Astronauts typically grow about 1 to 2 inches (2.5 to 5 cm) taller during spaceflight. The change happens because gravity is no longer compressing the spine, allowing the cushioning discs between vertebrae to expand and absorb extra fluid. It’s a temporary effect that reverses within a few months of returning to Earth.
Why the Spine Stretches in Microgravity
On Earth, your spine goes through a daily cycle of compression and expansion. When you’re upright during the day, gravity pushes down on the discs between your vertebrae, squeezing water out of them. At night, when you lie down, the load lifts and the discs rehydrate and swell. This is why you’re actually slightly taller in the morning than you are before bed, usually by about half an inch.
In space, that daily cycle breaks down. Without gravity constantly compressing the spine during waking hours, the discs keep absorbing water with nothing to push it back out. They swell beyond their normal daytime size and stay that way. The vertebrae themselves also spread apart slightly as the muscles and ligaments supporting the spine relax. The combined result is a spine that’s measurably longer than it was on the ground.
How the Growth Affects Spacesuits and Spacecraft
An extra inch or two might not sound like much, but it creates real engineering problems. NASA adds an arbitrary 1-inch (2.54 cm) adjustment to spacesuit torso length to account for spinal elongation, because some crew members have had trouble fitting into their suits after spending time in microgravity. The issue extends to spacecraft seating as well. In the Orion capsule, interior dimensions are fixed, so engineers have to carefully calculate how much headroom each crew member needs after growing taller in orbit. In one configuration, when a tall astronaut occupied the top seat, the person in the bottom seat had to be shorter than the 99th percentile for seated height once spinal elongation was factored in. Without the growth effect, any crew member could have fit in that bottom seat, gaining an extra 2.6 inches (6.6 cm) of clearance.
Back Pain and Disc Herniation Risks
The spinal stretching isn’t just a quirky fact. It frequently causes moderate to severe lower back pain while astronauts are in space. The swelling discs lose their normal wedge shape, which flattens the spine’s natural inward curve in the lower back (the lordotic curve). That flattening puts extra strain on the back wall of each disc, the area most vulnerable to bulging or tearing.
The consequences carry over after landing. Astronauts are three times more likely to herniate a disc in their lower back compared to people who haven’t been in space. A longitudinal study tracking 12 NASA astronauts before and after roughly six-month missions found evidence of disc changes at multiple checkpoints: within a week of landing and again one to two months later. The spine also loses flexibility in microgravity, with a measurable decrease in range of motion that persists during the recovery period.
How Long the Height Gain Lasts
The extra height begins to disappear as soon as gravity starts compressing the spine again. Within the first days back on Earth, astronauts lose a noticeable portion of the gain. Full reversal to pre-flight height takes a few months as the discs gradually shed excess fluid and return to their normal hydration cycle. This timeline is roughly the same regardless of whether the mission lasted a few weeks or six months, though longer missions are associated with greater disc changes and a longer adjustment period for spinal health overall.
The growth in space is not actual bone growth or permanent change in stature. No new tissue is created. It’s purely a mechanical expansion of soft tissue between existing bones, which is why it’s completely reversible. For comparison, the same basic process happens to you every night in bed. Space just removes the reset button.