Chris Cassidy is a former Navy SEAL who left the service to become an astronaut. When asked which one was harder, Cassidy says that the training to become a Navy SEAL and an astronaut are two different things. According to him, training to become a Navy SEAL requires more physical work, while training to become an astronaut is more academic. He also added that his background in the military has helped him a lot in becoming an astronaut.
The Crossroads of Special Operations