Long space missions may leave astronauts with weaker hips: Study links 90-plus-day flights to higher fracture rates

Space may be weightless, but the effects of spending months in microgravity can follow astronauts back to Earth. A new study has found that astronauts who spent more than 90 days in space had a higher rate of hip fractures after returning to Earth compared with periods following shorter spaceflights and with astronauts who had not experienced spaceflight. The research, published in Mayo Clinic Proceedings, points to a potential long-term consequence of spaceflight-induced bone loss. The finding comes as space agencies prepare for missions that will keep humans away from Earth for increasingly long periods, including future lunar and Mars exploration. Why does spaceflight affect astronauts' bones? The human skeleton is built to work against Earth's gravity. In space, astronauts live in microgravity, dramatically reducing the mechanical load placed on their bones. This causes bone and muscle loss as the body adapts to prolonged disuse. Importantly, astronauts may not feel this loss happening. Jean D. Sibonga, the study's lead investigator at NASA's Johnson Space Center, said astronauts may feel stable after returning to Earth and resume physically demanding activities even when their skeleton has not completely recovered from changes caused by prolonged disuse. Earlier NASA research has also found that some astronauts experience persistent loss of bone tissue after long-duration missions. More detailed imaging, including quantitative computed tomography (QCT), can detect changes that may not be captured by conventional bone-density measurements. What did the new astronaut fracture study find? The researchers examined fracture records from US-based astronauts and analysed the data using Bayesian probabilistic modelling, a statistical approach that can help deal with uncertainty when studying relatively small and unusual populations. The researchers did not find an increased incidence rate for all types of fractures. However, they found that the rate of hip fractures was higher after long-duration spaceflight -- defined in the study as more than 90 days -- than after shorter missions. The hip fractures also occurred at a younger age than would normally be expected in the general population. The researchers compared fracture data within the astronaut population, including periods before and after long-duration missions, rather than simply comparing astronauts with the general public. This was important because astronauts are already a highly selected group in terms of health and physical fitness. The study's findings build on earlier NASA analyses that had also identified a relationship between longer spaceflight exposure and increased hip and spine fracture rates. The problem could become bigger as missions get longer The timing of the research is significant because NASA and other space agencies are preparing for missions that could keep astronauts away from Earth for much longer than traditional missions. NASA's plans include missions lasting more than six months, along with lunar exploration and preparations for future human missions to Mars. That raises a key medical question: what happens when astronauts with partially recovered bones have to operate in environments where medical help is far away? A fracture on or near the Moon could be difficult to manage. A serious skeletal injury during a Mars mission would pose an even greater challenge because astronauts would not have the option of quickly returning to Earth. Why regular bone scans may not tell the whole story NASA currently monitors astronauts' bone health using dual-energy X-ray absorptiometry (DXA) to measure bone mineral density. But researchers say DXA alone may not provide a complete picture of fracture risk. NASA has begun using quantitative computed tomography (QCT) scans before and after spaceflight to examine the structure of bones in greater detail and determine whether astronauts have fully recovered. This matters because astronauts could otherwise return to normal physical activity while some parts of their skeleton remain weaker than before the mission. Research published in Osteoporosis International in 2026 similarly found that QCT can detect persistent loss of hip trabecular bone that may not be fully captured by conventional DXA measurements. Can astronauts prevent bone loss? The researchers say the next step is to develop better ways of preserving an astronaut's skeletal health before, during and after a long-duration mission. Potential countermeasures include carefully designed exercise programmes, dietary interventions and medications. The aim is to prevent astronauts from losing too much bone during spaceflight in the first place, rather than trying to deal with the consequences after they return. NASA is already studying potential treatments for microgravity-associated bone loss. One NASA investigation is examining whether targeting biological pathways involved in bone formation and breakdown could help protect astronauts during long missions. What does this mean for future Moon and Mars missions? The study does not mean that astronauts cannot safely undertake long-duration missions. Instead, it highlights a health risk that becomes increasingly important as missions get longer. For astronauts heading to the Moon or eventually Mars, maintaining skeletal strength could be just as important as maintaining cardiovascular fitness or muscle strength. As humans spend longer periods away from Earth's gravity, scientists will need to understand not only how much bone astronauts lose in space, but also how completely and how quickly their skeletons recover after returning to gravity. The study, "Increased Rates of Hip Fractures Associated With Longer Spaceflight Durations," was published in Mayo Clinic Proceedings.