Comet

A comet is a ball of mostly ice that moves around in outer space. Comets are often described as "dirty snowballs". They are very different from asteroids. The orbital inclinations of comets are usually high and not near the ecliptic where most solar system objects are found. Most of them are long-period comets and come from the Kuiper belt. That is very far away from the Sun, but some of them also come near enough to Earth for us to see at night.

They have long "tails", because the Sun melts the ice. A comet's tail does not trail behind it, but points directly away from the Sun, because it is blown by the solar wind. The hard centre of the comet is the nucleus. It is one of the blackest things (lowest albedo) in the solar system. When light shone on the nucleus of Halley's Comet, the comet reflected only 4% of the light back to us.

Periodic comets visit again and again. Non-periodic or single-apparition comets visit only once.

Comets sometimes break up, as Comet Biela did in the 19th century. Comet Shoemaker-Levy 9 broke up, and the pieces hit Jupiter in 1994. Some comets orbit (go around) together in groups. Astronomers think these comets are broken pieces that used to be one object.