Your skeleton may seem hard and stiff, but your bones actually are living tissue, in which cells called osteoclasts break down old, worn bone material and release calcium into your bloodstream, while other cells, called osteoblasts, lay down new minerals along the bone surfaces.
On Earth, these two processes work together in a healthy young person, so that bone is replaced at the same rate that it's broken down. As a person gets older, the balance sometimes gets out of kilter, and the bones weaken in a condition called osteoporosis. Unfortunately, an extended stay in the microgravity environment of space has an effect similar to aging, scientists say.
Studies of Russian cosmonauts who spent several months in space, for example, found that they lost as much as 20 percent of their bone mass. The effects of microgravity can leave bones so weak that they can't support astronauts' bodies when they return to Earth, putting them at risk of suffering fractures from stress. Researchers currently are trying to figure out whether tinkering with astronauts' diet, exercise and hormonal levels while in space could stave off some of the damage [sources: NASA, BBC ]. If that's not bad enough, the microgravity environment also causes changes in the spinal disks that give astronauts really bad backaches
[source: Sayson et al.].