Main-belt comets are bodies orbiting within the main asteroid belt which have shown cometary activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with a semi-major axis (average distance from the Sun) of more than 2 AU but less than 3.2 AU, and a perihelion (closest approach distance to the Sun) of no less than 1.6 AU.[1]
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Unlike comets, which spend most of their orbit at Jupiter-like or greater distances from the Sun, main-belt comets follow near-circular orbits within the asteroid belt that are undistinguishable from the orbits of many standard asteroids. While quite a few short-period comets have semimajor axes well within Jupiter's orbit, main-belt comets differ in having small eccentricities and inclinations similar to main-belt asteroids. The first three identified main-belt comets all orbit within the outer part of the asteroid belt.[2]
It is not known how an outer Solar System body like the other comets could have made its way into a low-eccentricity orbit typical of the asteroid belt, which is only weakly perturbed by the planets. Hence it is assumed that unlike other comets, the main-belt comets are simply icy asteroids, which formed in an inner Solar System orbit close to their present positions, and that many outer asteroids may be icy.[2]
Some main-belt comets display a cometary dust tail only for a part of their orbit near perihelion. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia.[3] The activity persists for a month or several[2] out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years.[2] These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation.[2]
When discovered in January 2010, P/2010 A2 (LINEAR) was initially given a cometary designation and considered a member of this group,[4] but P/2010 A2 is now believed to the remnant of an asteroid-on-asteroid impact.[5][6]. Observations of 596 Scheila indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter.
It has been hypothesized that main-belt comets may have been the source of Earth's water, since the deuterium-hydrogen ratio of the Earth's oceans is too low for classical comets to have been the principal source.[7]
The term 'main-belt comet' is a classification based on orbit and the presence of an extended morphology. It does not imply that these objects are comets or that the material surrounding their nuclei was ejected by the sublimation of volatiles, as on comets.
Identified members of this morphology class include:
| Full Name |
Hsieh[2] |
Jewitt[3] |
Semi-major axis (a) |
Perihelion (q) |
Perihelion date |
|---|---|---|---|---|---|
| 133P/Elst–Pizarro | Y | Y | 3.15 | 2.64 | 2013-Feb-08 |
| 176P/LINEAR | Y | Y | 3.19 | 2.57 | 2011-Jul-02 |
| 238P/Read (P/2005 U1) | Y | Y | 3.16 | 2.36 | 2011-Mar-10 |
| P/2008 R1 (Garradd) | Y | Y | 2.72 | 1.79 | 2013-Jan-25 |
| P/2010 A2 (LINEAR) | Y | 2.29 | 2.00 | 2009-Dec-04 | |
| P/2010 R2 (La Sagra) | Y | 3.09 | 2.62 | 2010-Jun-25 | |
| 596 Scheila | Y | 2.92 | 2.44 | 2012-May-19 | |
| (300163) 2006 VW139 | Y | 3.05 | 2.44 | 2011-Jul-18 |
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