On Earth, gravity is constantly squeezing your skeleton downward - most noticeably your spine, which has cushioning discs of cartilage between each vertebra. In space, with no gravity to compress those discs, they spread out slightly. After several months on the International Space Station, astronauts grow up to 2 inches taller.
It’s not just height. The whole body changes in zero-g. Fluids shift upward, making faces look puffy. Muscles weaken because they don’t have to fight gravity. Bones lose minerals at a rate that worries doctors - losing about 1% of their density every month if astronauts don’t exercise.
The good news is that most of these changes reverse when astronauts return to Earth. The spine compresses back down within a few weeks, so the extra height disappears. Bones and muscles rebuild gradually, though some changes - particularly bone loss - can take months or years to fully recover, and may never quite return to pre-flight levels.
