Main-belt comets are bodies orbiting within the asteroid belt that have shown comet-like activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with an axis of more than 2 AU but less than 3.2 AU. The first main-belt comet discovered is 7968 Elst–Pizarro and it was discovered in 1979 and was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Although quite a few comets have semimajor axes well within Jupiters orbit, main-belt comets differ in having small eccentricities. The first three identified main-belt comets all orbit within the part of the asteroid belt. 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, 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, the activity persists for a month or several 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. These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation, observations of Scheila indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter. In October 2013, observations of P/2013 R3, taken with the 10.4 m Gran Telescopio Canarias on the island of La Palma showed that this comet was breaking apart. The brightest A fragment was also detected at the position in CCD images obtained at the 1.52 m telescope of the Sierra Nevada Observatory in Granada on October 12. NASA reported on a series of images taken by the Hubble Space Telescope between October 29,2013 and January 14,2014 that show the separation of the four main bodies. The Yarkovsky–OKeefe–Radzievskii–Paddack effect, caused by sunlight, increased the rate until the centrifugal force caused the rubble pile to separate. The term main-belt comet is a 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, identified members of this morphology class include, Centaur Extinct comet Henry Hsiehs Main-Belt Comets page has extensive details on Main-belt comets David Jewitt. J. Licandro New images obtained with the GTC
Asteroid (596) Scheila displaying a comet-like appearance on December 12, 2010.
Image: 14060 Asteroid P2013R3 Disintegration 20140306