What is a come

Comets are heavenly bodies. They are large, loose collections of material that penetrate into the inner regions of the solar system from outside. They have a bright head and a long tail. The head is made up of heavy particles and the tail is made up of gases like ammonia and methane and always points away from the sun. They move in elliptical orbits. They are small (of about 10 km diameter) when away from the sun but when they reach near the sun, the solar radiations heats up the material which expands and forms a head of 10,000 km diameter. Once in many years one gets a chance to see a comet with a naked eye.The name of the prominent comet seen in early 1986 is Halley's comet.