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What are comets?

Comet McNaught, seen in the night skies of Chile in January 2007Comets are small lumps of dust and ice. Like the planets, comets orbit the Sun. When they come near it, the ice melts and long tails of gas and dust stretch out, always pointing away from the Sun. Sometimes, we can spot a comet from Earth: It looks like a smear of light hanging in the night sky. Comets probably start out from the outer edge of the Solar System. Here, there may be a cloud of icy bodies, some of which fall in towards the Sun.



Close-up of a comet nucleus. It measures a few kilometres across. Jets of dust and gas burst through the crust as the comet...Read More >>Close-up of a comet nucleus. It measures a few kilometres across. Jets of dust and gas burst through the crust as the comet approaches the Sun.

Nucleus

The main part of a comet, the lump of dust and ice, is called the nucleus. When the comet approaches the Sun, the ice starts to melt and the outer crust of the nucleus cracks open. Jets of dust and gas stream out to form a cloud called a coma. Nearer the Sun, the coma is swept back to form long tails of dust and gas.

The two tails of a comet

Halley's Comet was visible from Earth in 1066. It is shown in the Bayeux Tapestry which depicts the events leading up to the Norman conquest of England in that year.

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