Comets are small Solar System bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (atmosphere) or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across.

Comets have a variety of different orbital periods, ranging from a few years, to hundreds of thousands of years, while some are believed to pass through the inner Solar System only once before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt, or associated scattered disc, which lies beyond the orbit of Neptune. Long-period comets are believed to originate at a very much greater distance from the Sun, in a cloud (the Oort cloud) consisting of debris left over from the condensation of the solar nebula. Comets are thrown from these outer reaches of the Solar System inwards towards the Sun by gravitational perturbations from the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort Cloud objects), or as a result of collisions.

Comets leave a trail of debris behind them. If the comet's path crosses Earth's path, then at that point may be meteor showers as the Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of the comet Swift-Tuttle. Halley's comet is the source of the Orionid shower in October.

Comets are distinguished from asteroids by the presence of a coma or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids. Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System. Recent findings have, however, somewhat blurred the distinction between asteroids and comets.

There are a reported 3, 354 known comets as of November 2007, of which several hundred are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number of naked-eye comets averages to roughly one per year, though many of these are faint and unspectacular. When a historically bright or notable naked-eye comet is witnessed by many people, it is often considered a Great comet.

The word "comet" came to the English language through the Latin worrd "cometes" which is derived from the Greek word kome, meaning "hair of the head"; Aristotle first used the derivation kometes to depict comets as "stars with hair." The astronomical symbol for comets accordingly consists of a disc with a hairlike tail.

Solar System Links & Resources

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Comet Links & Resources

Comets & Meteor Showers Gary Knonk's site lists currently visible comets, periodic and sungrazing comets. One of the best comet resources on the web BAA Comet Section The latest comet news from the British Astronoical Association Central Bureau for Astronomical Telegrams Publishes the IAU Circulars--a must for people wanting to observe comets. The Circulars announce new comet and asteroid discoveries, publish their orbits, and provide predictions of where to find these objects The ICQ Comet Information Website This is a key place to begin looking for useful and accurate information regarding news, observations, orbital data, designations and names, and good links regarding comets and related topics Comet Chasing Comet chasing is all about the visual observation of telescopic comets. This site provides information on where to look for them along with finder charts Canadian Comet Catchers Comets discoverd by Canadian astronomers Observable Comets This page lists links to orbital elements and ephemerides of (potentially) observable comets Comets The Comet page from 'The Astronomer Online' showcases comet images taken from around the world Comet Shoemaker-Levy Collision with Jupiter Large archive of data on the comet that collided with Jupiter in July 1994 Comet Shoemaker-Levy 9 Collision with Jupiter Another large site achiving the data gathered on the comet that collided with Jupiter in July 1994 SOHO Comets On this website you can view and submit reports of suspected comets visible in LASCO images. You will find information on past comets found by the LASCO instrument aboard the SOHO satellite and from other spaceborne and ground-based instruments. There is also a wealth of information here designed to help newcomers discover their first SOHO comet! In addition, pictures and educational resources that can be used both in the classroom and at home are shown throughout the site Stardust - NASA's Comet Sample Return Mission News and information about NASA's Stardust Mission which collected samples of comet dust by passing through the tail of a comet and returned those samples to Earth Stardust@Home Help search for interstellar dust in the Stardust aerogel collector. The Stardust@home collaboration -- consisting of thousands of volunteers all over the world, and professional scientists at U. C. Berkeley and NASA/JSC -- has identified a number of intriguing features in the Stardust Interstellar Dust Collector

Comet eBooks & Articles

Stardust Comet Flyby NASA report in PDF format. Left-click to read in your browser or Right-click to download. The Comet With a Broken Heart Report in PDF format. Left-click to read in your browser or Right-click to download. The VLT Takes Images of Disintegrating Comet 73P/Schwassmann-Wachmann 3. Photometric Behavior of Comet Hale-Bopp (C/1995 O1) Before Perihelion Report in PDF format. Left-click to read in your browser or Right-click to download. Narrowband photometric observations of comet Hale-Bopp (C/1995 O1) between 25 July 1995 and 15 February 1997 indicated gas and dust production rates of 20 and 100 times greater, respectively, than observed at the same heliocentric distances for comet P/Halley in 1985. The Tau Herculid meteor shower and Comet 73P/Schwassmann-Wachmann 3 Report in PDF format. Left-click to read in your browser or Right-click to download. The Tau Herculid meteor shower has not shown any appreciable activity since 1930. However, it is associated with Comet 73P/Schwassmann-Wachmann 3, a Jupiter-family comet that split in 1995. The fragments will pass near the Earth on 2006 May 13, and could produce an outburst of the Tau Herculid shower.