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NASA solar system facts - Comet

A comet is a collection of gas and dust a few miles across. Normally it is far too faint to be seen even with a powerful telescope. But when it approaches the Sun it becomes brighter, and can then be seen with a telescope. Very occasionally a comet becomes bright enough to be seen with the unaided eye.

NASA solar system facts: a comet appears as a bright ball of light. When close to the Sun it may have a long, glowing tail. The Sun's rays cause the gas contained in the comet to glow and spread out to form the tail. Night after night the comet moves slowly across the background of stars until it finally fades away.

People used to think that a comet's sudden appearance in the sky foretold a terrible event on Earth. Later, astronomers discovered that comets are harmless bodies that travel in ellipses around the Sun.

Comets do not last for ever. Each time one passes the Sun it loses some of its gas until, eventually, only the dust particles are left. Sometimes the Earth crosses the path of a swarm of dust particles from a former comet which plunge through the Earth's atmosphere as 'shooting stars'. (See Meteors.)

NASA solar system facts: comet Alcock appeared in the sky in 1959. Below: The most famous comet of all, Halley's comet, appears every 76 years. It was last seen in 1910 and is expected again in 1986. It was seen as an omen before the Battle of Hastings in 1066.

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