Comet

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The term "comet" is derived from the Ancient Greek word komētēs (κομήτης), which means "long-haired", referring to the visual appearance of a comet's tail. In Hebrew, a comet is called חַרְטוֹם (khartom), which is derived from the Aramaic word for "star with a tail". In Arabic, a comet is known as السيارة (al-sayyārah), which translates to "the traveler". In Japanese, the term is 彗星 (suisei), which translates to "broom star". Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (directly around the nucleus) and tail (a long, thin stream of material extending away from the Sun). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Short-period comets or periodic comets are generally defined as those having orbital periods of less than 200 years. Long-period comets have orbital periods longer than 200 years. Single-apparition comets are sometimes called non-periodic comets. The orbit of a comet is determined by its close approach to the Sun, which is known as its perihelion.