A chunk of frozen gasses, ice, and rocky debris that orbits the Sun. A comet nucleus is about the size of a mountain on earth. When a comet nears the Sun, heat vaporizes the icy material producing a cloud of gaseous material surrounding the nucleus, called a coma. As the nucleus begins to disintegrate, it also produces a trail of dust or dust tail in its orbital path and a gas or ion tail pointing away from the Sun. Comet comas can extend up to a million miles from the nucleus and comet tails can be millions of miles long. There are thought to be literally trillions of comets in our solar system out past Neptune and Pluto, but only once per decade or so does one become near and bright enough to see easily without binoculars or a telescope.
Historical Notes
1066 A large comet is sighted. In Britain it is thought to be connected to the invasion of William the Conqueror from France. It will revisit Earth in 1985/6 - we know it today as Halley's comet.
1540 The book Astronomicon Caesareum by Peter Apian notes that the tails of comets point away from the Sun, a fact known by the Chinese since 635 AD.
1758 Halley's comet appears on 25th December, exactly in accordance with predictions.
1908 The "Tunguska event" - major damage to a forest region in Siberia caused by a comet or meteorite.
1994 Astronomers observed comet Shoemaker-Levy 9 (S-L 9) colliding with Jupiter.

See also: Coma, Palermo Scale, Sun, Torino Number.

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Subjects: Physics