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NASA
Comets
Not long ago, many people thought that comets were a portent that something bad was about to happen to them. Since people did not yet understand about the objects in the solar system and how they moved, the sight of a comet must have been very disturbing. There are many historical records and art works that record the appearance of comets and associate them with terrible events such as wars or plagues.
Now we know that comets are lumps of ice and dust that periodically come into the center of the solar system from somewhere in its outer reaches, and that some comets make repeated trips. The solid part or nucleus of comets can be oddly shaped and can be only a few miles across. When comets get close enough to the Sun, heat makes the nucleus start to sublimate. A cloud of gas and dust called the coma forms. The coma can reach a similar diameter to that of a giant planet! Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long.
In 1985-1986, a spacecraft called Giotto visited the most famous comet, Halley, on its most recent visit to the inner solar system. In 1994, comet Shoemaker-Levy became trapped by the gravity of Jupiter and plunged into Jupiter's atmosphere!
In 1996 and 1997 we saw comet Hyakutake, and comet Hale-Bopp. Hale-Bopp was one of the brightest comets ever seen from Earth. Comet Linear was discovered in 1999 and made its closest approach of the Sun in July 2000. The Stardust spacecraft flew by Comet Wild 2 in January 2004, collecting samples of the comet to return to Earth. The newest comet mission is Rosetta -- it will land on Comet Churyumov-Gerasimenko!
Now scientists have identified a class of comets known as small comets (though they originally were just called snowballs from space!)
Would you like to design your own custom comet? If you would, check out our interactive comet animation!
