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Main-belt comet
Main-belt comets (MBCs) 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 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.〔(【引用サイトリンク】publisher=JPL Solar System Dynamics )〕 David Jewitt from UCLA points out that these objects are most likely not comets with sublimating ice, but asteroids that exhibit dust activity, and hence he and others started calling these class of objects active asteroids.〔(【引用サイトリンク】publisher=Space Telescope Science Institute (STScI), official YouTube channel for the Hubble Space Telescope )〕
The first main-belt comet discovered is 7968 Elst–Pizarro. 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.〔
== Orbits ==
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. Although 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.〔
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.〔 ((older 2010 site) )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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