Comets are small astronomical bodies, typically a few kilometers across, that orbit the Sun. They contain icy chunks and frozen gases with bits of embedded rock and dust, and possibly a rocky core, aptly described as a "dirty snowball." Comets are leftovers from the formation of the Solar System and are believed to exist in vast numbers in the Oort Cloud and, to a lesser extent, in the Kuiper Belt. From these regions they can be perturbed by the gravitational influence of passing stars or interstellar clouds and thrown into new, highly elliptical orbits that bring them into the inner Solar System. As a comet draws nearer to the Sun, solar radiation causes the comet's frozen gases to sublime (turn directly from a solid into a gas) and be released, so that, in addition to the frozen nucleus, several new features develop. These include a coma, a luminous cloud of water vapor, carbon dioxide, and other neutral gases driven off the nucleus; a hydrogen cloud, a huge (millions of kilometers in diameter), tenuous envelope of neutral hydrogen; a dust tail (the most prominent part of a comet to the unaided eye) up to 10 million km long, composed of smoke-sized dust particles driven off the nucleus by escaping gases; and an ion tail, as much as several hundred million km long, composed of plasma and laced with rays and streamers caused by interactions with the solar wind.

(For further details see Comet »)