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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?

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    $\begingroup$ We don't have enough information to answer this. Our data set is 12 men on the surface of the Moon for a total of 14 days. Unless part of the spacecraft for the trip spins for simulated gravity, the 14 to 16 months in space lengthens the time in low gravity to over 3 years. $\endgroup$ – kim holder Sep 30 '15 at 14:04
  • $\begingroup$ Our data set also includes the astronauts/cosmonauts who spent prolonged periods in Earth orbiting space stations in zero gravity. $\endgroup$ – Fred Sep 30 '15 at 14:42
  • $\begingroup$ @Fred ~ 0 != 3.711 m/s² $\endgroup$ – TildalWave Sep 30 '15 at 15:23
  • $\begingroup$ @Fred yeah, the thing is microgravity effects are likely very different from reduced gravity effect. $\endgroup$ – kim holder Sep 30 '15 at 16:19
  • $\begingroup$ There must be some overlap though, right? $\endgroup$ – Premier Bromanov Sep 30 '15 at 18:26
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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.

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