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Made of dust, ice, carbon dioxide, ammonia and methane, comets resemble dirty snowballs. You may remember them as blurry smudges in the sky. Comets orbit the Sun, but most are believed to inhabit in an area known as the Oort Cloud, far beyond the orbit of Pluto. Occasionally a comet streaks through the inner solar system (see Comet Wild 2); some do so regularly, some only once every few centuries.
Heads and tails
Meteor showersWhen Earth crosses the path of a comet, even if the comet hasn't been around for a few years, leftover dust and ice can create increased numbers of meteors. Comet Temple-Tuttle is a good example of this. It passed by in early 1998, created a dazzling meteor shower in November. ez Explore NASA's Stardust Mission, set to intercept a comet and bring dust particles back to Earth. Photographs:
NASA/NSSDC |
As
a comet nears the Sun, its icy core boils off, forming a cloud of
dust and gas called a head, or coma. Comets become visible when sunlight
reflects off this cloud. As the comet gets closer to the sun, more
gas is produced. 
The
gas and dust is pushed away by charged particles known as the solar
wind, forming two tails. Dust particles form a yellowish tail, and
ionized gas makes a bluish ion tail. A comet's tails, like these on
comet Halley, always points away from the Sun. 