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; some do so regularly,
some only once every few centuries.
Heads and tails
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
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.
When 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.