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.