Why astronauts get taller

Your spine isn't one solid bone. Between the vertebrae are discs that are more than half water. As you stand, sit, and walk, your body weight presses down on them, and some fluid gradually leaves. By evening, you're a little shorter than you were in the morning: at night, when your spine is lying down without a load, the discs absorb water again and decompress. The difference is about 1-1.5 cm, and you usually don't notice it.

In weightlessness, that load is completely gone - day and night. The discs swell to their limit, and the spine's natural curves - the inward curves in your lower back and neck - partially straighten. Your body stretches out like a string. NASA reports a gain of up to 3% of height, and almost all of it happens in the first 2-3 days in orbit. This isn't growth in the usual sense: bone length doesn't change by a single millimeter.

Stretching out comes at a cost. Muscles and ligaments that have held your spine in its usual shape for years become stretched, while nerve roots end up in an unfamiliar position. That's why many astronauts complain of nagging lower back pain during the first days of a flight. It's not an injury: your body has become longer than it's used to, and your muscles are holding it in a new geometry.

It all reverses just as quickly. After landing, body weight returns, the discs release excess water within 1-2 days, and your height goes back to normal. Something else is more concerning: in the first year after long missions, herniated discs are found more often than usual. There are few crew members and the samples are small, so exactly how much higher the risk is remains debated, but the pattern itself is seen consistently.

Engineers plan for this in advance: seat liners and spacesuits are made with extra length, and spacecraft have height limits for crew members - someone who stretches out in orbit has to return to the same seat she launched in.