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Main article: Oort cloud The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)[85] to as far as 50,000 AU (0.79 ly)[66] from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).[85] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.[66] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.[86] Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;[86][87][88] it is seen as a possible source of new comets to 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.[89] Exocomets Main article: Exocomet Exocomets beyond our Solar System have also been detected and may be common in the Milky Way Galaxy.[90] The first exocomet system detected was around Beta Pictoris, a very young type A V star, in 1987.[91][92] A total of 10 such exocomet systems have been identified as of 2013, using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.[90][91] Effects of comets Connection to meteor showers Diagram of Perseids meteors 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.[93] If the comet's path crosses the path the Earth follows in orbit around the Sun, then at that point there are likely to be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower, for example, occurs every year between August 9 and August 13, when Earth passes through the orbit of Comet Swift–Tuttle.[94] Halley's comet is the source of the Orionid shower in October.[94] Comets and impact on life Many comets and asteroids collided into Earth in its early stages. Many scientists believe 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. Other researchers have cast doubt on this theory.[95] The detection of organic molecules in significant quantities in comets has led some to speculate that comets or meteorites may have brought the precursors of life—or even life itself—to Earth.[96] 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.[97] 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.[98] Comet and meteoroid impacts are also believed responsible for the existence of tektites and australites.[99] Fate of comets Departure (ejection) from Solar System If a comet is traveling fast enough, it may leave the Solar System; such is the case for hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter.[100] Volatiles exhausted Main article: Extinct comet 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.[33] Eventually most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid.[101] Some asteroids in elliptical orbits are now identified as extinct comets.[102] Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer emit gas.[33] Breakup Breaking up of 73P/Schwassmann–Wachmann in 1995. This animation covers three days. Disintegration of asteroid P/2013 R3 observed by the Hubble Space Telescope (6 March 2014).[103] The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.[104] A significant cometary disruption was that of Comet Shoemaker–Levy 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 atmosphere—the first time astronomers had observed a collision between two objects in the Solar System.[105][106] Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006.[107] Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC.[108] Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.[109] 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.[110] Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. 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 lesser meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.[111] Collisions Brown spots mark impact sites of Comet Shoemaker–Levy on Jupiter Some comets meet a more spectacular end – either falling into the Sun[112] 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 Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.[113] Nomenclature Main article: Naming of comets Halley's Comet, named after the astronomer Edmund Halley for successfully calculating its orbit. 1910 photo. 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, that comet became known as Halley's Comet.[114] Similarly, the second and third known periodic comets, Encke's Comet[115] and Biela's Comet,[116] 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 apparition.[117] 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.[117] History of study Main article: Observational history of comets Early observations and thought Halley's Comet appeared at the Battle of Hastings in 1066 (Bayeux Tapestry). From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.[118] 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.[119][120] 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.[121] Pliny the Elder believed that comets were connected with political unrest and death.[122] 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.[123][124] Orbital studies The orbit of the comet of 1680, fitted to a parabola, as shown in Isaac Newton's Principia Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of his inverse square law of universal gravitation 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.[125] In 1705, Edmond Halley (1656–1742) 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 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 1758–9.[126] 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.[127] 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.[128] Studies of physical characteristics "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." James Thomson The Seasons (1730; 1748)[129] 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.[130] 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.[131] 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.[132] In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.[133] 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.[134] Spacecraft missions See also: List of comets visited by spacecraft View from the impactor in its last moments before hitting the comet in the Deep Impact mission File:NASA Developing Comet Harpoon for Sample Return.ogv NASA is developing a comet harpoon for returning samples to Earth. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between 26 to 71 °C (79 to 160 °F), and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly's.[135] In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1.[136] Renamed EPOXI, it made a flyby of Comet Hartley 2 on November 4, 2010. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire," at extremely high temperatures of over 1,000 °C (1,830 °F).[137][138] Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk,[139] so comets also contain crystalline grains that formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials".[140] These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.[141] The Rosetta probe is presently en route to Comet Churyumov–Gerasimenko; in 2014 it will go into orbit around the comet and place a small lander on its surface.[142] |
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