Question: Do you know anything about what happens to astronauts bones when they are in space?

Mark Burnley answered on 14 Jun 2011:

There has been a lot of research in this area, because we have always thought that NASA would one day commit to going to Mars. The first step in this process would be to make sure the human body can cope with a prolonged stay in space, and one of the systems affected by space flight is the skeleton. Some of these experiments have been done on astronauts staying on the old Mir space station or more recently the ISS. When we can’t do these experiments (they are quite costly) we can “simulate” microgravity by unweighting people (using bed rest), or by immobilisation (by putting a cast on a joint and making the subjects walk on crutches). All of these experiments show the same thing: not using muscles and bones normally results in a loss of muscle tissue (called atrophy) and bone (known as remodelling). Astronauts returning from prolonged space flight, even if they have done exercise during that flight, have great difficulty standing up on their return to earth, and are at greater risk of fractures in their daily lives. Therefore, one issue about a trip to Mars (assuming the astronauts survive the dose of radiation they’ll receive on the way) is whether they will be physically capable of doing anything when they get there!

The last bit is even more challenging than you might think. I went to Penn State University to give a talk a few years ago and met this guy: http://en.wikipedia.org/wiki/James_A._Pawelczyk He told me that the space suits used on the Moon had a few problems. First, they are pressured to 4 times the normal atmospheric pressure. This makes moving around alone quite challenging. The hardest thing, though, is using your hands: grasping an object for a few seconds is exhausting! Not only that, but the CO2 “scrubbers” in the suit can only remove at most 1.0-1.5 litres of CO2 every minute. This is about enough to walk briskly, which wasn’t a problem on the Moon but would be on Mars because of the greater gravitational force. And remember, this is in astronauts already knackered by 6 months of inactivity and radiation exposure!

Basically, living in space is rubbish for your whole body!