The Partial Gravity of the Moon and Mars Appears Insufficient to Maintain Human Health

Astronauts who spend many weeks or months in space in microgravity suffer serious
health problems including muscle atrophy, cardiovascular deconditioning, bone calcium loss,
impaired vision, and immune system changes. The debilitating effects of weightlessness were
first demonstrated on the early Skylab, Salyut, and Mir missions, but it was then hoped that
countermeasures including in-flight exercise and resistance training could reduce most of
these problems. Similar effects are anticipated in the partial gravity of the Moon and Mars.
Direct evidence of the long-term effects of partial gravity on humans is not yet available, but
indirect evidence suggests that partial gravity exposure below 0.4 g will be insufficient to
maintain musculoskeletal and cardiopulmonary conditioning over the long term. Some studies
show a strong correlation between heart rate, oxygen consumption, net metabolic rate, and
simulated gravity from 0 to 1 g. Exposure to moon and Mars gravities will probably cause less
severe physiological deconditioning than microgravity, but the benefit of partial gravity seems
likely to be roughly proportional to the level of gravity experienced. As in microgravity,
exercise countermeasures seem useful but insufficient to preserve all physiological systems as
they would be in Earth gravity.