Comets
Comets are objects—relatively small compared to planets—that are composed of dust and ices of various compounds. Comets orbit the Sun in elongated elliptical (eccentric, elongated circle) or parabolic orbits. Accordingly, these objects spend the majority of time in the outer regions of the solar system, in some cases well beyond the orbits of Neptune and Pluto. Short-period comets are those with less exaggerated elliptical orbits that carry them out only as far as the region of space between the orbits of Jupiter and Neptune. Comets make periodic, brief, but sometimes-spectacular transits through the inner solar system as they approach the Sun. Comet orbits may be prograde, in the same direction as the planets, or retrograde, in the opposite direction. With the aid of a telescope, a comet is usually visible from Earth.
The term "comet" derives from the Greek aster kmetes (translated literally as "hairy" or long-haired star)—a reference to a sometimes-visible comet tail. If a comet's path takes it close enough to the Sun, the heating causes melting and emission of gases (out gassing) and dust that are then swept behind the comet's orbital path (away from the Sun) by the solar wind to form the characteristic tail.
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