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Comets

Comets are relatively small icy bodies, often only a few kilometers in extent, that formed in the outer solar system where temperatures are cold enough to sustain (predominately water) ices. They represent the leftover bits and pieces from the outer solar system formation process that took place some 4.6 billion years ago.

Over long time periods, some comets are perturbed from their distant orbits and sent close enough to the sun that their ices begin to vaporize. This out-gassing of gas and dust from a comet's nucleus produces an atmosphere (i.e., coma) often extending many hundreds of thousands of kilometers. Because of the reflection of sunlight from its dust particles and the fluorescence of its excited gases, this atmosphere glows with a "fuzzy" appearance when viewed from the ground. As this coma material continues to expand away from the solid cometary nucleus, the gas component is eventually "blown" away from the sun by a high speed stream of charged particles from the sun (solar wind). The comet's dust component is also blown away from the sun - this time by the pressure of sunlight on the tiny dust particles. Thus a comet can have both a gas tail and a dust tail.

Comets originate in the outer regions of our planetary system with one group forming in the region near the current orbits of Uranus and Neptune and another group, called the Kuiper belt objects, forming somewhat more distant to the sun - beyond the orbit of Neptune. As a result of their interactions with the outer major planets, the comets in the first group can be thrown out to the distant Oort cloud some 50,000 to 150,000 times further from the sun than the Earth. Close passing single stars and the gravitational interaction with our Milky Way disk of stars can then nudge these comets back into the inner solar system where they can arrive with any inclination with respect to the Earth's orbital plane. Sometimes these objects can be seen as impressive, long-period comets like comet Hale-Bopp that was easily observable to the naked-eye in 1997.

So-called long-period comets orbit the sun with periods ranging from 200 to several million years. Comets that form in the so-called Kuiper belt (or Edgeworth-Kuiper belt after the two researchers who hypothesized these comets in the mid twentieth century) are also acted upon gravitationally by the massive outer planets and they often evolve into the short-periodic comets, whose orbital inclinations are usually relatively close to the Earth's orbital plane. With their orbital periods of about 5-7 years, these short-period comets orbit the sun frequently, lose much of their volatile ices, and are often far less visually impressive than their long-period cometary cousins that arrive fresh from the Oort cloud.

Read this short article by Don Yeomans to learn why comets are particularly interesting and why we should study these primitive bodies. Then, learn about some of the great comets of the past in this article by Don Yeomans.

Orbits: Diagrams & Elements

The orbit of any comet (or asteroid) can be viewed using our java-based orbit applet. Start with our small-body browser to find the asteroid of interest, then select the Orbit Diagram link. For example, here is the orbit diagram for comet 1P/Halley.

Orbital elements and related parameters are also available for any comet (or asteroid) using our small-body browser. In addition, custom tables of orbital elements and/or physical parameters are available using our small body database search engine. We also provide fixed-format ASCII tables of elements.

Warning: If you intend to use cometary orbital elements in a two-body propagation to compute future/past position (ephemerides), your results will be inaccurate and in some cases, completely incorrect. The motion of comets is affected by their so-called non-gravitational forces (the rocket-like force from outgassing of material from the comet while close to the sun). Thus, it is especially important to use HORIZONS to compute comet ephemerides.

Physical Parameters

Physical parameters for comets are not well known primarily because these bodies are too small for ground-based observing when the comet is far enough from the sun that its coma does not shroud its surface. The only parameters determined for nearly all comets are their magnitude parameters (M1,K1 and/or M2,K2). However, a few comets have other parameters determined including rotation period, and geometric albedo. Known physical parameters for any given small body are are available from our small-body browser.

Ephemerides

Comet ephemerides are available using JPL's HORIZONS system.

Discovery Circumstances

Discovery circumstances for many comets are also available using our small-body browser. Discovery data include the date of discovery, who discovery the comet, and where it was discovered.

Mission Targets

Spacecraft missions to small-bodies provide valuable scientific data ultimately improving our understanding of these primitive solar system bodies. A list of asteroids and comets targeted by spacecraft missions (past, present, and future) is presented on this page.

Radar Astrometry

Radar astrometry for selected comets is available in tabular format. A table showing data for only comets is presented on this page.

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