Bone loss causes an increase in the concentrations of calcium throughout the rest of the body, which can lead to secondary problems like kidney stones - treatable on Earth, but potentially dangerous in space. Astronauts can recover bone mass lost during brief stays in space, but it is not known whether they can fully recover from long flights. Bone recovery is slow, and astronauts are very weak when they return to Earth after a long flight. This is perhaps the most difficult problem of space flight for the human body - and so far we have no solution. Our cells also find it hard to cope without gravity.
Most cells have a scaffolding made up of 'microtubules', made from fibres of the protein tubulin.
On Earth these tubules form weight-bearing struts at right angles to gravity. In microgravity they grow in all directions. Malformed tubules interfere with proper cell division, and may impair the workings of the immune system. The arrival of the first crew on the International Space Station marks the beginning of a permanent human presence in space. How they and their successors cope with the problems of microgravity will be crucial in determining its future.