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Comets are small astronomical bodies, typically a few kilometers across, that orbit the Sun. They contain icy chunks and frozen gases with bits of embedded rock and dust, and possibly a rocky core, aptly described as a "dirty snowball." Comets are leftovers from the formation of the Solar System and are believed to exist in vast numbers in the Oort Cloud and, to a lesser extent, in the Kuiper Belt. From these regions they can be perturbed by the gravitational influence of passing stars or interstellar clouds and thrown into new, highly elliptical orbits that bring them into the inner Solar System. As a comet draws nearer to the Sun, solar radiation causes the comet's frozen gases to sublime (turn directly from a solid into a gas) and be released, so that, in addition to the frozen nucleus, several new features develop. These include a coma, a luminous cloud of water vapor, carbon dioxide, and other neutral gases driven off the nucleus; a hydrogen cloud, a huge (millions of kilometers in diameter), tenuous envelope of neutral hydrogen; a dust tail (the most prominent part of a comet to the unaided eye) up to 10 million km long, composed of smoke-sized dust particles driven off the nucleus by escaping gases; and an ion tail, as much as several hundred million km long, composed of plasma and laced with rays and streamers caused by interactions with the solar wind.

(For further details see Comet »)

  • Deep Impact-EPOXI (Comets) Featured Article Deep Impact-EPOXI (Comets) Deep Impact-EPOXI (Comets)

    A radical mission to excavate the interior of a comet, work on Deep Impact began in January 2000, as part of NASA's Discovery Program. The spacecraft launched from Cape Canaveral... More »

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Comets Last Updated on 2009-12-03 14:49:59 Throughout history, people have been both awed and alarmed by comets, stars with "long hair" that appeared in the sky unannounced and unpredictably. We now know that comets are dirty-ice leftovers from the formation of our solar system around 4.6 billion years ago. They are among the least-changed objects in our solar system and, as such, may yield important clues about the formation of our solar system. We can predict the orbits of many of them, but not all. Around a dozen "new" comets are discovered each year. Short-period comets are more predictable because they take less than 200 years to orbit the Sun. Most come from a region of icy bodies beyond the orbit of Neptune. These icy bodies are variously called Kuiper Belt Objects, Edgeworth-Kuiper Belt Objects, or trans-Neptunian objects. Less predictable are long-period comets, many of which arrive from a distant... More »
Deep Impact-EPOXI (Comets) Last Updated on 2009-09-28 00:00:00 A radical mission to excavate the interior of a comet, work on Deep Impact began in January 2000, as part of NASA's Discovery Program. The spacecraft launched from Cape Canaveral on January 12, 2005, beginning its 268-million-mile journey to Comet Tempel 1. The two-part spacecraft consisted of a larger flyby spacecraft carrying a smaller impactor spacecraft. On July 2, 2005, at 11:07 PDT, the impactor was successfully released at a distance of about 547,000 miles from the comet. The battery-powered, 770-lb impactor was designed to operate independently for just one day, taking over its own navigation and maneuvering into the path of the comet. Nearly 24 hours later, at 10:52 pm PDT on July 3, traveling at a speed of 23,000 miles per hour, the impactor successfully placed itself into the path of comet Tempel 1. A camera on the impactor captured and relayed images of the comet nucleus... More »