FILE - This Thursday, Feb. 14, 2008 image provided by NASA shows astronaut Alan Poindexter exercising on a bicycle ergometer on the space shuttle Atlantis while docked with the International Space Station. Astronauts can become as weak as 80-year-olds after six months at the International Space Station, according to a new study that raises serious health concerns as NASA contemplates prolonged trips to asteroids and Mars. (AP Photo/NASA)

FILE - This Thursday, Feb. 14, 2008 image provided by NASA shows astronaut Alan Poindexter exercising on a bicycle ergometer on the space shuttle Atlantis while docked with the International Space Station. Astronauts can become as weak as 80-year-olds after six months at the International Space Station, according to a new study that raises serious health concerns as NASA contemplates prolonged trips to asteroids and Mars. (AP Photo/NASA)