Chuck began his service as an astronaut. He studied his whole life and busted his ass to be the perfect candidate for the astronaut program and he dreamed of going to space. Unfortunately, a training accident exposed Chuck to a large dose of radiation. Before he ever flew in space, Hicks had exceeded the lifetime limit for radiation exposure allowed by NASA.
He thought his life was over. But someone at the SGC became aware of his situation and his skills. They decided not to let his training to to waste, and they brought him on board.