A body that orbits within the main belt of asteroids yet shows activity similar to that of a cometary nucleus. The first such body to be recognized was 133P/Elst–Pizarro. When originally discovered in 1979 it appeared to be an ordinary main-belt asteroid and was numbered 7968, but in 1996 it was seen to be exhibiting cometary activity; as a result, it was reclassified as a periodic comet. This comet and 176P/LINEAR have orbits that show them to be members of the Themis family of asteroids. The cometary activity exhibited by main-belt comets is thought to be due to pockets of subsurface ice exposed by impacts. Currently, only a handful of main-belt comets are known.