University of Mississippi Medical Center researchers are using an advanced computer program to help better understand what happens when astronauts are reintroduced to gravity. When NASA astronauts return to Earth’s gravity, about a fourth of them, mostly women, become so weak they have trouble standing. Others adjust to gravity with few problems. The longer a person is in space, the more difficult the transition. That makes the prospect of a six-month stay on the space station or a trip to Mars that could take over a year a daunting one.