Long space missions linked to higher hip fracture risk in astronauts, study finds

Astronauts who spend more than 90 days in space face a higher rate of hip fractures after they return to Earth than astronauts who flew shorter missions or than people who have never left the planet, according to a study published Thursday in Mayo Clinic Proceedings. The research, led by Jean D. Sibonga of NASA's Human Research Program at the Johnson Space Center, is among the first to connect long-duration spaceflight directly to a measurable rise in a specific, serious fracture rather than to bone density loss alone.

The paper, titled "Increased Rates of Hip Fractures Associated With Longer Spaceflight Durations," surveyed a cohort of United States-based astronauts for fractures reported during their annual clinical exams and analyzed the results using Bayesian probabilistic modeling, a statistical method suited to small sample sizes. The astronaut corps numbers only in the hundreds, and long-duration missions remain comparatively rare, which has long complicated efforts to draw firm conclusions about rare outcomes such as fractures.

The numbers

The researchers did not find an elevated rate for fractures overall. What they did find was narrower and, in clinical terms, more concerning: the incidence rate for hip fractures specifically rose after missions longer than 90 days, compared with the rate after shorter flights or with no spaceflight exposure at all. Those hip fractures also tended to occur at a younger age than would be expected in a non-astronaut population on Earth.

• No increase was detected in the overall fracture rate across all bone types.
• Hip fracture incidence rose specifically after spaceflights exceeding 90 days.
• Affected astronauts were younger, on average, than typical hip-fracture patients on Earth.
• NASA has begun adding pre- and post-flight quantitative CT scans to its existing bone-density screening.

How bone loss happens in orbit

Muscle and bone loss in microgravity is a well-documented physiological response to disuse: without the constant mechanical loading that gravity provides on Earth, the skeleton sheds mass, particularly in weight-bearing regions such as the hip. For decades NASA has tracked this loss mainly through dual-energy X-ray absorptiometry, or DXA, which measures bone mineral density but says relatively little about the three-dimensional structure and strength of bone. The new paper argues that density measurements alone have understated how much the mechanical competence of the hip can be compromised after a long flight, and that the gap between feeling recovered and being structurally recovered is where the danger lies.

An accompanying editorial, "Bone Loss in Astronauts," was co-written by Matthew T. Drake, who chairs the Endocrinology and Metabolic Bone Disease Service at the Hospital for Special Surgery in New York after a decade leading the equivalent group at the Mayo Clinic, and his HSS colleague Moshe Gertzulin. Drake, who was not involved in the underlying study, said the finding fits a pattern researchers have struggled to pin down with confidence. "While data to date on bone loss associated with space flight are consistent, a major limitation is accuracy, as astronaut cohort sample sizes are small and spaceflights are rare," he said.

Who is affected

The immediate population is small: the pool of people who have flown spaceflights longer than 90 days is limited to NASA, partner-agency and a handful of commercial astronauts, most of whom have served aboard the International Space Station. But the stakes are rising. NASA is planning a growing number of missions longer than six months, including expanded commercial activity in low Earth orbit, the Artemis IV mission to land astronauts near the lunar south pole in 2028, and the buildup of sustained lunar surface operations that the agency has described as a proving ground for an eventual Mars expedition. Each of those missions will keep crew members in microgravity, or in the roughly one-sixth gravity of the moon, for longer stretches than most of the astronauts in the current study experienced.

"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 of the Hospital for Special Surgery.

The study's authors frame the risk as partly behavioral. Returning astronauts often feel stable within weeks of landing and resume demanding physical routines, including the resistive exercise regimens NASA prescribes as a countermeasure to bone and muscle loss, before imaging confirms that their skeleton has regained its preflight strength. The researchers argue that this mismatch between how astronauts feel and what their bones can actually bear may be contributing to fractures that occur earlier in life than would otherwise be expected.

What happens next

Sibonga's team is pressing NASA to widen its post-flight bone surveillance beyond standard DXA scans. The agency has already begun ordering quantitative computed tomography scans before and after missions, a technique that can characterize different regions of bone and flag whether a given astronaut has actually recovered preflight strength, rather than simply regained average density. The study's authors are also calling for prophylactic countermeasures, administered before fractures become likely rather than after, that could include medication, modified exercise protocols or dietary changes tailored to preserve an astronaut's baseline skeletal health going into a mission.

The authors acknowledge that a definitive fix will require a fuller characterization of exactly how spaceflight reshapes bone mass, density, structure and microarchitecture, work that is ongoing within NASA's Human Research Program. Additional details on the research were reported by Phys.org and distributed more broadly through a publisher summary carried by Mirage News. With NASA's timeline for lunar and eventual Mars missions pushing crews toward longer stays away from Earth's gravity, the question the study leaves open is how quickly the agency's screening and countermeasure programs can catch up to the duration of the flights it is now planning.