Prolonged exposure to microgravity causes astronauts to lose bone and muscle, so they have to exercise for hours a day to stay healthy. NASA is working on a new strategy that could involve just laying down and going for a spin - in a short-radius centrifuge. 32 test subjects will spend 21 days in bed rest, simulating the effects of microgravity. Some will spend an hour a day in a centrifuge that simulates 2.5 times the Earth's gravity. It's hoped that this treatment can reverse the loss of bone and muscle mass.