When you think of an astronaut, you often think of people in white suits, floating in outer space, but have you ever thought about what's happening to their bodies? When you're in space, you experience what feels like weightlessness. Since your body was built for earth gravity, not zero gravity. Because of this it stretches out and weakens, meaning your muscle mass lowers and your and your bones weaken. Your immune system suffers too. There are numerous reasons for this, due to microgravity, radiation, isolation or changes in cardiovascular rhythm. Due to all of the negative effects of the being star-bound, no wonder the astronauts go through the training they take.