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. 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.

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.

Asteroid belt

The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids. About half the mass of the belt is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. The total mass of the asteroid belt is approximately 4% that of the Moon, which is significantly less than that of Pluto and roughly twice that of Pluto's moon Charon (whose diameter is 1200 km).

Ceres, the asteroid belt's only dwarf planet, is about 950 km in diameter, whereas Vesta, Pallas, and Hygiea have mean diameters of less than 600 km. The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form an asteroid family whose members have similar orbital characteristics and compositions. Individual asteroids within the asteroid belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).