Astronauts break hips younger and more often after spaceflights longer than 90 days

Astronaut bone research has a denominator problem. "While data to date on bone loss associated with spaceflight are consistent, a major limitation is accuracy, as astronaut cohort sample sizes are small and spaceflights are rare," said Matthew T. Drake of the Hospital for Special Surgery in New York, a co-author of the editorial that accompanies the paper ^[6][14]. The team, led by Jean D. Sibonga at NASA's Johnson Space Center, surveyed a cohort of U.S.-based astronauts about every fracture reported at their annual clinical exams and analyzed the counts with Bayesian probabilistic modeling ^[2][3].

The controls are what I find most persuasive. Astronauts are chosen for physical fitness and optimal health ^[12], so a comparison with the general public alone would be stacked against finding any excess. The study also compared the same people before a long-duration mission, and astronauts who never flew ^[15]. Those two references hold the selection roughly constant, and the hip signal held against both. The null result for fractures overall ^[4] keeps the claim to one site.

The release does not give the cohort size, the number of hip fractures or the size of the rate difference, and it does not say whether risk keeps rising past the 90-day line ^[1]. The editorialists still read the result causally. "Taken together, the finding of hip fracture occurrence at an age earlier than expected after long-duration spaceflight is clinical evidence that spaceflight-induced bone loss can lead to real long-term fracture consequences, which should be addressed accordingly," said Moshe Gertzulin, another co-author of the editorial at the Hospital for Special Surgery ^[7][14]. I think the design supports a confident association. Drake's point about accuracy bears hardest on how large the risk is ^[6].

People do not feel bone loss, Sibonga noted. Astronauts who feel stable or recovered after landing may go back to physically demanding activities on a skeleton still altered by prolonged disuse ^[8]. NASA's bone surveillance has relied on dual-energy X-ray absorptiometry measurements of bone mineral density, and Sibonga wants the program expanded past them ^[9]. Quantitative CT scans taken before and after flight show different areas of bone in more detail and can indicate whether an astronaut has fully recovered ^[10]. Without that information, according to the release, astronauts could put too much strain on bones that have not regained their preflight condition ^[16].

As the release describes NASA's plans, several missions will run longer than six months, including commercial low Earth orbit activity, Artemis IV's first human mission to the lunar south pole in 2028, and lunar surface capabilities built in preparation for Mars ^[11]. Six months is about 180 days, roughly double this study's threshold ^[1]. The investigators want preventive measures that hold astronauts at their preflight skeletal baseline. They say fuller measurement of bone mass, density, structure and microarchitecture will show whether drugs, exercise or dietary changes help most ^[13].