A comet is an object orbiting the Sun that displays a visible coma (a fuzzy surrounding region which is the comet's atmosphere) and possibly a tail, on the side away from the Sun. They are icy, i.e., made of substances solid but that can be turned to gas at temperatures reached in the inner part of their orbit (i.e., volatile material substances), creating the coma and possible tail. Classically, they have long orbital periods (tens to thousands of years) that are highly elliptical so a brief portion of the orbit is visible and displays the coma and tail effects. At the far end of their orbits, they move slowly such that they remain in that region for a very long time, and it is thought that there are always many long-period comets are in that state, which together are known as the Oort Cloud.
Inspection has revealed rocky surfaces. Volatiles such as water, carbon dioxide, carbon monoxide, methane, and ammonia make up the ice, which may be inside.
Two well-known comets are Comet Halley (recognized in 1705 by Edmund Halley as orbiting approximately every 76 years) and Comet P/Shoemaker-Levy 9 which was observed colliding with Jupiter in 1994.