Elon Musk talks a lot about Mars. Once a Mars colony is established, we should be able to make a trip to/from Mars every two years when the planets align. However, Mars' gravity is 38% that of the Earth. Won't the deterioration in muscle and bone mass, as a result of residing on Mars for two years, be a problem when returning to Earth gravity? If so, what are the possible solutions?
We don't know for sure. We can make some guesses based on our experience with weightlessness.
- in 1/3 G, muscles and bone will weaken over time, although not as rapidly as they would in weightlessness. This can be mitigated with lots of exercise: on the ISS, a 6 month stay will result in a ~8% loss of bone mass when the astronaut exercises 2.5 hours a day, versus ~20% when he doesn't exercise.
- Other effects we see on the ISS (e.g. redistribution and subsequent loss of fluids, changes in the shape of the eyeball) will be less of an issue.
We haven't done any long-term experiments where humans (or any living creature) was exposed to less than 1 G. There have been proposals, e.g. the Mars Gravity Biosatellite and a centrifuge module for the ISS, but those have been canceled.