Spaceflight stretches the human body. In orbit, astronauts often measure up to 2 inches taller than they did on the ground, only to return to their original height soon after landing.
The key lies in the spine. On Earth, gravity constantly loads the vertebral column, compressing the intervertebral discs that sit between each vertebra. These discs, made of cartilage and water-rich gel, act as shock absorbers. Under continuous axial load, fluid is squeezed out and the spine shortens slightly over the course of a normal day.
In microgravity, that axial load almost disappears. Without sustained compression, fluid gradually moves back into the intervertebral discs by osmosis, and the discs increase in volume. The spinal column elongates, the natural curves of the spine flatten somewhat, and total body height can increase by as much as about 2 inches during a mission.
The effect reverses when astronauts return to Earth. Gravity reintroduces constant mechanical stress, restoring spinal compression and driving fluid out of the discs again. Over a relatively short period, the vertebral column settles back toward its previous length, and measured height returns to preflight values as the body readapts to the familiar pull of Earth.
