A comet is a celestial object composed of water ice, frozen gases and small particles. It has nucleus or head, which is enveloped by ammonia and carbon dioxide gas that form a long tail-like beam called "coma". Comets orbit around the Sun in an elliptical pattern. The last comet seen from Earth was Hale-Bopp in 1997. But the most famous comet is Halley's comet, which was last seen from Earth in 1986.