Comet

Comet A small Solar System body made of ice and dust. Comets are asteroidal in appearance at distances of many astronomical units from the Sun (when they consist of a bare, inactive nucleus), and are often spectacularly active when nearer to the Sun. The characteristic bright head (coma) and streaming tails (both dust and ions) are created by solar heating, with emission of light from gas molecules and scattered light from the dust. The comet nucleus is a few km in size, irregularly shaped and very dark; dust in the coma contains carbon and silica; there is evidence of polymerized organic molecules; the gases of the coma include water, carbon monoxide, carbon dioxide, ammonia, methane, and hydrocarbons. The source of comets is believed to be a spherical halo cloud about the Sun called the Oort Cloud. Observable comets are occasionally scattered into the inner Solar System by the gravitational fields of nearby stars and giant molecular clouds. Some (eg Halley's comet) are captured into relatively short period orbits as a result of gravitational interactions with giant planets. A small 'armada' of spacecraft (Sakigake and Suisei, VEGA 1 and 2, Giotto) encountered Halley's comet in 1986. >>