are bodies orbiting within the (main
) asteroid belt which show cometary activity during a part of their orbit.
Unlike most 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
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 .
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.
It has been hypothesised that they may have been the source of Earth's water.
Presently there are five identified members: