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Study conducted by Nature magazine, with high-resolution CT revealed significant bone loss in the tibia of the crew members of the SpaceX Polaris Dawn and Fram2 missions after just 3 to 5 days in microgravity, with measurable trabecular deterioration and differences between men and women that surprised researchers.
The idea that space weakens bones is not new, but what a recent study has just demonstrated changes the understanding of the speed of this process. The 8 crew members of the Polaris Dawn and Fram2 missions from SpaceX returned to Earth with measurable signs of bone loss in the tibia after just 3 to 5 days in microgravity, a time frame that until now was considered too short to produce detectable skeletal changes. The data were obtained through high-resolution peripheral quantitative computed tomography, a technique that allows for the assessment of not only density but the entire internal microarchitecture of the bone.
The most unexpected result was the difference between the sexes. Male crew members showed more than double the trabecular deterioration observed in females, with an average bone density loss of 0.95% compared to 0.24% in the same period. Women, in turn, showed a greater increase in cortical porosity. The data suggest that the male skeleton may be more sensitive to the absence of mechanical load, while the female responds differently, with vulnerability concentrated in the outer layer of the bone. The study involved four men and four women and was published based on examinations conducted before and after the flights.
What high-resolution CT revealed about the crew members’ bones
The research team used HR-pQCT, an imaging modality that surpasses traditional bone densitometry by evaluating the three-dimensional microarchitecture of the cortical and trabecular compartments. The examinations performed on the tibia of the crew members showed significant reductions in total bone density and trabecular density, accompanied by a decrease in trabecular thickness and an increase in separation between them, clear indicators of structural weakening.
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In contrast, the distal radius, a forearm bone that does not support body weight, showed no significant changes. This pattern confirms what previous studies have indicated: bone loss in microgravity concentrates on bones that typically bear load, such as the tibia and the spine. The difference between the two skeletal sites reinforces the hypothesis that the absence of mechanical stimulus is one of the main factors behind the deterioration, although the exact mechanisms are still not fully understood.
Why men lost more bone than women in such a short time
The analysis separated by sex provided data that the researchers themselves classified as surprising. In the tibia, male crew members showed more than double the bone density loss recorded in women, along with more significant reductions in the fraction of trabecular bone volume and a greater increase in the separation between trabeculae. The estimated breaking load, which measures the bone’s resistance to fractures, decreased in men and showed a slight increase in women.
A possible explanation is that the male skeleton, being typically denser and supporting greater body weight, may be more responsive to both mechanical stimulus and its absence. The men in the sample were, on average, 14.9 kilograms heavier and 15 years older than the women, making it difficult to completely isolate the effect of sex from the effect of age. Women, although relatively protected in the trabecular compartment, showed an increase of 8.66% in cortical porosity, compared to 3.37% in men, suggesting that female cortical bone may be more susceptible to degradation under short-term exposure to microgravity.
What differentiated the Polaris Dawn and Fram2 missions and how that affected the results
The crew members of Polaris Dawn remained in orbit for almost five days on September 10, 2024, and returned on September 15, 2024, totaling 118 hours and 13 minutes, while the Fram2 team launched in March/April 2025 spent approximately three and a half days in space, with 86 hours and 33 minutes. Both missions used SpaceX’s Crew Dragon spacecraft, but followed different orbital trajectories: Polaris Dawn operated in a high-altitude elliptical orbit, while Fram2 operated in a polar orbit.
The exposure to radiation was drastically different between the two missions. The dosimeters recorded 1,378 mrem for Polaris Dawn and 149 mrem for Fram2, a difference of almost nine times. As radiation can compromise bone integrity, the combination of longer time in microgravity and greater exposure to radiation may have amplified bone loss in the crewmembers of Polaris Dawn. Researchers observed slightly more pronounced deterioration in this group, although the small sample size prevents definitive conclusions about the isolated contribution of each factor.
How the researchers discounted that bone loss was caused by natural aging
A methodological aspect significantly strengthened the study’s conclusions. The Polaris Dawn mission had originally been scheduled for September 2022 but was only launched in September 2024. This allowed the crewmembers to undergo two CT scans before the flight, spaced two years apart, creating a baseline that shows how their bones behaved under normal Earth conditions during that interval.
The results were clear: over the two years on Earth, no significant changes were detected in the density, microarchitecture, or bone strength of the crewmembers. The median variation in trabecular density before the flight was +0.28%, while after the space flight it dropped to -1.41%. This contrast proves that the changes observed after return cannot be attributed to aging but rather to exposure to microgravity, even for a period as short as three to five days.
What these results mean for the future of space travel
The main implication of the study is practical. If bone loss begins in days, and not weeks or months as previously thought, then short-duration missions can be used as testbeds to evaluate countermeasures before applying them to long journeys, such as a potential mission to Mars. The researchers suggest that future studies combine the CT protocol with exercises on just one limb during flight, allowing for internal comparisons within the same individual.
The crew of Fram2, for example, performed 30 minutes of resistance band exercises during the mission aboard SpaceX’s Crew Dragon, while the Polaris Dawn team participated in a blood flow restriction experiment without simultaneous exercise. The ability to test interventions on short missions accelerates the development of solutions and reduces reliance on long missions to validate each new approach. Researchers acknowledge, however, that the sample size is a limitation and that future studies with larger groups and gender balance will be essential to confirm the observed patterns.
Did you know that just a few days in space is enough for bones to start weakening, or did you think it took months? Tell us in the comments what you think about the physical risks of space travel and whether you believe science will find a solution before we send humans to Mars.
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