchange”. This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.[56]

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[57][58] but these detections have been questioned.[59][60] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the type II or dust tail.[52] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[61] On occasions – such as when the Earth passes through a comet’s orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.[62]

The observation of antitails contributed significantly to the discovery of solar wind.[63] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an “induced magnetosphere” around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called “pick-up ions”) congregate and act to “load” the solar magnetic field with plasma, such that the field lines “drape” around the comet forming the ion tail.[64]

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a “tail disconnection event”.[64] This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke’s Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[65]

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions.”[66][67]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[68] These streams of gas and dust can cause the nucleus to spin, and even split apart.[68] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[69] Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.[70]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[71] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.[72] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[73] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley’s Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet’s orbit are called its “family”.[74] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[75]

At the shorter orbital period extreme, Encke’s Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called Jupiter-family comets (JFCs).[76][77] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[78][79] As of 2017[update], only 87 HTCs have been observed, compared with 567 identified JFCs.[80]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[81]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[82] Short-period comets have a tendency for their aphelia to coincide with a giant planet’s semi-major axis, with the JFCs being the largest group.[77] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[83][84]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[85] a disk of objects in the trans-Neptunian regionwhereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[86] 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.[87]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[88] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[89] 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 further from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having “periods”. The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[88] 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.[90] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[91] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[92] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

No comets with an eccentricity significantly greater than one have been observed,[92] so there are no confirmed observations of comets that are likely to have originated outside the Solar System. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[93] 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),[94] whereas others use it to mean exclusively short-period comets.[88] 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.[95]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[97] to as far as 50,000AU (0.79ly)[78] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[97] 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).[98] The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune.[78] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[99] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[99][100][101] 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.[102]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[103] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[104][105] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[103][104]

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[106] If the Earth’s orbit sends it though that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle.[107]Halley’s Comet is the source of the Orionid shower in October.[107]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth’s oceans, or at least a significant portion of it. Others have cast doubt on this idea.[108] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[16] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[109] 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.[110] 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.[111]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth’s Moon, some of which may have survived as lunar ice.[112] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[113]

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.[114] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[115]

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[30] Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[116] Some asteroids in elliptical orbits are now identified as extinct comets.[117] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[30]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[118] 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.[119][120] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[121] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[122] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[123]

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.[124]

Some comets have been observed to break up during their perihelion passage, including great comets West and IkeyaSeki. Biela’s Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela’s Comet.[125]

Some comets meet a more spectacular end either falling into the Sun[126] 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.[127]

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.[129] Similarly, the second and third known periodic comets, Encke’s Comet[130] and Biela’s Comet,[131] 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.[132]

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.[132]

From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.[133] 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.[134][135]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[136]Pliny the Elder believed that comets were connected with political unrest and death.[137]

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.[138]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth’s atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[139][140]

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.[141]

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.[142] 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.[143][144] 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.[145]

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

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.[147]

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.[148] 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.[149]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[150] 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.[151]

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.[152] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[153] 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.”[153] 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, H 2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[154][155]

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.[122] 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.[164] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[165]Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[166]

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.[167]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[168] 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.[169]

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.[170] 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.[171]

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.[172]2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[173] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[174](See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[175] 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,[176] 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.[177]

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.[178] SOHO’s 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[179] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a “new” comet is discovered, and calculation of its orbit shows it to be an old “lost” comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[180] There are at least 18 comets that fit this category.[181]

NASA is developing a comet harpoon for returning samples to Earth.

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.[183] 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)[183] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson”.[183]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley’s Comet in 1910, the Earth passed through the comet’s tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[184] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[185]

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).[183] 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.[186]

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Comet – Wikipedia

Comet Facts – Interesting Facts about Comets – Space Facts

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

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 Facts – Interesting Facts about Comets – Space Facts

Comets to play Crunch in preseason game in Rome – Times Telegram – The Times Telegram

Ben Birnell

The Utica Comets are set to return to a familiar place in the Mohawk Valley for a preseason game.

Their opponent also is well-known to hockey fans in Central New York.

The Comets announced Thursday the team will host the rival Syracuse Crunch at 5 p.m. Saturday, Sept. 30, at Kennedy Arena in Rome to begin out a home-and-home exhibition series.

Tickets are $10 and will go on sale at 2 p.m. Friday, Sept. 8, at the Kennedy Arena box office.A special Season Ticket member presale will begin at 10 a.m.All proceeds will go to Kennedy Arena.

The preseason game is being played in Rome because of the construction project at Utica Memorial Auditorium. The contest will mark the third time since the Comets began play in 2013 that the team will have a preseason game at the Rome rink. The Comets played preseason contests in Rome in 2013 and 2014.

I like Kennedy Arena, Comets president Rob Esche said. “Im happy we were able to work … with the city of Rome and the AHL to highlight such a great hockey community, and to kick off what is to be a very special fifth anniversary season.

This will be the third consecutive preseason that the Comets have taken on Syracuse, which is Tampa Bay Lightnings top affiliate. Last season, a home-and-home series between teams included a game at the Aud. In 2015, the teams traveled to France for training camp and preseason.

The teams will close out the preseason series at 5 p.m. Sunday, Oct. 1, at War Memorial arena in Syracuse. Including the regular season, the North Division foes will meet a total of 14 times in 2017-18. This season, the series has some added intrigue as new Comets coach Trent Cull and associate coach Gary Agnew previously were coaches at different times with Syracuse.

The Comets open the regular season the first full weekend of October. Utica travels to North Division rival Toronto for a two-game set on Saturday, Oct. 7, and Sunday, Oct. 8. Both games are set for 3 p.m. starts. It is the first of seven consecutive road games for the Comets to start the season.

Cometssign goaltender

The Utica Comets signed goaltender Michael Garteigto a one-year American Hockey League contract,general manager Ryan Johnson announced Thursday.

Garteig appeared in eight games with the Comets last season. He went 0-4-1 with a .897 save percentage and a 3.01 goals against average in net for Utica.

The 25-year-old British Columbia native and former Quinnipiac University skater posted an 11-6-2 record for the Alaska Aces last season in the ECHL.Standing at6-foot-1, 183-pound, Garteig dressed in 22 games for Alaska, recording a .906 save percentage and a 3.11 goals against average.

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Comets to play Crunch in preseason game in Rome – Times Telegram – The Times Telegram

Comets spikers down Dan River High School 3-1 in opening game – YourGV.com

The Halifax County High School varsity volleyball team got its season off to a good start Tuesday night with a 3-1 win over Dan River High School at Dan River High School in Ringgold.

Dan River High School topped the Comets 25-19 in the opening game, but the Comets bounced back to win three games in a row by scores of 25-13, 25-16 and 25-18 to win the best-of-five-game match.

Overall it was a great first game and a good win, said Comets Head Coach Tiffaney Bratton.

We saw that we needed to work on some things to compete in our district and beyond. The girls had a great time on the court and came home with a win.

The Comets struggled with service errors at times in the opening game, opening the door for Dan River High School to pull off the win.

We missed key serves in the first game that cost us that one, Bratton explained.

We controlled the ball well in the first game. but we made some small mistakes that cost at critical moments. We also let some balls drop without making a play on them.

The Comets bounced back with a better effort in the second game, improving their ability to place their shots.

The team played much smarter in the second game, Bratton pointed out.

They figured out that if they controlled the ball more on our side we could put the ball in spots that Dan River couldnt return it.

The Comets continued to control the ball on both sides of the net in the third and fourth games with several players making big plays at key points, and came away with solid wins in the two games to seal the match win.

I was really pleased to see the overall team effort that the girls gave, Bratton remarked.

We really had some unselfish play out there, and that made the team concept a reality for us.

Several players drew praise from Bratton for their play.

We had some big plays from seniors Mackenzie Lawter and Rose Spainhour, Bratton noted.

Both Mackenzie and Rose led the team both on the front row and on the back row. Leigh-Anne McCormick stepped up and made some great sets and big hits to lead us on the front row.

Leigh-Anne is a great player, Bratton continued.

She is so versatile as a player that she helps the team immensely with power plays and great sets. Outside hitter Savanna Cabaniss had some big hits on the front row and held her own on the back row as well. Katie Cole stepped into several roles on the court to help with the win.

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Comets spikers down Dan River High School 3-1 in opening game – YourGV.com

Comets win opener – Waupaca County News

August 24, 2017

Medford’s Sam Hallgren (left) collides with Waupaca’s Jack Snider during the second half of a nonconference boys’ soccer game at Waupaca High School. Greg Seubert Photo

By Greg Seubert

Waupacas boys soccer team opened its season with a win.

Jack Sniders goal broke a 1-1 tie and the Comets went on to hand Medford a 2-1 nonconference loss Aug. 22 in the season opener for both teams.

Damian Johnson gave Waupaca a 1-0 lead in the 34th minute off of an assist from Keenin Polebitski and Matt Marquette.

Medford tied the game in the 68th minute on a goal from Alex Veal, but Snider found the net for the game-winning goal two minutes later.

Passing was excellent today, coach Cory Nagel said. Great looks and finding the open man. Medford played an excellent offsides trap, which frustrated our forwards most of the game. Dawson Patzke, a freshman, played the entire game in his first-ever high school soccer game. It wont show up on the stat sheet, but he was our rock today.

Bailey Colden had 10 saves in goal.

Waupaca will play its first game at Comet Field at 6:30 p.m. Thursday, Aug. 31, as the Comets host Wrightstown in the North Eastern Conference opener.

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Comets win opener – Waupaca County News

Colts defeat Comets 9-0 in soccer action – Stanly News & Press

The mens soccer teams of North Stanly and West Stanly met in a non-conference matchup on Monday in Red Cross on the Colts home field.

West was playing in its second match of the season after losing 4-0 to Union Academy a week before, while North had losses last week to Mount Pleasant 5-0 and Piedmont 10-1.

Scoring three times in the first 10 minutes, West ended the match early with the mercy rule midway through the second half, winning 9-0 over the Comets.

Logan Brown led the offense for the Colts (1-1) with three goals while Aldo Cruz added a goal and two assists. Manuel Osorio and Noel Medina each added a goal and an assist, while Elvin Lonas and Leonardo Martinez each scored goals. Damian Talley, Caleb Feere and Arturo Zelaya each added an assist for West as well.

West Stanly continues non-conference road play today at Monroe while North Stanly was scheduled to open conference play Wednesday at South Davidson.

To submit story ideas, call Charles Curcio at (704) 982-2121, ext. 26, email charles@stanlynewspress.com or contact him via Twitter (@charles_curcio).

Charles Curcio is sports editor of The Stanly News & Press. Contact him at (704) 982-2121 ext. 26, charles@stanlynewspress.com or PO Box 488, Albemarle, NC 28002.

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Colts defeat Comets 9-0 in soccer action – Stanly News & Press

Comets face final test Thursday – YourGV.com

The Halifax County High School varsity and junior varsity football teams will face their final pre-season test Thursday night with a scrimmage against Colonial Heights High School.

Thursdays action at Tuck Dillard Memorial Stadium starts with a JV scrimmage at 5 p.m. followed by the varsity scrimmage at 6 p.m.

Both Comets teams were successful in their respective opening pre-season tests Friday night, with the varsity squad topping Buckingham County High School 34-14 in the annual Virginia High School League Benefit Game and the JV team outscoring Buckingham County High School four touchdowns to none in its scrimmage.

Thursdays scrimmage against Colonial Heights will be a different kind of test for the Comets varsity squad in terms of preparation, format and the level and style of the competition.

As far as preparation goes, it is quite a bit different because it cuts a day of preparation out in your normal routine, so everything is off kilter, explained Comets Head Coach Grayson Throckmorton.

Youre scrimmaging on a night you are not accustomed. You have to adjust your practice plans and your overall mindset. In the NFL, it doesnt make much difference. In college, it doesnt make much difference. With high school, it makes a lot of difference because the kids cant adjust as well as those seasoned veterans can.

Its just something weve got to do out of necessity, Throckmorton added.

The Comets are expected to see the level of the competition ramp up Thursday night when they face the Richmond-area school.

As far as team size, the number of players and the number of students in the school, Buckingham County and Colonial Heights are about the same, Throckmorton noted.

But, Colonial Heights competes in a much larger district, and they compete against the likes of Thomas Dale, Dinwiddie, Meadowbrook, Matoaca, and the list goes on. When they get into the (post-season) playoffs, they compete at the Division 3 level, but they compete mostly against Division 5 and Division 6 teams during the regular season. So, just with regard to the level of the competition they are used to competing against, Colonial Heights is going to be a better squad.

Colonial Heights, Throckmorton said, will play a different style of offense than his team saw Friday night with Buckingham County High School.

They (Colonial Heights High School) are a true spread team which we havent seen yet, explained Throckmorton.

They are going to be looking to throw the ball out in the perimeter and try to screen on us with wide receiver screens and screens in the backfield. They will be looking to use their passing game as an advantage, which is going to be good for us.

The result, Throckmorton says, is that the Comets defense will be stretched more and better play will be needed in the defensive secondary.

Were not going to be able to play as run heavy as we did Friday night, he pointed out.

Were going to have to play more 50-50 versus the run and the pass, where the other night we were playing 80 percent run and 20 percent pass. Were going to have to be more evenly balanced Thursday night.

Throckmorton and the Comets kept everything very simple in Friday nights contest against Buckingham County High School. The Comets Head Coach says he plans to expand a few things Thursday night.

Offensively, we are going to add a little more offense in, some stuff we have been working on that we didnt use and a couple of things that are brand new, Throckmorton pointed out.

Defensively, were going to add a few stunts in that we didnt use the other night. We didnt use any stunts Friday night and we are going to add some of that in on defense.

Thursday night is also going to offer Throckmorton and his coaching staff another opportunity to evaluate personnel.

We are going to continue to look at personnel, Throckmorton pointed out, and see who can play what positions and who can play what and where in the future to help us. We have a real good idea of who is where now. Where before there were some bigger adjustments the last week and a half, now its a matter of one or two (players) here and there.

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Comets face final test Thursday – YourGV.com

VOLLEY: Comets show grit in 5-set win – Kokomo Tribune

GREENTOWN When it mattered most Tuesday night, Easterns volleyball team was dialed in.

The Comets won the first two games against visiting Kokomo, then errors crept into Easterns game and Kokomos serving got sharper. The VolleyKats won games three and four to even the match and set up a decisive fifth set.

Thats when Eastern responded. The Comets took a 9-8 lead when middle hitter Hailey Holliday floored a Kokomo overpass and the Comets never trailed again, finishing off a 25-19, 26-24, 23-25, 19-25, 15-11 victory over Kokomo.

I think we finally dug deep and decided not to let up, Eastern coach Missy Mavrick said. We have a bad habit of letting up. Once we get ahead we feel comfortable, and we decided to make sure we kept pushing that [fifth] set.

Holliday and Isabel Kelly each floored 13 kills to lead the Comets and Bailey Johnson added seven kills. Each of those net players came up with important finishes in the fifth set. Johnson had three kills including a tip kill on an overpass to end the match and scored on a block. Holliday had three kills and Kelly two.

I thought our hitters played really well at the net, Mavrick said. I thought we saw the floor really well, all the way around, all of our hitters. Weve still got some kinks to work out in the back row, thats really the key to our offense is weve got to be able to pass better than what were passing. Once we fix one thing weve got to keep being aggressive and not let something else fall apart.

Kokomo had done damage in the third and fourth sets with effective service, especially from Alliyah Hochstedler and Kylee Lauderbaugh. As the fifth set played out, the service rotation worked out ideally for the Kats as Hochstedler and Lauderbaugh were Kokomos last two servers. But neither got on a run.

A strong Hochstedler serve almost aced the Comets but the home team recovered and scored on a Johnson block for a 13-10 lead. Then after a Kokomo point, Lauderbaughs serve was fielded cleanly and Kelly smoked a kill for a 14-11 lead. Johnson settled the match on the next point.

We really made sure that on receive on those two girls that we stayed focused and make sure that we pushed to get the ball back, Mavrick said.

To illustrate how much cleaner Easterns play was in the decisive set, look at the errors. In the third set, Kokomo scored points on 14 Eastern attack, serve or technique errors. The Comets gave up eight more points in the fourth game on their own errors. In the fifth set, Eastern gave away just one point via a service error and the Kats had to earn all their other points that set.

On the other side, errors took a toll.

We had a lot of momentum going into the fifth set we had all the momentum, but our inexperience in those situations showed its head with hitting errors and kind of playing safe instead of playing to win, Kokomo coach Jason Watson said.

We had five hitting errors, two serve receive errors and a block error. Thats eight of their 15 points were directly points that we give them.

When the Kats (1-6) served well, they had the advantage, but early when they struggled, serving was the problem.

Early on we struggled serving, Watson said. In the first and second set, our serving cost us in my opinion the match. At one point we missed four out of five serves and you cant beat anybody with that lack of consistency.

Gabby Cooper led the Kat attack with 16 kills, and added seven digs. Chiara Minor had 11 kills and Hochstedler had seven kills and 14 digs. Lauderbaugh finished with 41 assists, 12 digs, four kills from her setter spot and six aces. Molly Fisher added seven digs and Madison Wood six.

I liked our energy and I think we definitely have a lot to build on, Watson said.

Maci Weeks had 25 assists and Grace Kuhlman had 16 for the Comets (3-3). Casey Clark had 31 serve receives and 25 digs. Torie Bratcher served 11 points and 16 serve receives.

I thought Hailey Holliday did a great job, Mavrick said. She played very consistent through the whole night. And Isabel Kelly, shes just been our go-to. Shes been very consistent.

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VOLLEY: Comets show grit in 5-set win – Kokomo Tribune

Comet – Wikipedia

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

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

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun’s light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets.

As of November 2014[update] there are 5,253 known comets,[5] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] Particularly bright examples are called “Great Comets”. Comets have been visited by unmanned probes such as the European Space Agency’s Rosetta, which became the first ever to land a robotic spacecraft on a comet,[9] and NASA’s Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek (“wearing long hair”), and the Oxford English Dictionary notes that the term () already meant “long-haired star, comet” in Greek. was derived from (“to wear the hair long”), which was itself derived from (“the hair of the head”) and was used to mean “the tail of a comet”.[10][11]

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

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[13] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[14] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[15] Research conducted in 2014 suggests that comets are like “deep fried ice cream”, in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[16]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[17][18] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[19] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[20][21]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley’s Comet reflects about four percent of the light that falls on it,[22] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0% of the light that falls on it;[22] by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.[23]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[24] but ascertaining their exact size is difficult.[25] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[26] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[27] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[28] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[29]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[30] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/ChuryumovGerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[31][32] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1km (0.62mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[33][34] Instruments on the Philae lander found at least sixteen organic compounds at the comet’s surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[35][36][37]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the “coma”. The force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous “tail” to form pointing away from the Sun.[46]

The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000km; 280,000,000 to 370,000,000mi) of the Sun.[47] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[47] Larger dust particles are left along the comet’s orbital path whereas smaller particles are pushed away from the Sun into the comet’s tail by light pressure.[48]

Although the solid nucleus of comets is generally less than 60 kilometres (37mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[49] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[50] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[51] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[51] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[51] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[50]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects Sunlight directly while the gases glow from ionisation.[52] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[53] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[54]

In 1996, comets were found to emit X-rays.[55] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, “stealing” one or more electrons from the atom in a process called “charge exchange”. This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.[56]

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[57][58] but these detections have been questioned.[59][60] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the type II or dust tail.[52] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[61] On occasions – such as when the Earth passes through a comet’s orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.[62]

The observation of antitails contributed significantly to the discovery of solar wind.[63] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an “induced magnetosphere” around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called “pick-up ions”) congregate and act to “load” the solar magnetic field with plasma, such that the field lines “drape” around the comet forming the ion tail.[64]

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a “tail disconnection event”.[64] This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke’s Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[65]

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions.”[66][67]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[68] These streams of gas and dust can cause the nucleus to spin, and even split apart.[68] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[69] Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.[70]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[71] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.[72] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[73] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley’s Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet’s orbit are called its “family”.[74] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[75]

At the shorter orbital period extreme, Encke’s Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called Jupiter-family comets (JFCs).[76][77] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[78][79] As of 2017[update], only 87 HTCs have been observed, compared with 567 identified JFCs.[80]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[81]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[82] Short-period comets have a tendency for their aphelia to coincide with a giant planet’s semi-major axis, with the JFCs being the largest group.[77] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[83][84]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[85] a disk of objects in the trans-Neptunian regionwhereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[86] 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.[87]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[88] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[89] 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 further from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having “periods”. The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[88] 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.[90] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[91] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[92] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

No comets with an eccentricity significantly greater than one have been observed,[92] so there are no confirmed observations of comets that are likely to have originated outside the Solar System. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[93] 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),[94] whereas others use it to mean exclusively short-period comets.[88] 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.[95]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[97] to as far as 50,000AU (0.79ly)[78] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[97] 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).[98] The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune.[78] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[99] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[99][100][101] 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.[102]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[103] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[104][105] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[103][104]

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[106] If the Earth’s orbit sends it though that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle.[107]Halley’s Comet is the source of the Orionid shower in October.[107]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth’s oceans, or at least a significant portion of it. Others have cast doubt on this idea.[108] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[16] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[109] 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.[110] 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.[111]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth’s Moon, some of which may have survived as lunar ice.[112] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[113]

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.[114] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[115]

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[30] Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[116] Some asteroids in elliptical orbits are now identified as extinct comets.[117] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[30]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[118] 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.[119][120] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[121] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[122] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[123]

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.[124]

Some comets have been observed to break up during their perihelion passage, including great comets West and IkeyaSeki. Biela’s Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela’s Comet.[125]

Some comets meet a more spectacular end either falling into the Sun[126] 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.[127]

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.[129] Similarly, the second and third known periodic comets, Encke’s Comet[130] and Biela’s Comet,[131] 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.[132]

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.[132]

From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.[133] 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.[134][135]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[136]Pliny the Elder believed that comets were connected with political unrest and death.[137]

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.[138]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth’s atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[139][140]

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.[141]

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.[142] 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.[143][144] 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.[145]

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

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.[147]

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.[148] 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.[149]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[150] 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.[151]

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.[152] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[153] 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.”[153] 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, H 2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[154][155]

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.[122] 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.[164] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[165]Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[166]

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.[167]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[168] 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.[169]

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.[170] 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.[171]

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.[172]2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[173] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[174](See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[175] 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,[176] 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.[177]

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.[178] SOHO’s 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[179] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a “new” comet is discovered, and calculation of its orbit shows it to be an old “lost” comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[180] There are at least 18 comets that fit this category.[181]

NASA is developing a comet harpoon for returning samples to Earth.

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.[183] 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)[183] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson”.[183]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley’s Comet in 1910, the Earth passed through the comet’s tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[184] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[185]

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).[183] 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.[186]

Read more:

Comet – Wikipedia

Comet Goldfish, Sarasa Comet Goldfish Information, Care …

Black Moor Goldfish Bubble Eye Goldfish Celestial Eye Goldfish Comet Goldfish Common Goldfish Fantail Goldfish Lionhead Goldfish Oranda Goldfish Pearlscale Goldfish Ranchu Goldfish Redcap Oranda Goldfish Ryukin Goldfish Shubunkin Goldfish Telescope Goldfish Veiltail Goldfish Comet Goldfish look just like regular goldfish but with a much longer andmore deeplyforked tail fin!

Contents:

The Comet Goldfish is also called the Comet-tail Goldfish or Pond Comet. This fish was the first variety of single-tail goldfish to be developed with a long caudal (tail) fin. It was developed in the United States from the Common Goldfish in the early 19th century, presumably by Hugo Mullert of Philadelphia, who then introduced them in quantity into the market.

Being a further development of the Common Goldfish, the Comet is sometimes confused for its close relative. The Comet Goldfish and Common Goldfishhave an almost identical body shape. However, the fins on the Comet Goldfish are much longer, especially the caudal (tail) fin. Its caudal fin is also more deeply forked. On both these fish,the caudal fin is held fully erect.

The adult size of the Comet Goldfish is also smaller than the Common Goldfish. Yet they are every bit as durable and can be kept in either an aquarium or outdoor pond. Both fish are inexpensive and readily available.

The Comet is generally more reddish orange in color while the Common Goldfish is more orange. While the Comet Goldfish is typically areddish orange, this fish isalso available in yellow, orange, white, and red. They can also be found as a bi-color red/white combination, and occasionally they are available with nacreous (pearly) scales, giving them a variegated color.

Other types of Comet include the Sarasa Comet. This variety has long flowing fins and is characterized by a red-and-white coloration that holds a resemblance to a koi color pattern called ‘Kohaku.’Additionally, the Tancho Single-tail Comet is a silvervariety with a red patch on its head.

One of the hardiest of the goldfish varieties, Comet Goldfish are recommended for beginners. They are an easy fish to keep as they are not picky and will readily eat what is offered.

These fish can be quite personable and are delightful to watch. They are some of the most graceful of the elongated goldfish, and this quality isemphasized by their long tails. They are active, rapid swimmers and tend to leap out of the water occasionally, so having a lid on an aquarium is good idea. They are also very social and thrive well in a community.

Along with the other elongated goldfish, such as the Common Goldfish and the Shubunkin Goldfish, the Comet varieties make good pond fish. They are hardy and can tolerate cold water temperatures. They are moderate in size but are active and fast, so will get along well with Koi. Also, Comets usually won’t uproot plants, but they will readily spawn. Care should be taken, so they don’t quickly overpopulate your pond.

Comet Goldfish – Quick Aquarium Care

Habitat: Distribution / Background

The goldfish of today are descendants of a species of wild carp known as the Prussian Carp, Silver Prussian carp, or Gibel Carp Carassius gibelio (syn: Carassius auratus gibelio), which was described by Bloch in 1782. These wild carp originated in Asia; Central Asia (Siberia). They inhabit the slow moving and stagnant waters of rivers, lakes, ponds, and ditches feeding on plants, detritus, small crustaceans, and insects.

For many years, it was believed that goldfish had originated from the Crucian Carp Carassius carassius described by Linnaeus in 1758. This fish has a wide range across the waters of the European continent, running west to east from England to Russia, north to Scandinavian countries in the Arctic Circle and as far south as the central France and the Black Sea. However, recent genetic research points to C. gibelio as a more likely ancestor.

Goldfish were originally developed in China. The first goldfish werenormally a silver or gray color, but early in the Jin Dynasty, somewhere between the years 265 – 420, breeders noted a natural genetic mutation thatproduceda yellowish orange color. It became common practice to breed this pretty golden fish for ornamental garden ponds.

By the 1500’s goldfish were traded to Japan, to Europe in the 1600’s, and to America by the 1800’s. The majority of the fancy goldfish were being developed by Asianbreeders. The results of this centuries-long endeavor is the wonderful goldfish colors and forms we see today. Domesticated goldfish are now distributed world-wide.

The Comet Goldfish was the first variety of the single-tail goldfish to be developed with a long caudal (tail) fin. It was developed in the United States from the Common Goldfish in the early 19th century, presumably by Hugo Mullert of Philadelphia, who then introduced them in quantity into the market. The Comet Goldfishis one of more than 125 captive-bred varieties of goldfish that have been developed.

Description

The Comet Goldfish is an elongated, flat-bodied variety of goldfish. The head is wide but short, and its body tapers smoothlyfrom its back and belly to the base of its caudal fin (tail fin). The caudal fin is long anddeeply forked and generally stands fully erect. Comets have a natural life span of up to 14 years, though possibly longer if kept in optimal conditions.

The Comet Goldfish is a bit smaller than the Common Goldfish, but even so, the environment it is kept in will mostly determinewhether your pet grows to its full potential size. In an average 15 gallon tank, if well cared for and not crowded, they can grow up to about 4 inches (10 cm), while in a larger, uncrowded tank, they can grow larger, generally reaching about 7 or 8 inches (17.78 – 20.32 cm). If kept in a spacious pond, they can reach over 12 inches (30+ cm).

They are primarily a reddish orange color, but they are also available in yellow, orange, white, and red. Some Comet Goldfish come in abi-color red/white combination, and occasionally they are available with nacreous (pearly) scales, giving them a variegated color.

Comet Goldfish can and do naturally change color, but color changes are believed to be influenced by diet and the amount of light. Aquarists often report the reds and oranges of their goldfish changing to white. A fresh dietalong with good lighting and available shadeare suggested as the best ways to maintain the original coloration. Even so, these measuresare not always successful.

Other types of Comet Goldfish include the Sarasa Comet. This variety has long flowing fins and is characterized by a red-and-white coloration that holds a resemblance to a koi color pattern called ‘Kohaku.’The Tancho Single-tail Comet is a silver variety with a red patch on its head.

Fish Keeping Difficulty

Comet Goldfish are some of the hardier species of goldfish. They are very undemanding of water quality and temperature. They can do well in a goldfish aquariumor even a pond as long as the environment is safe and their tankmates are not competitive.

Many people will keep goldfish in an aquarium with no heater or filtration, but for the best success, provide them the same filtration, especially biological filtration, that other aquarium residents enjoy.

Foods and Feeding

Since they are omnivorous, the Comet Goldfish will generally eat all kinds of fresh, frozen, and flake foods. To keep a good balance, give them a high quality flake food every day. To care for your goldfish, feed brine shrimp (either live or frozen), blood worms, Daphnia, or tubifex worms as a treat. It is usually better to feed freeze-dried foods as opposed to live foods to avoid parasites and bacterial infections that could be present in live foods.

Aquarium Care

These goldfish are hardy and easy to keep in a well maintained tank. Minimum tank size is 15 gallons, so make sure water changes are frequent in such as small tank. Regular weekly water changes of 1/4 to 1/3 is strongly recommended to keep these fish healthy. Snails can be added as they reduce the algae in the tank, helping to keep it clean.

Aquarium Setup

Setting up a goldfish aquarium in a manner that will keep your fish happy and healthy is the first step to success. The shape and size of the aquarium is important and depends upon the number of goldfish you are going to keep. These fish need a lot of oxygen and produce a lot of waste. Good filtration, especially biological filtration, is very helpful in maintaining the water quality of the aquarium. A filtration system will remove much of the detritus, excess foods, and waste, which keeps the tank clean and maintains the general health of the goldfish.

Goldfish are a cold water fish and will do best at temperatures between 65 – 72 F (18- 22 C). The Comet Goldfish are one of the most hardy varieties and can tolerate temperatures a few degrees above freezing, as long as the cooling drops only a few degrees a day. A quick temperature drop can kill them, so if you live in a very cold climate,a heater is advisable.

Provide a gravel substrate to help create a natural and comfortable environment for your fish. You can add some decor, but make sure that all ornamentation is smooth with no protruding points or sharp edges. Smooth rocks or driftwood should be used sparingly if at all. Aquarium plants would be the best choice of aquarium decor for goldfish, but unfortunately these fish are diggers. Consequently live plants may be uprooted. Artificial plants make a good substitute and silk plants are safer than plastic ones.

Most aquariums come with a cover that includes lighting. A cover for the tank is desirable as it reduces evaporation and though they are not prone to jumping, on occasion some gold fish will jump out. Lighting is not essential for goldfish, but does make the aquarium a nice showpiece and lighting will help if you have live plants.

Social Behaviors

Goldfish are very social animals and thrive in a community. Not only are they a great community fish, but they are great scavengers as well. It is really not necessary to add other scavengers or other bottom feeders to the aquarium when you have goldfish.

Most fancy goldfish will thrive in both freshwater and tropical aquariums as long as there are no aggressive or territorial fish in the tank. Some good tankmates for fancy goldfish are the Chinese Blue Bitterling and the Northern Redbelly Dace. Comet Goldfish can be kept with other varieties of elongated goldfish, such as the Common Goldfish and the Shubunkin, and they also do fine with Koi.

Sex: Sexual differences

During the breeding season, the male has white prickles, called breeding tubercles, on its gill covers and head. Seen from above, a female will have a fatter appearance when she is carrying eggs. It is impossible to sex Goldfish when they are young and not in breeding season, but generally the male is smaller and more slender than the female.

Breeding / Reproduction

Comet Goldfish are egg layers that spawn readily in the right conditions. They can be bred in groups as small as five individuals, but they are very social animals and likely to breed in larger groups as well. The only time Goldfish will spawn in the wild is when spring arrives. To spawn them in the aquarium, you will need to mimic the conditions found in nature.

Provide an aquarium that is at least 20 gallons and make sure the fish are healthy and disease free. Some breeders suggest you treat them for parasites. Many breeders will also separate the males and females for a few weeks prior to breeding to help increase their interest in spawning. Introduce the fish into the breeding tank at the same time. The tank will need a lush environment with solid surfaces for the spawning process and for the eggs to adhere to. Bushy, oxygenating plants, such as Anacharis, work well for this, though artificial plants or fibrous spawning mops can also be used.

To induce spawning, the temperature can be slowly dropped to around 60 F (11 C) and then slowly warmed at a rate of 3 F (2 C) per day until they spawn. Spawning generally begins when the temperatures are between 68 and 74 F (20-23 C). Feeding lots of high protein food such live brine shrimp and worms during this time will also induce spawning. Feed small amounts three times a day, but don’t overfeed. Uneaten scraps will sink to the bottom and foul the water. Maintain the breeding tank with partial water changes of up to about 20% per day.

Before spawning, as the temperature increases, the male will chase the female around the aquarium in a non-aggressive way. This can go on for several days, and the fish will intensify in color. During the spawn, the fish will gyrate from side to side, and the male will push the female against the plants. This stimulates the female to drop tiny eggs which the male will then fertilize. The eggs will adhere by sticky threads to the plants or spawn mop. Spawning can last two or three hours and can produce up to 10,000 eggs.

At this point, the parents will start to eat as many eggs as they can find. For this reason, it is best to remove the parents after spawning is complete. The fertilized eggs will hatch in 4 to 7 days, depending on the temperature. You can feed the newly hatched goldfish specialty fry foods until they become big enough to eat flake or brine shrimp, or you can offer the same food as you feed the parents as long as it is crushed very small. At first, the fry are a dark brown or black color in order to better hide and not be eaten by larger fish. They gain their adult color after several months and can be put in with larger fish once they reach about 1 inch long. See Breeding Freshwater Fish – Goldfish for more information on breeding Goldfish.

Fish Diseases

In properly maintained goldfish aquariums or ponds, goldfish illness is largely preventable. Even so, goldfish illnesses can occur, and if left untreated, may prove fatal. Goldfish are hardy, though, and if treated in a timely manner, most will make a full recovery.

When treating individuals, it is usually best to move the afflicted fish into a separate tank with no gravel or plants and do regular partial water changes. However, if the disease is apparent throughout the main tank, it may be best to do the treatments there. Whether treating in a hospital tank or your main tank, read and follow the manufacturer’s instructions for any medication. Some medications can adversely affect the water quality by destroying beneficial bacteria. You may also need to remove the carbon from the filtration system, as carbon will absorb many medications, making the treatment ineffective.

Goldfish diseases are mostly the same as those that afflict other freshwater fish, and the symptoms and treatment of goldfish are also similar. The main types of fish diseases include bacterial infections, fungal infections, parasites, and protozoa. There are also other ailments caused by injury, poor nutrition, or bad water conditions.

One of the more common problems is Ich, which is a protozoan disease. Ich is easy to identify because your fish looks like it is sprinkled with salt. Though Ich is easily treated, like other protozoan diseases, it can be fatal if not caught quickly. Some other protozoan diseases are Costia, which causes a cloudiness of the skin, and Chilodonella, which will cause a blue-white cloudiness on the skin.

External parasites are fairly common, too, but pretty easy to treat and usually not fatal when treated. These include flukes, which are flatworms about 1 mm long with hooks around their mouths. They infest the gills or body of the fish. Another type of parasite is fish lice (Argulus), flattened, mite-like crustaceans about 5 mm long that attach themselves to the body of the goldfish. Lastly, anchor worms look like threads coming out of the fish.

Some bacterial infections include Dropsy, an infection in the kidneys that can be fatal if not treated quickly. Fish Tuberculosis is indicated by the fish becoming emaciated (having a hollow belly). For this illness, there is no absolute treatment, and it can be fatal. Tail/Fin Rot may also be bacterial, though the reduced tail or fins can be caused by a number of factors as well. There is also fungus, a fungal infection, and Black Spot or Black Ich, which is a parasitic infection.

Swim Bladder Disease is an ailment indicated by fish swimming in abnormal patterns and having difficulty maintaining their balance. This can be caused by a number of things: constipation, poor nutrition, a physical deformity, or a parasitic infection. Feeding frozen peas (defrosted) has been noted to help alleviate the symptoms and correct the problem in some cases.

Other miscellaneous ailments include Cloudy Eye, which can be caused by a variety of things ranging from poor nutrition, bad water quality, and rough handling. It can also be the result of other illnesses, such as bacterial infections. Constipation is indicated by a loss of appetite and swelling of the body, and the cause is almost always diet. Then there are wounds and ulcers. Wounds can become infected, creating ulcers. Wounds can develop either bacterial or fungal infections, or both, and must be treated. There are treatments for each of these diseases individually and treatments that handle both. For more in-depth information about goldfish diseases and illnesses, see Goldfish Care; Fancy Goldfish and Goldfish Diseases.

Availability

The Comet Goldfish is inexpensive andreadily available in fish stores and online.

References

See more here:

Comet Goldfish, Sarasa Comet Goldfish Information, Care …

Halley’s Comet – Wikipedia

Halley’s Comet or Comet Halley, officially designated 1P/Halley,[3] is a short-period comet visible from Earth every 7479 years.[3][10][11][12] Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and the only naked-eye comet that might appear twice in a human lifetime.[13] Halley last appeared in the inner parts of the Solar System in 1986 and will next appear in mid-2061.[14]

Halley’s returns to the inner Solar System have been observed and recorded by astronomers since at least 240 BC. Clear records of the comet’s appearances were made by Chinese, Babylonian, and medieval European chroniclers, but were not recognized as reappearances of the same object at the time. The comet’s periodicity was first determined in 1705 by English astronomer Edmond Halley, after whom it is now named.

During its 1986 apparition, Halley’s Comet became the first comet to be observed in detail by spacecraft, providing the first observational data on the structure of a comet nucleus and the mechanism of coma and tail formation.[15][16] These observations supported a number of longstanding hypotheses about comet construction, particularly Fred Whipple’s “dirty snowball” model, which correctly predicted that Halley would be composed of a mixture of volatile ices such as water, carbon dioxide, and ammonia and dust. The missions also provided data that substantially reformed and reconfigured these ideas; for instance, it is now understood that the surface of Halley is largely composed of dusty, non-volatile materials, and that only a small portion of it is icy.

Comet Halley is commonly pronounced , rhyming with valley, or , rhyming with daily.[17][18] Spellings of Edmond Halley’s name during his lifetime included Hailey, Haley, Hayley, Halley, Hawley, and Hawly, so its contemporary pronunciation is uncertain.[19]

Halley was the first comet to be recognized as periodic. Until the Renaissance, the philosophical consensus on the nature of comets, promoted by Aristotle, was that they were disturbances in Earth’s atmosphere. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon. Many were still unconvinced that comets orbited the Sun, and assumed instead that they must follow straight paths through the Solar System.[20]

In 1687, Sir Isaac Newton published his Philosophi Naturalis Principia Mathematica, in which he outlined his laws of gravity and motion. His work on comets was decidedly incomplete. Although he had suspected that two comets that had appeared in succession in 1680 and 1681 were the same comet before and after passing behind the Sun (he was later found to be correct; see Newton’s Comet),[21] he was unable to completely reconcile comets into his model.

Ultimately, it was Newton’s friend, editor and publisher, Edmond Halley, who, in his 1705 Synopsis of the Astronomy of Comets, used Newton’s new laws to calculate the gravitational effects of Jupiter and Saturn on cometary orbits.[22] This calculation enabled him, after examining historical records, to determine that the orbital elements of a second comet that had appeared in 1682 were nearly the same as those of two comets that had appeared in 1531 (observed by Petrus Apianus) and 1607 (observed by Johannes Kepler).[22] Halley thus concluded that all three comets were, in fact, the same object returning about every 76 years, a period that has since been found to vary between 7479 years. After a rough estimate of the perturbations the comet would sustain from the gravitational attraction of the planets, he predicted its return for 1758.[23] Halley died in 1742 before he could observe this himself.[24]

Halley’s prediction of the comet’s return proved to be correct, although it was not seen until 25 December 1758, by Johann Georg Palitzsch, a German farmer and amateur astronomer. It did not pass through its perihelion until 13 March 1759, the attraction of Jupiter and Saturn having caused a retardation of 618 days.[25] This effect was computed prior to its return (with a one-month error to 13 April)[26] by a team of three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute.[27] The confirmation of the comet’s return was the first time anything other than planets had been shown to orbit the Sun. It was also one of the earliest successful tests of Newtonian physics, and a clear demonstration of its explanatory power.[28] The comet was first named in Halley’s honour by French astronomer Nicolas Louis de Lacaille in 1759.[28]

Some scholars have proposed that first-century Mesopotamian astronomers already had recognized Halley’s Comet as periodic.[29] This theory notes a passage in the Bavli Talmud[30] that refers to “a star which appears once in seventy years that makes the captains of the ships err.”[31]

Researchers in 1981 attempting to calculate the past orbits of Halley by numerical integration starting from accurate observations in the seventeenth and eighteenth centuries could not produce accurate results further back than 837 due to a close approach to Earth in that year. It was necessary to use ancient Chinese comet observations to constrain their calculations.[32]

Halley’s orbital period has varied between 7479 years since 240 BC.[28][11] Its orbit around the Sun is highly elliptical, with an orbital eccentricity of 0.967 (with 0 being a circle and 1 being a parabolic trajectory). The perihelion, the point in the comet’s orbit when it is nearest the Sun, is just 0.6 AU.[33] This is between the orbits of Mercury and Venus. Its aphelion, or farthest distance from the Sun, is 35 AU (roughly the distance of Pluto). Unusual for an object in the Solar System, Halley’s orbit is retrograde; it orbits the Sun in the opposite direction to the planets, or, clockwise from above the Sun’s north pole. The orbit is inclined by 18 to the ecliptic, with much of it lying south of the ecliptic. (Because it is retrograde, the true inclination is 162).[34] Due to the retrograde orbit, it has one of the highest velocities relative to the Earth of any object in the Solar System. The 1910 passage was at a relative velocity of 70.56km/s (157,838mph or 254,016km/h).[35] Because its orbit comes close to Earth’s in two places, Halley is associated with two meteor showers: the Eta Aquariids in early May, and the Orionids in late October.[36] Halley is the parent body to the Orionids. Observations conducted around the time of Halley’s appearance in 1986 suggested that the comet could additionally perturb the Eta Aquarid meteor shower, although it might not be the parent of that shower.[37]

Halley is classified as a periodic or short-period comet; one with an orbit lasting 200 years or less.[38] This contrasts it with long-period comets, whose orbits last for thousands of years. Periodic comets have an average inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, so Halley’s orbit is atypical.[28] Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 2030 degrees or less) are called Jupiter-family comets. Those resembling 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.[38][39] As of 2015[update], only 75 Halley-type comets have been observed, compared with 511 identified Jupiter family comets.[40]

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System.[38] If Halley was once a long-period comet, it is likely to have originated in the Oort Cloud,[39] a sphere of cometary bodies that has an inner edge of 20,00050,000 AU. Conversely the Jupiter-family comets are generally believed to originate in the Kuiper belt,[39] a flat disc of icy debris between 30 AU (Neptune’s orbit) and 50 AU from the Sun (in the scattered disc). Another point of origin for the Halley-type comets was proposed in 2008, when a trans-Neptunian object with a retrograde orbit similar to Halley’s was discovered, 2008 KV42, whose orbit takes it from just outside that of Uranus to twice the distance of Pluto. It may be a member of a new population of small Solar System bodies that serves as the source of Halley-type comets.[41]

Halley has probably been in its current orbit for 16,000200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations.[42] The non-gravitational effects can be crucial;[42] as Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path. These orbital changes cause delays in its perihelion of four days, average.[43]

In 1989, Boris Chirikov and Vitaly Vecheslavov performed an analysis of 46 apparitions of Halley’s Comet taken from historical records and computer simulations. These studies showed that its dynamics were chaotic and unpredictable on long timescales.[44] Halley’s projected lifetime could be as long as 10million years. More recent work suggests that Halley will evaporate, or split in two, within the next few tens of thousands of years, or will be ejected from the Solar System within a few hundred thousand years.[45][39] Observations by D.W. Hughes suggest that Halley’s nucleus has been reduced in mass by 8090% over the last 20003000 revolutions.[16]

The Giotto and Vega missions gave planetary scientists their first view of Halley’s surface and structure. Like all comets, as Halley nears the Sun, its volatile compounds (those with low boiling points, such as water, carbon monoxide, carbon dioxide and other ices) begin to sublime from the surface of its nucleus.[46] This causes the comet to develop a coma, or atmosphere, up to 100,000km across.[2] Evaporation of this dirty ice releases dust particles, which travel with the gas away from the nucleus. Gas molecules in the coma absorb solar light and then re-radiate it at different wavelengths, a phenomenon known as fluorescence, whereas dust particles scatter the solar light. Both processes are responsible for making the coma visible.[13] As a fraction of the gas molecules in the coma are ionized by the solar ultraviolet radiation,[13] pressure from the solar wind, a stream of charged particles emitted by the Sun, pulls the coma’s ions out into a long tail, which may extend more than 100millionkilometers into space.[46][47] Changes in the flow of the solar wind can cause disconnection events, in which the tail completely breaks off from the nucleus.[15]

Despite the vast size of its coma, Halley’s nucleus is relatively small: barely 15kilometers long, 8kilometers wide and perhaps 8kilometers thick.[b] Its shape vaguely resembles that of a peanut.[2] Its mass is relatively low (roughly 2.21014kg)[4] and its average density is about 0.6g/cm3, indicating that it is made of a large number of small pieces, held together very loosely, forming a structure known as a rubble pile.[5] Ground-based observations of coma brightness suggested that Halley’s rotation period was about 7.4 days. Images taken by the various spacecraft, along with observations of the jets and shell, suggested a period of 52hours.[16] Given the irregular shape of the nucleus, Halley’s rotation is likely to be complex.[46] Although only 25% of Halley’s surface was imaged in detail during the flyby missions, the images revealed an extremely varied topography, with hills, mountains, ridges, depressions, and at least one crater.[16]

Halley is the most active of all the periodic comets, with others, such as Comet Encke and Comet Holmes, being one or two orders of magnitude less active.[16] Its day side (the side facing the Sun) is far more active than the night side. Spacecraft observations showed that the gases ejected from the nucleus were 80% water vapor, 17% carbon monoxide and 34% carbon dioxide,[48] with traces of hydrocarbons[49] although more-recent sources give a value of 10% for carbon monoxide and also include traces of methane and ammonia.[50] The dust particles were found to be primarily a mixture of carbonhydrogenoxygennitrogen (CHON) compounds common in the outer Solar System, and silicates, such as are found in terrestrial rocks.[46] The dust particles decreased in size down to the limits of detection (~0.001m).[15] The ratio of deuterium to hydrogen in the water released by Halley was initially thought to be similar to that found in Earth’s ocean water, suggesting that Halley-type comets may have delivered water to Earth in the distant past. Subsequent observations showed Halley’s deuterium ratio to be far higher than that found in Earth’s oceans, making such comets unlikely sources for Earth’s water.[46]

Giotto provided the first evidence in support of Fred Whipple’s “dirty snowball” hypothesis for comet construction; Whipple postulated that comets are icy objects warmed by the Sun as they approach the inner Solar System, causing ices on their surfaces to sublimate (change directly from a solid to a gas), and jets of volatile material to burst outward, creating the coma. Giotto showed that this model was broadly correct,[46] though with modifications. Halley’s albedo, for instance, is about 4%, meaning that it reflects only 4% of the sunlight hitting it; about what one would expect for coal.[51] Thus, despite appearing brilliant white to observers on Earth, Halley’s Comet is in fact pitch black. The surface temperature of evaporating “dirty ice” ranges from 170 K (103C) at higher albedo to 220K (53C) at low albedo; Vega 1 found Halley’s surface temperature to be in the range 300400 K (30130C). This suggested that only 10% of Halley’s surface was active, and that large portions of it were coated in a layer of dark dust that retained heat.[15] Together, these observations suggested that Halley was in fact predominantly composed of non-volatile materials, and thus more closely resembled a “snowy dirtball” than a “dirty snowball”.[16][52]

Halley may have been recorded as early as 467BC, but this is uncertain. A comet was recorded in ancient Greece between 468 and 466 BC; its timing, location, duration, and associated meteor shower all suggest it was Halley.[53] According to Pliny the Elder, that same year a meteorite fell in the town of Aegospotami, in Thrace. He described it as brown in colour and the size of a wagon load.[54] Chinese chroniclers also mention a comet in that year.[55]

The first certain appearance of Halley’s Comet in the historical record is a description from 240BC, in the Chinese chronicle Records of the Grand Historian or Shiji, which describes a comet that appeared in the east and moved north.[56] The only surviving record of the 164BC apparition is found on two fragmentary Babylonian tablets, now owned by the British Museum.[56]

The apparition of 87BC was recorded in Babylonian tablets which state that the comet was seen “day beyond day” for a month.[57] This appearance may be recalled in the representation of Tigranes the Great, an Armenian king who is depicted on coins with a crown that features, according to Vahe Gurzadyan and R. Vardanyan, “a star with a curved tail [that] may represent the passage of Halley’s Comet in 87BC.” Gurzadyan and Vardanyan argue that “Tigranes could have seen Halley’s Comet when it passed closest to the Sun on August 6 in 87BC” as the comet would have been a “most recordable event”; for ancient Armenians it could have heralded the New Era of the brilliant King of Kings.[58]

The apparition of 12BC was recorded in the Book of Han by Chinese astronomers of the Han Dynasty who tracked it from August through October.[10] It passed within 0.16AU of Earth.[59] Halley’s appearance in 12BC, only a few years distant from the conventionally assigned date of the birth of Jesus Christ, has led some theologians and astronomers to suggest that it might explain the biblical story of the Star of Bethlehem. There are other explanations for the phenomenon, such as planetary conjunctions, and there are also records of other comets that appeared closer to the date of Jesus’ birth.[60]

If, as has been suggested, the reference in the Talmud to “a star which appears once in seventy years that makes the captains of the ships err”[61] (see above) refers to Halley’s Comet, it may be a reference to the 66AD appearance, because this passage is attributed to the Rabbi Yehoshua ben Hananiah. This apparition was the only one to occur during ben Hananiah’s lifetime.[62]

The 141AD apparition was recorded in Chinese chronicles.[63] It was also recorded in the Tamil work Purananuru, in connection with the death of the south Indian Chera king Yanaikatchai Mantaran Cheral Irumporai.[64]

The 374AD and 607 approaches each came within 0.09AU of Earth.[59] The 684AD apparition was recorded in Europe in one of the sources used by the compiler of the 1493 Nuremberg Chronicles. Chinese records also report it as the “broom star”.[65]

In 837, Halley’s Comet may have passed as close as 0.03AU (3.2million miles; 5.1million kilometers) from Earth, by far its closest approach.[59] Its tail may have stretched 60degrees across the sky. It was recorded by astronomers in China, Japan, Germany, The Byzantine Empire, and the Middle East.[10] In 912, Halley is recorded in the Annals of Ulster, which state “A dark and rainy year. A comet appeared.”[66]

In 1066, the comet was seen in England and thought to be an omen: later that year Harold II of England died at the Battle of Hastings; it was a bad omen for Harold, but a good omen for the man who defeated him, William the Conqueror. The comet is represented on the Bayeux Tapestry as a fiery star, and the surviving accounts describe it as appearing to be four times the size of Venus and shining with a light equal to a quarter of that of the Moon. Halley came within 0.10AU of Earth at that time.[59]

This appearance of the comet is also noted in the Anglo-Saxon Chronicle. Eilmer of Malmesbury may have seen Halley previously in 989, as he wrote of it in 1066: “You’ve come, have you?… You’ve come, you source of tears to many mothers, you evil. I hate you! It is long since I saw you; but as I see you now you are much more terrible, for I see you brandishing the downfall of my country. I hate you!”[67]

The Irish Annals of the Four Masters recorded the comet as “A star [that] appeared on the seventh of the Calends of May, on Tuesday after Little Easter, than whose light the brilliance or light of The Moon was not greater; and it was visible to all in this manner till the end of four nights afterwards.”[66]Chaco Native Americans in New Mexico may have recorded the 1066 apparition in their petroglyphs.[68]

The 1145 apparition was recorded by the monk Eadwine. The 1986 apparition exhibited a fan tail similar to Eadwine’s drawing.[65] Some claim that Genghis Khan was inspired to turn his conquests toward Europe by the 1222 apparition.[69] The 1301 apparition may have been seen by the artist Giotto di Bondone, who represented the Star of Bethlehem as a fire-colored comet in the Nativity section of his Arena Chapel cycle, completed in 1305.[65] Its 1378 appearance is recorded in the Annales Mediolanenses[70] as well as in East Asian sources.[71]

In 1456, the year of Halley’s next apparition, the Ottoman Empire invaded the Kingdom of Hungary, culminating in the Siege of Belgrade in July of that year. In a Papal Bull, Pope Calixtus III ordered special prayers be said for the city’s protection. In 1470, the humanist scholar Bartolomeo Platina wrote in his Lives of the Popes that,[72]

A hairy and fiery star having then made its appearance for several days, the mathematicians declared that there would follow grievous pestilence, dearth and some great calamity. Calixtus, to avert the wrath of God, ordered supplications that if evils were impending for the human race He would turn all upon the Turks, the enemies of the Christian name. He likewise ordered, to move God by continual entreaty, that notice should be given by the bells to call the faithful at midday to aid by their prayers those engaged in battle with the Turk.

Platina’s account is not mentioned in official records. In the 18th century, a Frenchman further embellished the story, in anger at the Church, by claiming that the Pope had “excommunicated” the comet, though this story was most likely his own invention.[73]

Halley’s apparition of 1456 was also witnessed in Kashmir and depicted in great detail by rvara, a Sanskrit poet and biographer to the Sultans of Kashmir. He read the apparition as a cometary portent of doom foreshadowing the imminent fall of Sultan Zayn al-Abidin (AD 1418/14201470).[74]

After witnessing a bright light in the sky which most historians have identified as Halley’s Comet, Zara Yaqob, Emperor of Ethiopia from 1434 to 1468, founded the city of Debre Berhan (tr. City of Light) and made it his capital for the remainder of his reign.[75]

Halley’s periodic returns have been subject to scientific investigation since the 16th century. The three apparitions from 1531 to 1682 were noted by Edmond Halley, enabling him to predict its 1759 return.[76] Streams of vapour observed during the comet’s 1835 apparition prompted astronomer Friedrich Wilhelm Bessel to propose that the jet forces of evaporating material could be great enough to significantly alter a comet’s orbit.[77]

The 1910 approach, which came into naked-eye view around 10 April[59] and came to perihelion on 20 April,[59] was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained.[15] Furthermore, the comet made a relatively close approach of 0.15 AU,[59] making it a spectacular sight. Indeed, on 19 May, Earth actually passed through the tail of the comet.[78][79] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,[80] which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas “would impregnate the atmosphere and possibly snuff out all life on the planet.”[81] His pronouncement led to panicked buying of gas masks and quack “anti-comet pills” and “anti-comet umbrellas” by the public.[82] In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.[81]

The comet added to the unrest in China on the eve of Xinhai Revolution that would end the last dynasty in 1911. As James Hutson, a missionary in Sichuan Province at the time, recorded,

The people believe that it indicates calamity such as war, fire, pestilence, and a change of dynasty. In some places on certain days the doors were unopened for half a day, no water was carried and many did not even drink water as it was rumoured that pestilential vapour was being poured down upon the earth from the comet.”[83]

The 1910 visitation is also recorded as being the travelling companion of the first known English Muslim to make the Haj pilgrimage to Mecca. However, his explanation of its scientific predictability did not meet with favour in the Holy City.[84]

The comet was also fertile ground for hoaxes. One that reached major newspapers claimed that the Sacred Followers, a supposed Oklahoma religious group, attempted to sacrifice a virgin to ward off the impending disaster, but were stopped by the police.[85]

American satirist and writer Mark Twain was born on 30 November 1835, exactly two weeks after the comet’s perihelion. In his autobiography, published in 1909, he said,

I came in with Halley’s comet in 1835. It is coming again next year, and I expect to go out with it. It will be the greatest disappointment of my life if I don’t go out with Halley’s comet. The Almighty has said, no doubt: ‘Now here are these two unaccountable freaks; they came in together, they must go out together.'[86][87]

Twain died on 21 April 1910, the day following the comet’s subsequent perihelion.[88] The 1985 fantasy film The Adventures of Mark Twain was inspired by the quotation.

Halley’s 1910 apparition is distinct from the Great Daylight Comet of 1910, which surpassed Halley in brilliance and was actually visible in broad daylight for a short period, approximately four months before Halley made its appearance.[89][90]

Halley’s 1986 apparition was the least favorable on record. The comet and Earth were on opposite sides of the Sun in February 1986, creating the worst viewing circumstances for Earth observers for the last 2,000 years.[91] Halley’s closest approach was 0.42 AU.[92] Additionally, with increased light pollution from urbanization, many people failed to even see the comet. It was possible to observe it in areas outside of cities with the help of binoculars.[93] Further, the comet appeared brightest when it was almost invisible from the northern hemisphere in March and April.[94] Halley’s approach was first detected by astronomers David Jewitt and G. Edward Danielson on 16 October 1982 using the 5.1m Hale telescope at Mount Palomar and a CCD camera.[95] The first person to visually observe the comet on its 1986 return was amateur astronomer Stephen James O’Meara on 24 January 1985. O’Meara used a home-built 24-inch telescope on top of Mauna Kea to detect the magnitude 19.6 comet.[96] On 8 November 1985, Stephen Edberg (then serving as the Coordinator for Amateur Observations at NASA’s Jet Propulsion Laboratory) and Charles Morris were the first to observe Halley’s Comet with the naked eye in its 1986 apparition.[97][98]

The development of space travel gave scientists the opportunity to study the comet at close quarters, and several probes were launched to do so. The Soviet Vega 1 started returning images of Halley on 4 March 1986, and the first ever of its nucleus,[16] and made its flyby on 6 March, followed by Vega 2 making its flyby on 9 March. On 14 March, the Giotto space probe, launched by the European Space Agency, made the closest pass of the comet’s nucleus.[16] There were also two Japanese probes, Suisei and Sakigake. The probes were unofficially known as the Halley Armada.[99]

Based on data retrieved by Astron, the largest ultraviolet space telescope of the time, during its Halley’s Comet observations in December 1985, a group of Soviet scientists developed a model of the comet’s coma.[100] The comet was also observed from space by the International Cometary Explorer. Originally International Sun-Earth Explorer 3, the probe was renamed and freed from its L1 Lagrangian point location in Earth’s orbit to intercept comets 21P/Giacobini-Zinner and Halley.[101]

Two Space Shuttle missions the ill-fated STS-51-L (ended by the Challenger disaster)[102] and STS-61-E were scheduled to observe Halley’s Comet from low Earth orbit. STS-51-L carried the Shuttle-Pointed Tool for Astronomy (SPARTAN-203) satellite, also called the Halley’s Comet Experiment Deployable (HCED).[103] STS-61-E was a Columbia mission scheduled for March 1986, carrying the ASTRO-1 platform to study the comet.[104] Due to the suspension of America’s manned space program after the Challenger explosion, the mission was canceled, and ASTRO-1 would not fly until late 1990 on STS-35.[105]

On 12 February 1991, at a distance of 14.4AU (2.15109km) from the Sun, Halley displayed an outburst that lasted for several months, releasing a cloud of dust 300,000km across.[46] The outburst likely started in December 1990, and then the comet brightened from magnitude 24.3 to magnitude 18.9.[106] Halley was most recently observed in 2003 by three of the Very Large Telescopes at Paranal, Chile, when Halley’s magnitude was 28.2. The telescopes observed Halley, at the faintest and farthest any comet has ever been imaged, in order to verify a method for finding very faint trans-Neptunian objects.[9] Astronomers are now able to observe the comet at any point in its orbit.[9]

The next predicted perihelion of Halley’s Comet is 28 July 2061,[1] when it is expected to be better positioned for observation than during the 19851986 apparition, as it will be on the same side of the Sun as Earth.[11] It is expected to have an apparent magnitude of 0.3, compared with only +2.1 for the 1986 apparition.[107] It has been calculated that on 9 September 2060, Halley will pass within 0.98AU (147,000,000km) of Jupiter, and then on 20 August 2061 will pass within 0.0543AU (8,120,000km) of Venus.[108] In 2134, Halley is expected to pass within 0.09AU (13,000,000km) of Earth.[108] Its apparent magnitude is expected to be 2.0.[107]

Halley’s calculations enabled the comet’s earlier appearances to be found in the historical record. The following table sets out the astronomical designations for every apparition of Halley’s Comet from 240BC, the earliest documented widespread sighting.[3][109] For example, “1P/1982U1, 1986III, 1982i” indicates that for the perihelion in 1986, Halley was the first period comet known (designated 1P) and this apparition was the first seen in half-month U (the second half of October)[110] in 1982 (giving 1P/1982 U1); it was the third comet past perihelion in 1986 (1986 III); and it was the ninth comet spotted in 1982 (provisional designation 1982i). The perihelion dates of each apparition are shown.[111] The perihelion dates farther from the present are approximate, mainly because of uncertainties in the modelling of non-gravitational effects. Perihelion dates of 1531 and earlier are in the Julian calendar, while perihelion dates 1607 and after are in the Gregorian calendar.[112]

The American rock band Bill Haley & His Comets was named after Halleys Comet. Although the name of the comet has traditionally been pronounced like Valley, historians have preferred to pronounce it as Haley, since the astronomer Edmond Halley was believed to have pronounced his own name in this way (though others claim he pronounced it as Hawley).

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Halley’s Comet – Wikipedia

Comets look to attack defensively – Kokomo Perspective

After completing 10 years as the head coach, Josh Edwards and his Comets put together their best season since 2011, with a 4-6 record last year. Now, Edwards plans to build on the teams success with his upperclassmen.

Though the Comets on paper did not look extremely impressive last year, part of that was due to only playing three quarters of the game. Of the six losses, two of them were lost in the fourth quarter, one at Oak Hill and one at Madison-Grant. Though the team has fallen short on some outcomes, its biggest blowout was by 40 points. With the capacity to outscore a team in that fashion, Edwards is counting on utilizing his strong-skilled veteran players to go above a .500 season.

Taking the snaps for Eastern will be quarterback Garrett Hetzner. Hetzner has put time in the weight room over the off-season to build his body frame. At 6-feet 3-inches and 180-pounds, he has the size and strength to pass or carry the ball where it needs to go.

This will be an exciting season for him. Hes been working hard in gym, asking a lot of good questions. Most importantly, hes trying to better his teammates, said Edwards.

At Hetzners disposal will be three returning senior wide receivers, Clayton McKillip, Braden Sparks, and Tyler TP Gilbert. With each receiver comes a unique skillset.

McKillip has the speed necessary to beat almost any cornerback off of the line. Sparks has the agility and mental know-how to beat off defenders. TP will be that guy who will make it happen. Between the three of them, itll be fun to watch, said Edwards.

On the other side of the ball, Edwards and his squad of nine returning starters do not have too much to stress over.

Last year we created a hybrid defense that would fit our kids the best, knowing we would have most of the kids back this year. Weve pretty much kept the scheme the same for the most part, minus a few adjustments, said Edwards. Overall, I think well have a lot of continuity that should carryover from last year to this year.

After implementing its hybrid defense, the Comets were able to shave 10 points allowed per game off of its average. With the majority of its defense returning, mixed in with several key players on offense, Edwards hopes to knockout Oak Hill in the season opener come Aug. 18.

Oak Hill has literally been circled on our schedule since last year after what happened. When we let them score 20 points in the fourth quarter to beat us, its all Ive been focused on since last year, said Edwards.

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Comets look to attack defensively – Kokomo Perspective

Comets face final test Thursday – YourGV.com

The Halifax County High School varsity and junior varsity football teams will face their final pre-season test Thursday night with a scrimmage against Colonial Heights High School.

Thursdays action at Tuck Dillard Memorial Stadium starts with a JV scrimmage at 5 p.m. followed by the varsity scrimmage at 6 p.m.

Both Comets teams were successful in their respective opening pre-season tests Friday night, with the varsity squad topping Buckingham County High School 34-14 in the annual Virginia High School League Benefit Game and the JV team outscoring Buckingham County High School four touchdowns to none in its scrimmage.

Thursdays scrimmage against Colonial Heights will be a different kind of test for the Comets varsity squad in terms of preparation, format and the level and style of the competition.

As far as preparation goes, it is quite a bit different because it cuts a day of preparation out in your normal routine, so everything is off kilter, explained Comets Head Coach Grayson Throckmorton.

Youre scrimmaging on a night you are not accustomed. You have to adjust your practice plans and your overall mindset. In the NFL, it doesnt make much difference. In college, it doesnt make much difference. With high school, it makes a lot of difference because the kids cant adjust as well as those seasoned veterans can.

Its just something weve got to do out of necessity, Throckmorton added.

The Comets are expected to see the level of the competition ramp up Thursday night when they face the Richmond-area school.

As far as team size, the number of players and the number of students in the school, Buckingham County and Colonial Heights are about the same, Throckmorton noted.

But, Colonial Heights competes in a much larger district, and they compete against the likes of Thomas Dale, Dinwiddie, Meadowbrook, Matoaca, and the list goes on. When they get into the (post-season) playoffs, they compete at the Division 3 level, but they compete mostly against Division 5 and Division 6 teams during the regular season. So, just with regard to the level of the competition they are used to competing against, Colonial Heights is going to be a better squad.

Colonial Heights, Throckmorton said, will play a different style of offense than his team saw Friday night with Buckingham County High School.

They (Colonial Heights High School) are a true spread team which we havent seen yet, explained Throckmorton.

They are going to be looking to throw the ball out in the perimeter and try to screen on us with wide receiver screens and screens in the backfield. They will be looking to use their passing game as an advantage, which is going to be good for us.

The result, Throckmorton says, is that the Comets defense will be stretched more and better play will be needed in the defensive secondary.

Were not going to be able to play as run heavy as we did Friday night, he pointed out.

Were going to have to play more 50-50 versus the run and the pass, where the other night we were playing 80 percent run and 20 percent pass. Were going to have to be more evenly balanced Thursday night.

Throckmorton and the Comets kept everything very simple in Friday nights contest against Buckingham County High School. The Comets Head Coach says he plans to expand a few things Thursday night.

Offensively, we are going to add a little more offense in, some stuff we have been working on that we didnt use and a couple of things that are brand new, Throckmorton pointed out.

Defensively, were going to add a few stunts in that we didnt use the other night. We didnt use any stunts Friday night and we are going to add some of that in on defense.

Thursday night is also going to offer Throckmorton and his coaching staff another opportunity to evaluate personnel.

We are going to continue to look at personnel, Throckmorton pointed out, and see who can play what positions and who can play what and where in the future to help us. We have a real good idea of who is where now. Where before there were some bigger adjustments the last week and a half, now its a matter of one or two (players) here and there.

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Comets face final test Thursday – YourGV.com

VOLLEY: Comets show grit in 5-set win – Kokomo Tribune

GREENTOWN When it mattered most Tuesday night, Easterns volleyball team was dialed in.

The Comets won the first two games against visiting Kokomo, then errors crept into Easterns game and Kokomos serving got sharper. The VolleyKats won games three and four to even the match and set up a decisive fifth set.

Thats when Eastern responded. The Comets took a 9-8 lead when middle hitter Hailey Holliday floored a Kokomo overpass and the Comets never trailed again, finishing off a 25-19, 26-24, 23-25, 19-25, 15-11 victory over Kokomo.

I think we finally dug deep and decided not to let up, Eastern coach Missy Mavrick said. We have a bad habit of letting up. Once we get ahead we feel comfortable, and we decided to make sure we kept pushing that [fifth] set.

Holliday and Isabel Kelly each floored 13 kills to lead the Comets and Bailey Johnson added seven kills. Each of those net players came up with important finishes in the fifth set. Johnson had three kills including a tip kill on an overpass to end the match and scored on a block. Holliday had three kills and Kelly two.

I thought our hitters played really well at the net, Mavrick said. I thought we saw the floor really well, all the way around, all of our hitters. Weve still got some kinks to work out in the back row, thats really the key to our offense is weve got to be able to pass better than what were passing. Once we fix one thing weve got to keep being aggressive and not let something else fall apart.

Kokomo had done damage in the third and fourth sets with effective service, especially from Alliyah Hochstedler and Kylee Lauderbaugh. As the fifth set played out, the service rotation worked out ideally for the Kats as Hochstedler and Lauderbaugh were Kokomos last two servers. But neither got on a run.

A strong Hochstedler serve almost aced the Comets but the home team recovered and scored on a Johnson block for a 13-10 lead. Then after a Kokomo point, Lauderbaughs serve was fielded cleanly and Kelly smoked a kill for a 14-11 lead. Johnson settled the match on the next point.

We really made sure that on receive on those two girls that we stayed focused and make sure that we pushed to get the ball back, Mavrick said.

To illustrate how much cleaner Easterns play was in the decisive set, look at the errors. In the third set, Kokomo scored points on 14 Eastern attack, serve or technique errors. The Comets gave up eight more points in the fourth game on their own errors. In the fifth set, Eastern gave away just one point via a service error and the Kats had to earn all their other points that set.

On the other side, errors took a toll.

We had a lot of momentum going into the fifth set we had all the momentum, but our inexperience in those situations showed its head with hitting errors and kind of playing safe instead of playing to win, Kokomo coach Jason Watson said.

We had five hitting errors, two serve receive errors and a block error. Thats eight of their 15 points were directly points that we give them.

When the Kats (1-6) served well, they had the advantage, but early when they struggled, serving was the problem.

Early on we struggled serving, Watson said. In the first and second set, our serving cost us in my opinion the match. At one point we missed four out of five serves and you cant beat anybody with that lack of consistency.

Gabby Cooper led the Kat attack with 16 kills, and added seven digs. Chiara Minor had 11 kills and Hochstedler had seven kills and 14 digs. Lauderbaugh finished with 41 assists, 12 digs, four kills from her setter spot and six aces. Molly Fisher added seven digs and Madison Wood six.

I liked our energy and I think we definitely have a lot to build on, Watson said.

Maci Weeks had 25 assists and Grace Kuhlman had 16 for the Comets (3-3). Casey Clark had 31 serve receives and 25 digs. Torie Bratcher served 11 points and 16 serve receives.

I thought Hailey Holliday did a great job, Mavrick said. She played very consistent through the whole night. And Isabel Kelly, shes just been our go-to. Shes been very consistent.

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VOLLEY: Comets show grit in 5-set win – Kokomo Tribune

Panthers slow down Comets – Kankakee Daily Journal

CLIFTON — Tuesday night’s volleyball game between Manteno and Central was a tale of two teams.

For the Panthers, it was a near-perfect way to kick off the season as they swept the Comets, 25-9 and 25-11, and only seemed to get stronger as the game went on.

For the Comets, it was a step backward in their second contest of the year. Yesterday, Central faced off against Armstrong at the Timberwolf Tip-Off, and though they ultimately lost that game, too, head coach Jill Luckenbill was much happier with their performance in that match.

“They played very well (on Monday), it was a completely different team than what I saw on the court today,” Luckenbill said. “Half the girls came to play, half of the girls did not, and that’s something that the girls themselves are going to have to change.”

Central seemed unsure of itself for the majority of the game, not communicating very well in either set. That resulted in eight dropped balls, one of the main reasons for the lopsided scores.

Manteno took advantage of that hesitancy, jumping out to a 3-0 lead and keeping their foot on the gas. After Central regained their composure after an early timeout, the Comets kept the game close for a few more volleys, but Manteno pulled away on a 5-0 run, and then went on another 9-0 run to end the game.

The second set wasn’t much better for Central, though it got off to a much better start than it did in the first one. The Panthers swapped out about half of their roster for the second game, but kept the same momentum going regardless of who was on the court.

Manteno’s depth likely will be one of their strong suits this season. The team as a whole is loaded with juniors (eight total), but there’s a lot of experience on the roster despite only having four seniors. It helped that the bench was able to contribute significantly in the second game, and senior Madie Monk said she was impressed with how everyone performed in their first game.

“It was nice to see everybody not only getting playing time, but doing well when they were playing too, which is a great factor,” Monk said. “We’ve got great people all up and down the bench. There’s not a single person that can’t help the team.”

As a result, there’s some competition going on for roster spots. Manteno coach Cheryl Davis mentioned the team has a few different setters who will be competing for playing time, and both their left and right sides are strong, as well.

“We’ve got several setters that are doing a nice job running the offense. Couple of juniors and a senior that are kind of battling it out right now, rights and lefts, all the outsides are really fighting for that playing time,” Davis said. “It’s a great problem to have.”

But even though the Panthers have a number of different players who could be game-changers on any given night, there’s one in particular who stood out against Central.

Manteno had a dominant lead in the first set, but outside hitter Kaycie Wenzel ended the match decisively with a spike that seemed to rattle the glass in the gym’s windows, and that wasn’t the only spike of hers with that kind of intensity; she finished with three kills and led the team with seven digs. Though she played just the one set on Tuesday, sitting in the second to allow others more playing time, Davis says that she’s one of their top players.

“She brings some consistency. She does a really good job out on that left side swinging and defensively,” Davis said. “She’s probably the most consistent we have right now, she’s just really smart with her swing.”

It was a bad game from the beginning for Central, who looked like they were sleepwalking through parts of the match. Call it a bad day or fatigue that carried over from Monday — Luckenbill wasn’t sure what to make of it.

Manteno’s fast start and long runs might have contributed to the team’s loss; whenever the Panthers accumulated multiple points, the team struggled to find their footing to change the tide.

“That is one thing we struggle with once we’re down, we can’t catch up,” Luckenbill said. “That’s our number one goal to have these girls overcome.”

It’s a younger team for Luckenbill this season, and though some of the players were teammate on the JV squad last season, not many of them have played together. Finding that team chemistry will be key, and how the group responds to this kind of performance might be indicative of what the season looks like for Central.

“All I want to see is the girls to turn around and play like they did yesterday because I know they’re capable (of it),” Luckenbill said.

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Panthers slow down Comets – Kankakee Daily Journal

Blue Devils beat Comets – hngnews.com

Lodi (1-0, 0-0) posted a convincing 34-6 non-conference win over Delavan-Darien/Williams Bay (0-1, 0-0) Aug. 18 at Lodi. The Blue Devils first quarter scoring opportunities were stopped short by turnovers, but the Capitol North team exploded for three second quarter points to take a 21-0 lead into the halftime intermission.

The Lodi defense kept the Comet offense off the field and in check throughout the first two quarters.

Week one is always interesting because you are never quite sure how quickly the players will get things going in the first full competition of the year. We were extremely pleased with the way the offense moved the ball down the field on the opening drive. The effort that was put forth from the very beginning of the game was impressive. The team played well but only showed a small glimpse of what they are capable, said Lodi Head Coach David Puls. The team will have to make many improvements throughout the season if they want to reach their goals. We need to have a good week of practice and correct a lot of blocking in the run game and the execution and decision making in the pass game, on defense we need to work on tackling, coverage, shedding blocks, staying low, and controlling our gaps. We have a great group of seniors and we expect them to keep things going in the right direction.

Lodis first score came early in the second quarter as senior Jacob Heyroth ran the ball in from 3 yards out (Savannah Curtis kick) to give the Blue Devils a 7-0 advantage.

A short time later, Heyroth sprinted 62 yards on a punt return (Curtis kick) to raise the Lodi advantage to 14-0.

Cameron McDonald connected with Kade Crissinger on an eight yard scoring toss (Curtis kick) later in the quarter to give the Blue devils a commanding 21-0 lead entering the third quarter.

McDonald teamed up with Dominic Scola on a 35-yard catch and run (Curtis kick) in the third quarter for a 28-0 Lodi lead.

Ben Rashid scored the Blue Devils final touchdown on a one yard run (Curtis kick unsuccessful) to raise the Lodi advantage to 34-0.

Delavan-Darien/Williams Bay finally lit up their side of the score board with an 18-yard touchdown run by Dakota Williams (Jaime Flores extra point unsuccessful) to close the final score to 34-6.

The Blue Devil offense posted 203 yards rushing and added another 77 yards through the air.

Heyroth gained 124 yards rushing in 22 carries and sophomore Colton Nicolay chipped in 45 yards rushing on 11 carries to pace the Blue Devil rushing game.

Both backs ran hard and made some big plays. Cameron McDonald had a couple of pass miscalculations early on, but then responded admirably with a couple of completions on screen passes and a couple touchdowns, Puls said.

McDonald completed five of seven passes for 77 yards and two touchdown passes. He was picked off twice by the Comet defense. Scola (35 yards), Hyroth (18 yards), Nicolay (12 yards), Crissinger (eight yards) and Rashid (four yards) each caught a pas during the game.

Lodi held the Comets to just 34 yards rushing in 26 carries and 118 yards passing.

Owen Jelinek led the team with five tackles and Max Barreau recorded four tackles (two TFLs, one sack, one forced fumble). Will Richards recorded 2 TFLs with a fumble recovery. Austin Soehle also had a fumble recovery, multiple quarterback pressures, and a pass defended. Twenty different Lodi players recorded at least one tackle in the game.

Overall, it was a great night for Parents Night and any win is a good win, Puls said. We learned a lot about our team Friday and now we need to move forward. We play the at Wisconsin Dells High School [Aug. 24] at 7 p.m. The Dells got a win against Thorp this past Friday and they are a much improved team. They are riding high after their win and will be looking to keep things rolling when they play us on Thursday. We expect our team to be ready and compete at the highest level,

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Blue Devils beat Comets – hngnews.com

Comets, Redskins travel to Reading – The Hillsdale Daily News

Matthew Lounsberry mlounsberry@hillsdale.net mlounsberryHDN

READING Jonesville and Camden-Frontier traveled to Reading on Tuesday night for a three-team match between local Hillsdale County schools.

The Comets came out the big winners, going 2-0.

Today, I was really impressed with the energy level and the [communication] amongst the players in the match and before [each] serve, said Jonesville coach Sydney Barrett. Thats something we talked a lot about at practice yesterday.

Jonesville opened the night against Camden-Frontier, and had their hands full with the Redskins.

Camden-Frontier opened the match with a 6-4 lead before a 9-4 run gave the Comets a 13-10 advantage. Following a Redskin timeout, Camden-Frontier battled back to force ties at 16, 19, 20 and 21 before Jonesville scored the final four points to take Game 1.

Camden gets so many balls up in their defense. They hustle, they dont let anything hit the floor, theyre smart and they know where to place the ball, Barrett said.

Game 2 provided another back-and-forth affair. The Comets grabbed an early 6-4 lead, but the teams played to ties at 7, 10, 14, and 15.

Jonesville rallied to take a 20-17 lead and never gave it up, winning 25-20 to take the match.

I told them, Defense is going to win the match. Camden was going to take points from us, they earned a lot of points as well. We needed to be ready on defense, Barrett said.

Camden-Frontier showed that they could play with the older Comet squad, but couldnt put together enough runs to finish Jonesville off.

One thing weve been working on is our consistency. In our Jonesville game, we just had way more mistakes than they had, said Redskins coach Dawn Follis.

Jonesville was hitting in, and making us make the play. They werent all kills, but we were making our own mistakes.

After dispatching Camden-Frontier, the Comets turned their attention to Reading, grabbing an early 8-1 lead in Game 1. Jonesville extended their lead to 17-5, forcing the Rangers into a timeout.

When we went up against Jonesville, we looked like deer in headlights, said Reading coach Nicole Bailey.

The Rangers were unable to grab any momentum, and Jonesville took the opening game by a 25-9 final score.

In the encore, Reading seemed to settle in a bit, grabbing a 10-7 lead. However, the Comets scored five straight points to take a 12-10 lead, and would never give up the advantage, cruising to a 25-13 win.

Barrett credited her seven seniors with helping her team get off to a strong start so far this season.

The group of seniors are all really exceptional leaders. I could have them all be captains. I cant have seven captains, but I could. Thats a good problem to have, she said.

They give a ton of feedback, so if theres a new face on the floor, theyre so good about explaining things. Theyre just really great leaders, they want to win and theyre sharing that love with the underclassmen, so thats really exciting.

Middle hitter Samantha Dunn led Jonesville with 11 kills, converting over 75 percent of her attempts. Outside hitter Hanna Purdy added 10 kills and 18 digs.

Koryn Playford had a solid performance on the outside with eight kills and 11 digs. Libero Lauren Mains led the team in digs with 21, while setter Teya Nichols had a strong night with 20 assists.

We passed pretty well, so we could run our middle a lot. Sam and Amber [Gordon] both got a lot of attempts in the middle and a lot of kills. That opens up the outside, and that played a huge role tonight, Barrett said.

When we pass well, were going to be able to spread the offense out. Teya did a nice job of that tonight.

Camden-Frontier and Reading squared off in the final match of the day, with the Redskins sweeping the Rangers 2-0.

In the opener, the teams were deadlocked early, with ties at 7-7, 10-10 and 13-13. Camden-Frontier then seized momentum, finishing Game 1 with a 12-6 run for a 25-19 win.

I spent a lot of time in timeouts saying, Yes, youve made mistakes, but how are you going to overcome them? We all make mistakes, we have to find a way to overcome them, Bailey said.

Against Camden, I thought we played with a little more confidence.

The Redskins got off to a quicker start in Game 2, seizing a 6-1 advantage early. Reading was unable to recover, as the lead grew to 11-6, 15-9 and 21-13 before Camden-Frontier put the Rangers away, 25-15.

Were still working on maturity in a lot of our positions. I know the end of the season will be very different than the beginning of the season, Follis said. They have the talent, its just that consistency to to it repetitively.

Camden-Frontier was led by sophomore Jordan Stump, who finished with 15 assists and 15 kills. Fellow sophomores Frances Churchwell (25 digs) and Alicia Fackler (18 assists) also made big contributions.

Even though they are sophomores, they do bring a lot of experience, Follis said. A lot of my team plays club all the time. Yes, theyre young, but I expect a lot out of them because theyve played together for so long.

The Redskins also got a boost from Hillsdale Academy transfer Brooklyn Gravel, who finished with 13 kills and six aces.

Shes got height, shes got hitting and she can pass. So, shes got the skills all the way around, Follis said. Shes had a ball in her hand for [most] of her life. When you have a 510 person walk in whos had a ball in her hand their whole life, you let them in.

She has played with a core of my team [in club volleyball] almost more than some of my regular players have played with them. With her walking on to my court, theres four of them who have played together a lot.

Rounding out the statistical impact for Camden-Frontier was Maddie Vondron with 9 kills, Marin Page with five kills and Layne Cooney with five kills.

In past years, we usually just had a couple [hitters], Follis said, noting that the Redskins were able to spread the offense around. This year I have five hitters that we can go to. Weve got more height than weve had in the past.

All three schools will travel to Jonesville on Saturday for the annual Sue Carlile tournament.

Read more:

Comets, Redskins travel to Reading – The Hillsdale Daily News

Comets JV gridders take good first step – YourGV.com

There was a lot of excitement among the coaches and players on the Halifax County High School junior varsity football team Friday night.

The Comets won their first pre-season test, outscoring Buckingham County High School four touchdowns to none in a scrimmage Friday afternoon at Tuck Dillard Memorial Stadium.

Overall Im pleased, but Im not going to get too far in front of myself, said Comets Head Coach Eugene Turbeville.

I dont want the kids to get too excited early because this was just the first time we matched up against somebody else. It was a good first step, but weve got a lot to work on.

The Comets got a touchdown on a 63-yard run by Shabazz Buster on the fifth play of their first possession of the scrimmage that had both teams run three 10-play series. William Davis snared a pass and raced to a touchdown on a 60-yard scoring play on the ninth play of the Comets second offensive series.

Zyliek Perkins intercepted a Buckingham County High School pass and returned it for a touchdown during the Knights final offensive series and Mekhjay Boyd added the Comets final score on a 67-yard run on the eighth play of the final series.

Along with Davis, Zyliek Perkins and Lamandre Adams also had catches.

At times we ran the ball pretty well, Turbeville pointed out.

We long two long runs from two different running backs. Both quarterbacks (Cody White and Zion Wilson) showed some promise. We werent sure which direction we were going to go. We have been working with both of them. Even though it is a run-based offense, our receivers caught the ball well and ran some decent routes.

As far as the defense went, Turbeville was pretty pleased with his teams effort.

We went primarily with a base defense, Turbeville noted.

We didnt try to do anything too fancy. We didnt want the kids to have to think about too much stuff the first time going out. We ran to the ball pretty well. We tackled pretty well.

We still have other things we need to put in, he added.

The Comets coach said one area the team still needs work is conditioning.

Weve got to get into a little better shape, Turbeville said.

The weather is hot this time of the year, and the kids werent quite ready for that.

The Comets jayvees will get the final pre-season test Thursday in a scrimmage against Colonial Heights High School. The scrimmage is scheduled for 5 p.m. at Halifax County High School.

See the article here:

Comets JV gridders take good first step – YourGV.com

Comet – Wikipedia

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

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

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun’s light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4] The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets.

As of November 2014[update] there are 5,253 known comets,[5] a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.[6][7] Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular.[8] Particularly bright examples are called “Great Comets”. Comets have been visited by unmanned probes such as the European Space Agency’s Rosetta, which became the first ever to land a robotic spacecraft on a comet,[9] and NASA’s Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

The word comet derives from the Old English cometa from the Latin comta or comts. That, in turn, is a latinisation of the Greek (“wearing long hair”), and the Oxford English Dictionary notes that the term () already meant “long-haired star, comet” in Greek. was derived from (“to wear the hair long”), which was itself derived from (“the hair of the head”) and was used to mean “the tail of a comet”.[10][11]

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

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia.[13] As such, they are popularly described as “dirty snowballs” after Fred Whipple’s model.[14] However, some comets may have a higher dust content, leading them to be called “icy dirtballs”.[15] Research conducted in 2014 suggests that comets are like “deep fried ice cream”, in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.[16]

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.[17][18] In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA’s Stardust mission.[19] In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.[20][21]

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley’s Comet reflects about four percent of the light that falls on it,[22] and Deep Space 1 discovered that Comet Borrelly’s surface reflects less than 3.0% of the light that falls on it;[22] by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.[23]

Comet nuclei with radii of up to 30 kilometres (19mi) have been observed,[24] but ascertaining their exact size is difficult.[25] The nucleus of 322P/SOHO is probably only 100200 metres (330660ft) in diameter.[26] A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330ft) across.[27] Known comets have been estimated to have an average density of 0.6g/cm3 (0.35oz/cuin).[28] Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.[29]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing,[30] including 14827 Hypnos and 3552 Don Quixote.

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/ChuryumovGerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals.[31][32] Further, the ALICE spectrograph on Rosetta determined that electrons (within 1km (0.62mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.[33][34] Instruments on the Philae lander found at least sixteen organic compounds at the comet’s surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.[35][36][37]

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the “coma”. The force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous “tail” to form pointing away from the Sun.[46]

The coma is generally made of H2O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000km; 280,000,000 to 370,000,000mi) of the Sun.[47] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry.[47] Larger dust particles are left along the comet’s orbital path whereas smaller particles are pushed away from the Sun into the comet’s tail by light pressure.[48]

Although the solid nucleus of comets is generally less than 60 kilometres (37mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun.[49] For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[50] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[51] Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000km; 140,000,000mi) from the Sun.[51] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail.[51] Ion tails have been observed to extend one astronomical unit (150 million km) or more.[50]

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects Sunlight directly while the gases glow from ionisation.[52] Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.[53] Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.[54]

In 1996, comets were found to emit X-rays.[55] This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, “stealing” one or more electrons from the atom in a process called “charge exchange”. This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.[56]

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope[57][58] but these detections have been questioned.[59][60] As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them.

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the type II or dust tail.[52] At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory.[61] On occasions – such as when the Earth passes through a comet’s orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.[62]

The observation of antitails contributed significantly to the discovery of solar wind.[63] The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an “induced magnetosphere” around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called “pick-up ions”) congregate and act to “load” the solar magnetic field with plasma, such that the field lines “drape” around the comet forming the ion tail.[64]

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a “tail disconnection event”.[64] This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke’s Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.[65]

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions.”[66][67]

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet’s nucleus, like a geyser.[68] These streams of gas and dust can cause the nucleus to spin, and even split apart.[68] In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.[69] Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.[70]

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder.[71] Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years.[72] They usually orbit more-or-less in the ecliptic plane in the same direction as the planets.[73] Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley’s Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet’s orbit are called its “family”.[74] Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.[75]

At the shorter orbital period extreme, Encke’s Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called Jupiter-family comets (JFCs).[76][77] Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs).[78][79] As of 2017[update], only 87 HTCs have been observed, compared with 567 identified JFCs.[80]

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.[81]

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations.[82] Short-period comets have a tendency for their aphelia to coincide with a giant planet’s semi-major axis, with the JFCs being the largest group.[77] It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.[83][84]

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc[85] a disk of objects in the trans-Neptunian regionwhereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence).[86] 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.[87]

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years.[88] An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.[89] 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 further from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having “periods”. The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years.

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories[88] 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.[90] The Sun’s Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)).[91] Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion[92] that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System.

No comets with an eccentricity significantly greater than one have been observed,[92] so there are no confirmed observations of comets that are likely to have originated outside the Solar System. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet.[93] 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),[94] whereas others use it to mean exclusively short-period comets.[88] 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.[95]

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000AU (0.03 and 0.08ly)[97] to as far as 50,000AU (0.79ly)[78] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000AU (1.58 and 3.16ly).[97] 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).[98] The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune.[78] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[99] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[99][100][101] 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.[102]

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way.[103] The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987.[104][105] A total of 10 such exocomet systems have been identified as of 2013[update], using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[103][104]

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind.[106] If the Earth’s orbit sends it though that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet SwiftTuttle.[107]Halley’s Comet is the source of the Orionid shower in October.[107]

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth’s oceans, or at least a significant portion of it. Others have cast doubt on this idea.[108] The detection of organic molecules, including polycyclic aromatic hydrocarbons,[16] in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of lifeor even life itselfto Earth.[109] 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.[110] 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.[111]

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth’s Moon, some of which may have survived as lunar ice.[112] Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.[113]

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.[114] An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.[115]

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages.[30] Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[116] Some asteroids in elliptical orbits are now identified as extinct comets.[117] Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.[30]

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[118] 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.[119][120] Other splitting comets include 3D/Biela in 1846 and 73P/SchwassmannWachmann from 1995 to 2006.[121] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372373 BC.[122] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[123]

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.[124]

Some comets have been observed to break up during their perihelion passage, including great comets West and IkeyaSeki. Biela’s Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela’s Comet.[125]

Some comets meet a more spectacular end either falling into the Sun[126] 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.[127]

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.[129] Similarly, the second and third known periodic comets, Encke’s Comet[130] and Biela’s Comet,[131] 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.[132]

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.[132]

From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.[133] 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.[134][135]

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days.[136]Pliny the Elder believed that comets were connected with political unrest and death.[137]

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.[138]

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth’s atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.[139][140]

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.[141]

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.[142] 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.[143][144] 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.[145]

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

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.[147]

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.[148] 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.[149]

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[150] 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.[151]

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.[152] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[153] 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.”[153] 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, H 2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).[154][155]

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.[122] 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.[164] Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.[165]Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.[166]

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.[167]

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres.[168] 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.[169]

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.[170] 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.[171]

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.[172]2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed.[173] Similarly, Comet ShoemakerLevy 2 was originally designated asteroid 1990 UL3.[174](See also Fate of comets, above)

Centaurs typically behave with characteristics of both asteroids and comets.[175] 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,[176] 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.[177]

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.[178] SOHO’s 2000th comet was discovered by Polish amateur astronomer Micha Kusiak on 26 December 2010[179] and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a “new” comet is discovered, and calculation of its orbit shows it to be an old “lost” comet. An example is Comet 11P/TempelSwiftLINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001.[180] There are at least 18 comets that fit this category.[181]

NASA is developing a comet harpoon for returning samples to Earth.

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.[183] 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)[183] and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song “Halley Came to Jackson”.[183]

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley’s Comet in 1910, the Earth passed through the comet’s tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions,[184] whereas the appearance of Comet HaleBopp in 1997 triggered the mass suicide of the Heaven’s Gate cult.[185]

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).[183] 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.[186]

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Comet – Wikipedia

Comet Goldfish, Sarasa Comet Goldfish Information, Care …

Black Moor Goldfish Bubble Eye Goldfish Celestial Eye Goldfish Comet Goldfish Common Goldfish Fantail Goldfish Lionhead Goldfish Oranda Goldfish Pearlscale Goldfish Ranchu Goldfish Redcap Oranda Goldfish Ryukin Goldfish Shubunkin Goldfish Telescope Goldfish Veiltail Goldfish Comet Goldfish look just like regular goldfish but with a much longer andmore deeplyforked tail fin!

Contents:

The Comet Goldfish is also called the Comet-tail Goldfish or Pond Comet. This fish was the first variety of single-tail goldfish to be developed with a long caudal (tail) fin. It was developed in the United States from the Common Goldfish in the early 19th century, presumably by Hugo Mullert of Philadelphia, who then introduced them in quantity into the market.

Being a further development of the Common Goldfish, the Comet is sometimes confused for its close relative. The Comet Goldfish and Common Goldfishhave an almost identical body shape. However, the fins on the Comet Goldfish are much longer, especially the caudal (tail) fin. Its caudal fin is also more deeply forked. On both these fish,the caudal fin is held fully erect.

The adult size of the Comet Goldfish is also smaller than the Common Goldfish. Yet they are every bit as durable and can be kept in either an aquarium or outdoor pond. Both fish are inexpensive and readily available.

The Comet is generally more reddish orange in color while the Common Goldfish is more orange. While the Comet Goldfish is typically areddish orange, this fish isalso available in yellow, orange, white, and red. They can also be found as a bi-color red/white combination, and occasionally they are available with nacreous (pearly) scales, giving them a variegated color.

Other types of Comet include the Sarasa Comet. This variety has long flowing fins and is characterized by a red-and-white coloration that holds a resemblance to a koi color pattern called ‘Kohaku.’Additionally, the Tancho Single-tail Comet is a silvervariety with a red patch on its head.

One of the hardiest of the goldfish varieties, Comet Goldfish are recommended for beginners. They are an easy fish to keep as they are not picky and will readily eat what is offered.

These fish can be quite personable and are delightful to watch. They are some of the most graceful of the elongated goldfish, and this quality isemphasized by their long tails. They are active, rapid swimmers and tend to leap out of the water occasionally, so having a lid on an aquarium is good idea. They are also very social and thrive well in a community.

Along with the other elongated goldfish, such as the Common Goldfish and the Shubunkin Goldfish, the Comet varieties make good pond fish. They are hardy and can tolerate cold water temperatures. They are moderate in size but are active and fast, so will get along well with Koi. Also, Comets usually won’t uproot plants, but they will readily spawn. Care should be taken, so they don’t quickly overpopulate your pond.

Comet Goldfish – Quick Aquarium Care

Habitat: Distribution / Background

The goldfish of today are descendants of a species of wild carp known as the Prussian Carp, Silver Prussian carp, or Gibel Carp Carassius gibelio (syn: Carassius auratus gibelio), which was described by Bloch in 1782. These wild carp originated in Asia; Central Asia (Siberia). They inhabit the slow moving and stagnant waters of rivers, lakes, ponds, and ditches feeding on plants, detritus, small crustaceans, and insects.

For many years, it was believed that goldfish had originated from the Crucian Carp Carassius carassius described by Linnaeus in 1758. This fish has a wide range across the waters of the European continent, running west to east from England to Russia, north to Scandinavian countries in the Arctic Circle and as far south as the central France and the Black Sea. However, recent genetic research points to C. gibelio as a more likely ancestor.

Goldfish were originally developed in China. The first goldfish werenormally a silver or gray color, but early in the Jin Dynasty, somewhere between the years 265 – 420, breeders noted a natural genetic mutation thatproduceda yellowish orange color. It became common practice to breed this pretty golden fish for ornamental garden ponds.

By the 1500’s goldfish were traded to Japan, to Europe in the 1600’s, and to America by the 1800’s. The majority of the fancy goldfish were being developed by Asianbreeders. The results of this centuries-long endeavor is the wonderful goldfish colors and forms we see today. Domesticated goldfish are now distributed world-wide.

The Comet Goldfish was the first variety of the single-tail goldfish to be developed with a long caudal (tail) fin. It was developed in the United States from the Common Goldfish in the early 19th century, presumably by Hugo Mullert of Philadelphia, who then introduced them in quantity into the market. The Comet Goldfishis one of more than 125 captive-bred varieties of goldfish that have been developed.

Description

The Comet Goldfish is an elongated, flat-bodied variety of goldfish. The head is wide but short, and its body tapers smoothlyfrom its back and belly to the base of its caudal fin (tail fin). The caudal fin is long anddeeply forked and generally stands fully erect. Comets have a natural life span of up to 14 years, though possibly longer if kept in optimal conditions.

The Comet Goldfish is a bit smaller than the Common Goldfish, but even so, the environment it is kept in will mostly determinewhether your pet grows to its full potential size. In an average 15 gallon tank, if well cared for and not crowded, they can grow up to about 4 inches (10 cm), while in a larger, uncrowded tank, they can grow larger, generally reaching about 7 or 8 inches (17.78 – 20.32 cm). If kept in a spacious pond, they can reach over 12 inches (30+ cm).

They are primarily a reddish orange color, but they are also available in yellow, orange, white, and red. Some Comet Goldfish come in abi-color red/white combination, and occasionally they are available with nacreous (pearly) scales, giving them a variegated color.

Comet Goldfish can and do naturally change color, but color changes are believed to be influenced by diet and the amount of light. Aquarists often report the reds and oranges of their goldfish changing to white. A fresh dietalong with good lighting and available shadeare suggested as the best ways to maintain the original coloration. Even so, these measuresare not always successful.

Other types of Comet Goldfish include the Sarasa Comet. This variety has long flowing fins and is characterized by a red-and-white coloration that holds a resemblance to a koi color pattern called ‘Kohaku.’The Tancho Single-tail Comet is a silver variety with a red patch on its head.

Fish Keeping Difficulty

Comet Goldfish are some of the hardier species of goldfish. They are very undemanding of water quality and temperature. They can do well in a goldfish aquariumor even a pond as long as the environment is safe and their tankmates are not competitive.

Many people will keep goldfish in an aquarium with no heater or filtration, but for the best success, provide them the same filtration, especially biological filtration, that other aquarium residents enjoy.

Foods and Feeding

Since they are omnivorous, the Comet Goldfish will generally eat all kinds of fresh, frozen, and flake foods. To keep a good balance, give them a high quality flake food every day. To care for your goldfish, feed brine shrimp (either live or frozen), blood worms, Daphnia, or tubifex worms as a treat. It is usually better to feed freeze-dried foods as opposed to live foods to avoid parasites and bacterial infections that could be present in live foods.

Aquarium Care

These goldfish are hardy and easy to keep in a well maintained tank. Minimum tank size is 15 gallons, so make sure water changes are frequent in such as small tank. Regular weekly water changes of 1/4 to 1/3 is strongly recommended to keep these fish healthy. Snails can be added as they reduce the algae in the tank, helping to keep it clean.

Aquarium Setup

Setting up a goldfish aquarium in a manner that will keep your fish happy and healthy is the first step to success. The shape and size of the aquarium is important and depends upon the number of goldfish you are going to keep. These fish need a lot of oxygen and produce a lot of waste. Good filtration, especially biological filtration, is very helpful in maintaining the water quality of the aquarium. A filtration system will remove much of the detritus, excess foods, and waste, which keeps the tank clean and maintains the general health of the goldfish.

Goldfish are a cold water fish and will do best at temperatures between 65 – 72 F (18- 22 C). The Comet Goldfish are one of the most hardy varieties and can tolerate temperatures a few degrees above freezing, as long as the cooling drops only a few degrees a day. A quick temperature drop can kill them, so if you live in a very cold climate,a heater is advisable.

Provide a gravel substrate to help create a natural and comfortable environment for your fish. You can add some decor, but make sure that all ornamentation is smooth with no protruding points or sharp edges. Smooth rocks or driftwood should be used sparingly if at all. Aquarium plants would be the best choice of aquarium decor for goldfish, but unfortunately these fish are diggers. Consequently live plants may be uprooted. Artificial plants make a good substitute and silk plants are safer than plastic ones.

Most aquariums come with a cover that includes lighting. A cover for the tank is desirable as it reduces evaporation and though they are not prone to jumping, on occasion some gold fish will jump out. Lighting is not essential for goldfish, but does make the aquarium a nice showpiece and lighting will help if you have live plants.

Social Behaviors

Goldfish are very social animals and thrive in a community. Not only are they a great community fish, but they are great scavengers as well. It is really not necessary to add other scavengers or other bottom feeders to the aquarium when you have goldfish.

Most fancy goldfish will thrive in both freshwater and tropical aquariums as long as there are no aggressive or territorial fish in the tank. Some good tankmates for fancy goldfish are the Chinese Blue Bitterling and the Northern Redbelly Dace. Comet Goldfish can be kept with other varieties of elongated goldfish, such as the Common Goldfish and the Shubunkin, and they also do fine with Koi.

Sex: Sexual differences

During the breeding season, the male has white prickles, called breeding tubercles, on its gill covers and head. Seen from above, a female will have a fatter appearance when she is carrying eggs. It is impossible to sex Goldfish when they are young and not in breeding season, but generally the male is smaller and more slender than the female.

Breeding / Reproduction

Comet Goldfish are egg layers that spawn readily in the right conditions. They can be bred in groups as small as five individuals, but they are very social animals and likely to breed in larger groups as well. The only time Goldfish will spawn in the wild is when spring arrives. To spawn them in the aquarium, you will need to mimic the conditions found in nature.

Provide an aquarium that is at least 20 gallons and make sure the fish are healthy and disease free. Some breeders suggest you treat them for parasites. Many breeders will also separate the males and females for a few weeks prior to breeding to help increase their interest in spawning. Introduce the fish into the breeding tank at the same time. The tank will need a lush environment with solid surfaces for the spawning process and for the eggs to adhere to. Bushy, oxygenating plants, such as Anacharis, work well for this, though artificial plants or fibrous spawning mops can also be used.

To induce spawning, the temperature can be slowly dropped to around 60 F (11 C) and then slowly warmed at a rate of 3 F (2 C) per day until they spawn. Spawning generally begins when the temperatures are between 68 and 74 F (20-23 C). Feeding lots of high protein food such live brine shrimp and worms during this time will also induce spawning. Feed small amounts three times a day, but don’t overfeed. Uneaten scraps will sink to the bottom and foul the water. Maintain the breeding tank with partial water changes of up to about 20% per day.

Before spawning, as the temperature increases, the male will chase the female around the aquarium in a non-aggressive way. This can go on for several days, and the fish will intensify in color. During the spawn, the fish will gyrate from side to side, and the male will push the female against the plants. This stimulates the female to drop tiny eggs which the male will then fertilize. The eggs will adhere by sticky threads to the plants or spawn mop. Spawning can last two or three hours and can produce up to 10,000 eggs.

At this point, the parents will start to eat as many eggs as they can find. For this reason, it is best to remove the parents after spawning is complete. The fertilized eggs will hatch in 4 to 7 days, depending on the temperature. You can feed the newly hatched goldfish specialty fry foods until they become big enough to eat flake or brine shrimp, or you can offer the same food as you feed the parents as long as it is crushed very small. At first, the fry are a dark brown or black color in order to better hide and not be eaten by larger fish. They gain their adult color after several months and can be put in with larger fish once they reach about 1 inch long. See Breeding Freshwater Fish – Goldfish for more information on breeding Goldfish.

Fish Diseases

In properly maintained goldfish aquariums or ponds, goldfish illness is largely preventable. Even so, goldfish illnesses can occur, and if left untreated, may prove fatal. Goldfish are hardy, though, and if treated in a timely manner, most will make a full recovery.

When treating individuals, it is usually best to move the afflicted fish into a separate tank with no gravel or plants and do regular partial water changes. However, if the disease is apparent throughout the main tank, it may be best to do the treatments there. Whether treating in a hospital tank or your main tank, read and follow the manufacturer’s instructions for any medication. Some medications can adversely affect the water quality by destroying beneficial bacteria. You may also need to remove the carbon from the filtration system, as carbon will absorb many medications, making the treatment ineffective.

Goldfish diseases are mostly the same as those that afflict other freshwater fish, and the symptoms and treatment of goldfish are also similar. The main types of fish diseases include bacterial infections, fungal infections, parasites, and protozoa. There are also other ailments caused by injury, poor nutrition, or bad water conditions.

One of the more common problems is Ich, which is a protozoan disease. Ich is easy to identify because your fish looks like it is sprinkled with salt. Though Ich is easily treated, like other protozoan diseases, it can be fatal if not caught quickly. Some other protozoan diseases are Costia, which causes a cloudiness of the skin, and Chilodonella, which will cause a blue-white cloudiness on the skin.

External parasites are fairly common, too, but pretty easy to treat and usually not fatal when treated. These include flukes, which are flatworms about 1 mm long with hooks around their mouths. They infest the gills or body of the fish. Another type of parasite is fish lice (Argulus), flattened, mite-like crustaceans about 5 mm long that attach themselves to the body of the goldfish. Lastly, anchor worms look like threads coming out of the fish.

Some bacterial infections include Dropsy, an infection in the kidneys that can be fatal if not treated quickly. Fish Tuberculosis is indicated by the fish becoming emaciated (having a hollow belly). For this illness, there is no absolute treatment, and it can be fatal. Tail/Fin Rot may also be bacterial, though the reduced tail or fins can be caused by a number of factors as well. There is also fungus, a fungal infection, and Black Spot or Black Ich, which is a parasitic infection.

Swim Bladder Disease is an ailment indicated by fish swimming in abnormal patterns and having difficulty maintaining their balance. This can be caused by a number of things: constipation, poor nutrition, a physical deformity, or a parasitic infection. Feeding frozen peas (defrosted) has been noted to help alleviate the symptoms and correct the problem in some cases.

Other miscellaneous ailments include Cloudy Eye, which can be caused by a variety of things ranging from poor nutrition, bad water quality, and rough handling. It can also be the result of other illnesses, such as bacterial infections. Constipation is indicated by a loss of appetite and swelling of the body, and the cause is almost always diet. Then there are wounds and ulcers. Wounds can become infected, creating ulcers. Wounds can develop either bacterial or fungal infections, or both, and must be treated. There are treatments for each of these diseases individually and treatments that handle both. For more in-depth information about goldfish diseases and illnesses, see Goldfish Care; Fancy Goldfish and Goldfish Diseases.

Availability

The Comet Goldfish is inexpensive andreadily available in fish stores and online.

References

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Comet Goldfish, Sarasa Comet Goldfish Information, Care …


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