As a result, the astronauts lost as much bone tissue as would have disappeared in a few decades of life.
The authors found that nine astronauts hadthe density of the tibias had not fully recovered even after a year on Earth - and they still lacked the equivalent of about ten years of bone mass.
Astronauts who went on the longest missions — four to seven months — recovered the slowest.
Boyd said this is a big problem for future missions to Mars, which could see astronauts spend years in space.
A simulation run in 2020 predicted that during a three-year space flight to Mars, 33% of astronauts would be at risk of developing osteoporosis.
The new study, published in the journal Scientific Reports, also showed how spaceflight changes the structure of the bones themselves.
Human bones in this state are similar to the structure of the Eiffel Tower, from which several connecting metal rods were pulled out.
“And when we return to Earth, the whole structure is compacted, but no new rods appear,” he said.
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