A large part of space research has focused on the physiological effects of microgravity
Free-fall has been shown to affect astronauts' ability to judge size and distance and to cause the loss of red blood cells and muscle mass. But the biggest toll may be on the bones.
Even with a rigorous exercise regimen, most people lose an average of about 1.5 per cent of their bone mass in certain body parts – such as the hips – for every month in space. That is about the same amount of bone mass that a post-menopausal woman loses in a year.
Researchers are working to reduce this effect, in part with vertical treadmills that simulate microgravity conditions.