Comets

Definition: Icy planetesimals formed in the outer solar system.
Composition: Mainly ice and dust
Orbits: Highly elliptical, taking them very close to the Sun and back out into deep space, often far beyond the orbit of Pluto.
Orbit duration: Less than 200 years to more than several millions of years.

*Short-period <><><><><> *Orbits range from a few years (Encke, Chiron) to about 200 years. Referred to as "periodic".
*Long-period <><><><><> *Gravitationally bound, with orbits over 200 years.
*Single-apparition <><><><><> *Parabolic and hyperbolic orbits (not bound).

Periodic comets: Those with short orbit durations
Nucleus: Main body of the comet, composed of dust particles trapped in a mixture of ices of water, carbon monoxide, carbon dioxide, methane, and ammonia. Typically only a few kilometers in diameter.
Coma: A halo of evaporated gas and dust which forms when the comet approaches the inner solar system. The coma grows larger as the comet gets closer to the sun.