An icy object in independent orbit about the Sun; smaller than a planet, usually having a highly elliptical orbit extending out to beyond Jupiter. At long intervals, of up to ten thousand years, they move close to the Sun, which partially melts them and evaporates some of the gases. The solar wind catches these luminous gas particles, and blows them out into a long streaming tail, stretching away from the Sun for millions of kilometers. Comets have long fascinated man, and were believed to be omens of revolutions and other catastrophic events. Halley's comet, a famous periodic comet which orbits the Sun in about 75 years, made an appearance in l986.