Science Puzzle
Why Do Astronauts Grow Taller in Space?
Astronauts who spend months on the International Space Station come home up to 3% taller: for an adult, several centimetres. After almost a year in orbit, NASA astronaut Scott Kelly returned about 5 cm taller than when he left.
The extra height doesn't last. Within days of landing, they shrink back to their old size. What's going on?
The Answer
Their spines stretch out. Without gravity squashing it, the backbone relaxes and lengthens.
Your spine is a stack of bones called vertebrae, with a spongy disc between each pair. The discs are tough rings around a soft, water-rich core, and they act as shock absorbers. On Earth, gravity presses down on them all day while you stand and sit, slowly squeezing out a little fluid. Lie down at night and the pressure comes off, so they soak the fluid back up.
That's why you're slightly taller first thing in the morning than at bedtime, often by around a centimetre. Try measuring yourself at both times.
In orbit nothing presses down at all. The discs swell and the spine's gentle S-shaped curve straightens a little, so astronauts gain height day after day for weeks. It isn't all good news: many astronauts get backache in space, and their discs and back muscles need careful exercise and recovery after they return. Back on Earth, gravity squashes them down to their old height again.
The principle: Gravity and the body. Your skeleton is shaped by the constant load of gravity; take the load away and soft tissues such as the spinal discs expand.