Astronauts Show Measurable Bone Changes After Just 3-5 Days in Space, Study Finds

Astronauts exercise in microgravity to help counter changes to muscles and bones during spaceflight.
Space missions have an impact on human physiology. Image credit: [ESA]

Space missions have an impact on human physiology. The absence of gravity leads to changes in bone structures, and longer missions cause known bone loss. However, how quick this process begin?

According to a 2026 study involving 8 astronauts, the process might be very fast. Published in the journal npj Microgravity, the article explains how 4 women and 4 men were under research for 3 to 5 days during the Fram2 and Polaris Dawn missions. The bone structures were studied with the help of high-level scanners at the lower leg and forearm before and after flight.

The tibia, a weight-bearing bone, showed the clearest changes. Total bone mineral density fell by a median of 0.42%, while trabecular bone mineral density fell by 0.88%. The trabecular thickness also decreased by 0.70%, and the spaces between these internal bone structures became 0.61% wider.

The results need to be taken with caution. The study does not show that astronauts' bones become dangerously weak within just a few days. What the researchers did find was that some changes in bone structure had already started to appear. At the same time, estimated bone strength remained largely unchanged, suggesting that these early changes may be part of how the skeleton responds to the space environment.

The small sample makes it difficult to draw conclusions about differences between men and women. There were only 4 participants of each sex, and all of the male participants were older than the female participants. This makes it difficult to separate the effects of age from those of sex.

One of the important things about this research is the timing of the study. The ability to measure changing bones after only a few days makes it possible for the researchers to investigate the beginning of the process of bone adaptation and to find ways of protection for astronauts during the long missions.

More research is needed to see what happens to these changes over time, whether they fade away or become more significant.

Reference:
Matheson, B. E., Walle, M., Bugbird, A. R., et al. (2026). "Early skeletal deteriorations following short-duration spaceflight." npj Microgravity 12, 28.