Main belt comets are bodies orbiting within the (main) asteroid belt which show cometary activity during a part of their orbit.
Orbits of main belt comets.
Unlike most comets which spend mot 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 just like main belt asteroids. The three known main belt comets all orbit within the outer part of the main 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 formed in an inner solar system orbit close to their present positions.
Cometary activity of main belt comets.
Main belt comets display a cometary dust tail only for a part of their orbit near perihelion. The activity persists for a month or several out of each 5-6 year orbit.
Composition of main belt comets.
It has been hypothesised that they may have been the source of Earth's water.
Members of main belt comets.
Presently there are three identified members: