Bone loss fear may rule out manned Mars mission after NASA tests indicate prolonged low gravity exposure untenable

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Medical tests indicate that prolonged exposure to low gravity would be untenable 

NASA is studying better ways for astronauts to exercise in space after long-running medical tests on International Space Station (ISS) crewmembers revealed significant bone loss rates.

Tests indicate bone loss rates of up to 1.5% a month, "or about as much as a post-menopausal woman loses per year", says Julie Robinson, acting ISS programme scientist at NASA Johnson Space Center. The results mean that, unless bone loss rates can be countered, a prolonged Mars mission would be virtually untenable for humans.

Results of more recent follow-up tests on crew who have had a year or more back on Earth after an ISS mission showed that bone material does rebuild, but in different places from where it was originally lost. In normal gravity, bones are constantly maintained by being broken down during regular activity and then refurbished.

Although bone loss in astronauts has been known about since the SkyLab missions of the 1970s, it is only since the advent of the ISS that the first serious experiments into the phenomenon have been possible. These have since taken on more urgency with the decision to return to the Moon and go to Mars. A transit from Earth to Mars is expected to be the equivalent of a typical six-month ISS mission.

The measured monthly losses were 1.4% on the femoral neck bone at the top of the thigh and 0.9% in the lumbar spine, creating the threat of a dangerous fracture "if you fall sideways", says Robinson, who adds that such an event would be a calamity on a Mars mission. In addition, she says bone loss adds calcium to the bloodstream, which can lead to higher risk of kidney stones, another hazard on a long-term mission. "So this shows we need a rework for the exercise hardware we'll take to Mars," she says.

Speaking at the American Institute for Aeronautics and Astronautics aerospace sciences meeting in Reno, Nevada, Robinson said: "Now, with our best efforts, the exercise can only give around 60% of the forces experienced on Earth while walking. They're essentially 'couch potatoes' when they're up there." Revised hardware is likely to include improved harnesses to tie astronauts more tightly to the exercise treadmill, she adds.