Lost in space: Astronauts struggle to regain bone density

Astronauts lose decades' worth of bone mass in space that many do not recover even after a year back on Earth, researchers said on June 30, warning that it could be a "big concern" for future missions to Mars.

Previous research has shown astronauts lose between one to two percent of bone density for every month spent in space, as the lack of gravity takes the pressure off their legs when it comes to standing and walking.

To find out how astronauts recover once their feet are back on the ground, a new study scanned the wrists and ankles of 17 astronauts before, during and after a stay on the International Space Station.

The bone density lost by astronauts was equivalent to how much they would shed in several decades if they were back on Earth, said study co-author Steven Boyd of Canada's University of Calgary and director of the McCaig Institute for Bone and Joint Health.

The researchers found that the shinbone density of nine of the astronauts had not fully recovered after a year on Earth and were still lacking around a decade's worth of bone mass.

The astronauts who went on the longest missions, which ranged from four to seven months on the ISS, were the slowest to recover.

"The longer you spend in space, the more bone you lose," Boyd told AFP.

Boyd said it is a "big concern" for planned for future missions to Mars, which could see astronauts spend years in space.

"Will it continue to get worse over time or not? We don't know," he said.

"It's possible we hit a steady state after a while, or it's possible that we continue to lose bone. But I can't imagine that we'd continue to lose it until there's nothing left."

A 2020 modelling study predicted that over a three-year spaceflight to Mars, 33...