Comets

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definition A small body of gas and dust which revolves around the Sun in a usually very elliptical or even parabolic orbit. It is seen to be composed of a head and often with a spectacular gaseous tail extending a great distance from the head. The rocky-icy head is called the comet nucleus. As the comet nears the Sun, the increased temperature causes the ice in the nucleus to sublimate and form a gaseous halo around the nucleus, called the coma. Comets often possess two tails, a dust tail that lies in the orbit behind the comet generated by surface activity, and a brighter, ionized gas tail, that points away from the Sun, driven by solar wind. Cometary reservoirs are thought to represent primordial solar system material. A comet with a dust coating on its surface that inhibits gas production might be classified as an asteroid. Because of this ambiguity, objects such as Chiron, Centaur asteroid, have been reclassified as comets. Comets are primarily composed of amorphous water ice, but also contain carbon dioxide, carbon monoxide, formaldehyde, methanol, methane at a few percent level (with respect to water), and many other molecules at a lower level.
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109P/Swift-Tuttle
P/Shoemaker-Levy 9
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