Why do astronauts grow taller in space?

On Earth, the weight of the upper body constantly compresses the intervertebral discs, those cartilage cushions filled with water wedged between the vertebrae. The volume of these discs actually varies throughout the day, between lying down asleep and standing upright. In orbit, that load disappears all at once: the discs are no longer compressed and the spine's natural curves, particularly the lumbar lordosis, flatten out. The spine then lengthens by a few centimeters.

This height gain is not a free bonus. The deep muscles that stabilize the trunk, constantly engaged against gravity, waste away quickly, and crews report back pain within the first few days of flight. Upon return, during the re-compression phase, flight surgeons closely monitor the risk of herniated discs. This is one of the reasons agencies require a daily exercise program on board, including a harnessed treadmill, as much for the back and postural muscles as for bone health.

The lengthening forces engineers to work with a body that changes dimensions: the seats of return spacecraft must leave room for the height gained in flight, or else crew members risk discomfort or compression during re-entry. On board, the spine is monitored by ultrasound to validate countermeasures. Fortunately, the phenomenon remains reversible: once back under gravity, the body re-compresses and returns to its pre-flight height, although the back sometimes takes longer to recover.