Ban on Religion in Space.
From the very beginnings of space exploration, NASA and the Soviet Space Program secretly decided to screen out overtly religious applicants from their space programs. Their psychologists were convinced that a combination of strong religious convictions and the sensory deprivation associated with long space flights could lead to very convincing hallucinations. As ocean mariners centuries before had seen mermaids, there was no way of knowing what an astronaut's imagination might conjure up and what life threatening decisions might be made as a result.
The ban wavered over the decades, but was reinforced after the religious uprising on Titan lead by John Howitt. The rebellion begun after a hallucinating Howitt thought he saw a snake in a greenhouse apple tree, leading him to declare Titan the “New Eden”. He and his followers attacked and murdered any non-Christians who would not undergo conversion. He was stopped when assassinated by Captain Patricia Symcox.