Ceres

Ceres is the largest asteroid in our solar system. Its neighbor, a smaller asteroid named Venice, has been colonized by human beings fleeing atoxic Earth environment. Ceres is also the chosen name of the colony's most preeminent psychiatrist. While he maintains a public professional decorum, Ceres is privately a very selfish and self-serving man, one who seeks immortality for himself. He plays with people.
He manipulates them. He forces his custom pharmaceuticals on them, all in his search for eternal life. An accident with one of his patients causes him to discover something almost as fantastic as immortality: one of his patients travels in time...backwards, to Earth's early human civilization...while she appears to be comatose. Further experimentation reveals that his new concoction allows his patients to swap personalities with people residing in that past world.
Another secret is revealed: the patients can only swap minds with their own incarnations from that world. After several successful transfers, Ceres decides to try it himself, only to discover that his incarnation in this time and world does not lead a life worth living. Frustration and bruised pride set Ceres off on other plots to influence and even harm the lives of the asteroid's denizens. He has gone mad.