Astronauts who spent more than 90 days in space had higher rates of hip fractures after returning to Earth, according to a study in Mayo Clinic Proceedings. The risk was higher than after shorter flights or in non-astronauts.

The research, led by Jean D. Sibonga of NASA’s Johnson Space Center, analysed fracture records of US-based astronauts using Bayesian modelling. The fractures also occurred at younger ages than expected.

Microgravity causes muscle and bone loss, a well-known risk of long space missions. Sibonga noted that people cannot feel bone loss. After returning and feeling recovered, astronauts may resume demanding activities despite hidden skeletal changes.

The findings matter now because NASA plans more missions lasting over six months. They include commercial low Earth orbit activity and Artemis IV, the first crewed lunar south pole mission in 2028, on the road to Mars.

Researchers say the study underlines the need to expand long-term skeletal health monitoring for astronauts as humans spend more time off Earth.