  Some asteroids eject dust, producing transient, comet-like comae and tails;
these are the active asteroids. The causes of activity in this newly-identified
population are many and varied. They include impact ejection and disruption,
rotational instabilities, electrostatic repulsion, radiation pressure sweeping,
dehydration stresses and thermal fracture, in addition to the sublimation of
asteroidal ice. These processes were either unsuspected or thought to lie
beyond the realm of observation before the discovery of asteroid activity.
Scientific interest in the active asteroids lies in their promise to open new
avenues into the direct study of asteroid destruction, the production of
interplanetary debris, the abundance of asteroid ice and the origin of
terrestrial planet volatiles.
