A comet is a phenomenon in the sky. It consist of a nucleus, a coma, and usually a dust tail, a plasma tail, and sometimes a neutral species tail (usually sodium atoms). The nucleus is the source of all cometary activity. It is composed of a low-density mixture of ice and dust. Typically comets move through the solar system in orbits with a period ranging from a few years to a few million years. Comets can provide spectacular views because of their tails of sublimating (vaporizing) ice that entrains very fine dust.

Long-period comet nuclei tend to be larger than short-period comet nuclei. Long-period comets are not comfined to the ecliptic. They can have retrograde motion around the Sun giving them a high relative velocity with respect to the Earth. Although collisions of nuclei of long-period coments are extremely rare, they are extremely dangerous because of their high relative velocity with respect to the Earth, their large mass, and their unknown orbits. They can appear suddenly, providing little warning time. Comet IRAS-Araki-Alcook passed the Earth in 1983 at a distance of less than 0.05 AU only two weeks after its discovery.

Southwest Research Institute: Space Instrumentation Division To report any problems, email: Walter Huebner
Last Updated: 3/12/2009