| 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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