More about comets

A comet is a minor planet made up of rock, dust and ice. It originates from a cloud of debris remaining from the condensation of the solar nebula. Comets are unique because they are created in the outer solar system, and are greatly affected by the planets they pass. While a comet is orbiting, its path is constantly being altered as it nears surrounding planets. These changes in orbit can send it on a path approaching the sun, where it will burn up, or can be cast completely out of the solar system.
The tail of a comet is actually called the coma, which is composed of gas and dust streams. When a comet passes through the inner solar system, the sun lights up these streams so that we are able to see it. This is how we have been able to see Halley’s Comet from Earth.
The orbital periods of comets vary, but have been divided into three categories: Short period comets; long period comets; and Single-apparition comets. While Short period comets orbit for 200 years or less, long period comets are bound by gravity to the sun, and remain much longer. Single-apparition comets have unusual orbits and are thrown out of the solar system forever.