A comet is a small, icy lump that travels in toward the Sun from the outer reaches of the Solar System. As it warms up, it develops a shining head and two tails.
Comets seem to be pieces left over from the formation of the Solar System. Some are in a belt beyond Neptune. Millions more form a giant spherical swarm, called the Oort Cloud, nearly a light-year from the Sun. When a comet travels in from the edge of the Solar System and is warmed by the Sun, the gas and dust it gives off make it look much brighter.
English astronomer Edmond Halley was the first to realize that the comet he saw in 1682 was a regular visitor to Earth’s skies, returning every 76 years or so. Historical records show that Halley’s Comet was spotted as long ago as 240 BC. It made its last visit in 1986 and is due to return in 2061.