Why do astronauts grow taller in space?

Astronauts grow taller in space because gravity stops squashing their spines. On Earth, your weight constantly presses down on your backbone, a stack of vertebrae separated by cushioning discs, and holds it in its natural curve. In orbit, that compression disappears. The spine straightens out and lengthens, and fluid may shift into the discs between the vertebrae. The result: during long missions on the International Space Station, crew members grow up to 3% taller.

It happens fast. Within about a month of weightlessness the skeleton is fully extended, adding roughly 2.5 centimetres. And it doesn't last: back on Earth, gravity takes over again and astronauts return to their normal height, often with back pain while their bodies readjust.

That extra height is a real engineering problem. A spacecraft's interior can't simply be stretched, so when NASA designed the seats of its Orion capsule, it had to build in extra clearance for spinal growth, to make sure crews could still fit into their seats and strap in for the ride home. Astronauts have even scanned each other's spines with portable ultrasound on the space station to understand exactly what is going on.