The government of their world had an unusual way of carrying out the death penalty on convicted criminals: they would be exiled via rockets to one of the glowing asteroids located in the system's asteroid belt. There, they would be supplied with food, water and other necessities until the asteroid exploded, killing everyone on it. Since it could not be predicted when an asteroid would explode, this added mental torment to the criminals' punishment.
In 2266, the USS Enterprise explored the Nabu system and made contact with a group of convicts led by Targu. A landing party led by Captain James T. Kirk was captured, but were eventually freed and escaped the asteroid shortly before it exploded. (TOS comic: "The Devil's Isle of Space")