Periodic comet

A periodic comet is a comet which orbits around the Sun. A short-period comet will complete its orbit in less than 200 years. A long-period comet will take more than 200 years to orbit. In most cases the comets are named after the person who discovered them. The Shoemaker-Levy comets were named after Eugene Shoemaker, Carolyn Shoemaker and David Levy. A few comets are named after the person who calculated the orbits, such as 2P/Encke and 27P/Crommelin. It is very difficult to work out the orbit of a long-period comet because the orbit changes. The gravity of planets, or the release of gas or material from the comet itself, changes the orbit. Short-period comets can be predicted very accurately.

Periodic comets sometimes have the same name, for example there are nine Shoemaker-Levy comets and twenty-four NEAT comets. The Minor Planet Center, a part of the International Astronomical Union gives comets a different number prefix or uses the full title such as 181P/Shoemaker-Levy and 192/Shoemaker-Levy which are both "Comet Shoemaker-Levy". Sometimes an informal numbering system is used for periodic comets so 181P is Comet Shoemaker-Levy 6 and 192P is Comet Shoemaker-Levy 1. Because non-periodic Shoemaker-Levy comets are included in the number sequences there appear to be gaps in the numbers, for example C/1991 B1 is between Shoemaker-Levy 2 and 3, C/1991 T2 is between 5 and 6, C/1993 K1 and D/1994 E2 after Shoemaker-Levy 9.

In naming a comet, the letter before the "/" is:
 "C" for a non-periodic comet
 "P" is a periodic comet
 "D" is a comet which has been lost or has disintegrated
 "X" is a comet for which no reliable orbit could be calculated –usually historical comets
 "A" for an object that was thought to be comet, but is actually a minor planet.

Some lists keep the "C" prefix for comets of with orbits longer than about 30 years until their return is confirmed.

References

Related pages
 List of comets

Other websites

Category:Comets

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