Small, icy body orbiting the Sun, usually on a highly elliptical path that takes it beyond the dwarf planet Pluto. A comet consists of a central nucleus a few kilometres across and is made mostly of ice mixed with gas and dust. As a comet approaches the Sun its nucleus heats up, releasing gas and dust, which form a coma (comet head) up to 100,000 km/60,000 mi wide, around the nucleus. Gas and dust stream away from the coma to form one or more tails, which may extend for millions of kilometres. Some comets, such as Halley's Comet, stay within Pluto's orbit for most of the time.
Comets are of many different types, characterized by their orbits, their composition (the ratio of ice to dust, and the amount of frozen volatiles other than water ice, such as methane and carbon monoxide), and their size. Most comets approach the Sun on a hyperbolic orbit and are seen only once; others (the periodic comets) return regularly in elliptical orbits. Famous examples of the periodic comets are Halley's Comet, which has a period of 76 years and is one of the largest comets known, with a nucleus about 15 × 7 × 7 km/9 × 4 × 4 mi across, and comet Encke, which has one of the shortest periods, at only 3.3 years.
Current thinking is that the nonperiodic comets, and those with very long periods, mostly originate in the Oort cloud, which lies far beyond the orbit of Pluto and which may contain billions of protocomets, only a few of which are gravitationally perturbed by passing stars into the inner Solar System each decade. The orbits of the periodic comets suggest a different source; this was previously thought to be the Kuiper belt, a zone extending from just beyond the orbit of Neptune to about 1.6 times the distance of Neptune. However, it is now thought that periodic comets are more likely to originate from the scattered disc, consisting of similar objects with much more elliptical orbits that extend from just beyond Neptune to several times that distance. Occasionally these objects interact with each other or are perturbed by Neptune, and some fall into the inner solar system as long-period comets.
Over 4,000 comets are known, although it is estimated that there could be a trillion orbiting in the Oort Cloud and the scattered disc, dormant and invisible. Over half of the known comets have been discovered by amateurs examining images from the SOHO spacecraft, launched in December 1995.
Names, visibility, and disintegration Most comets are first named after their discoverers and denoted by letters in order of discovery each year, but are subsequently numbered in order of their perihelion passage, that is the point at which they are closest to the Sun (for example, the Arend–Roland comet was 1956h but became 1957 III). Some of the orbits of the long-period comets so closely resemble each other that there is little doubt that they are traced out by fragments of a larger comet that broke into pieces as it passed around the Sun. One of the best known of these groups gave rise to the majority of the bright Sun-grazing comets; it includes the Great Comet of 1668, 1843 I, 1880 I, 1882 II, 1887 I, 1945 VII, 1963 V, 1965 VIII, 1970 VI, and possibly others. These comets passed through the solar corona, and near perihelion were bright enough to be observed in daylight. Few of the short-period comets show conspicuous tails, presumably because they have lost most of their volatile material. Some disintegrate and are not seen again, though some of the fragments may cause periodic meteor showers.
The famous periodic Halley's Comet was studied during its appearance in 1986 by the European spacecraft Giotto, two Soviet Vega probes, and two Japanese probes. In 1997 NASA launched rockets to study the brightest periodic comet, Hale-Bopp, as it flew within 196 million km/122 million mi of the Earth. Its icy core is estimated to be 40 km/25 mi wide. It was discovered by astronomers Alan Hale of Cloudcroft, New Mexico, and Thomas Bopp of Glendale, Arizona, in July 1995 when it was 1,050 million km/650 million mi away.
Recent research In 2004 NASA's Stardust mission, controlled from the Jet Propulsion Laboratory in Pasadena, California, collected samples of comet dust for the first time. The Stardust probe passed within 240 km/150 mi of the nucleus of comet Wild 2 and collected samples of comet dust using a lightweight glass foam mesh. The probe also obtained the highest-resolution pictures ever taken of a comet.
In July 2005 scientists successfully launched a probe into the core of comet Tempel 1. The 372 kg/820 lb probe launched by the NASA Deep Impact mission spacecraft recorded high-resolution images of the impact. The 37,000 kmph/23,000 mph collision took place 133 million km/83 million mi from Earth. Comets are thought to be unchanged since the formation of the Solar System and obtaining information from a comet's interior should provide new insight on the original composition of the Solar System.
The European Space Agency spacecraft Rosetta, launched in March 2004, is scheduled to go into orbit around comet 67P/Churyumov-Gerasimenko in May 2014, and send a lander to the comet's surface in November 2014. The mission plans to study the comet until December 2014.
Comets and the Kuiper Belt
Asteroids and Comets
Comet Shoemaker-Levy Collision with Jupiter
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