Question: what happens to an astronaut's bones once they are in space?

Simon Langley-Evans answered on 12 Nov 2013:

Great question that I sometimes get my degree students to think about.

Bone is an amazing living structure even though it looks dead. It is made of a scaffold of protein and we lay down minerals like calcium and phosphorus to add the strength and hardness. We lay down these minerals fastest when we are in our teens and growing fast. More mineral goes into bones that carry a lot of weight or stress. This means that if we do exercise like running our bones are stronger at the joints and when the running force is greatest.

On Earth even people who don’t exercise put their bones under stress because of gravity. Gravity pulls on our body mass and the bones have to resist it to stop us turning into wobbly goo. That causes calcium to go into bone. In space where gravity is weak the bones are not under stress in the same way, so instead of keeping calcium in bone, it is released and either used for other things that need calcium in our bodies, or pee’d out into the space toilet. This means that astronauts bones lose strength and mass.

Some astronauts who have been on the Space Station for a long time have lost so much bone on their stay that they can no longer stand up. Because of this they now follow exercise programmes with exercise bikes on the Station to slow down the bone loss.