Astronaut Grows 3.5 Inches in Space—May Not Fit in Spacecraft Seat for Ride Home

Japanese astronaut Norishige Kanai preparing to launch in December 2017. Kirill Kudryavtsev/AFP/Getty Images

Japanese astronaut Norishige Kanai tweeted on Monday that according to new body measurements taken earlier in the day, he has grown 9 centimeters, or about 3.5 inches, since he arrived in space three weeks ago. "I've only been in space for three weeks and haven't grown like this since middle/high school," he wrote. He joked that he's worried about fitting into his seat on the Soyuz capsule for the ride back to Earth.

Astronauts regularly grow in space—often as much as 2 inches—thanks to the microgravity experienced on the International Space Station. On Earth, the full force of gravity shapes our bodies during a day of standing or sitting, gently pushing down on the spinal column and shrinking our overall height. Overnight, as we sleep horizontally, the pressure of gravity on our spines weakens and we stretch out again, just a little bit. But in microgravity, a much more dramatic version of that overnight stretching takes place.

That phenomenon is why NASA has a long history of using bed rest studies here on Earth to understand risks astronauts face in space. These studies can entail spending more than two months entirely in bed—even to shower—with their heads slightly below their bodies. Other changes to the body caused by microgravity include loss of bone minerals, loss of muscle strength and changes in eye structure.

Kanai, who originally trained as a surgeon, joined the Japanese space agency in 2009 and became a certified astronaut in 2011. This is his first visit to the International Space Station, where he is serving as flight engineer in charge of ISS operations. He launched on December 17 with returning Russian astronaut Anton Shkaplerov and rookie NASA astronaut Scott Tingle. The trio is due to come home in April.

Read more: International Space Station: Tiny Bits of Space Junk Could Threaten ISS—This is How it is Being Protected

Height has always been a sticking point for astronauts. In 1959, when NASA began recruiting its very first class, the men who would become the seven Mercury astronauts, one of the key criteria was that each was shorter than 5 feet, 11 inches.

While the capsules astronauts used have grown a little over the decades, height is still a factor. The Soyuz capsules that are currently astronauts' exclusive rides to space can fit people up to 6 feet, 3 inches tall—three inches more than the previous model. Kanai's height isn't publicly available information, but he does appear to be the tallest of the three astronauts who launched last month.