As Sunita Williams and Barry "Butch" Wilmore prepare to return after nine months in microgravity, all eyes are on how their bodies will handle Earth’s gravity again.
The European Space Agency (ESA) confirmed the aging effect in space is minuscule but real. “Funny you should say that... astronauts actually age more slowly in orbit than on Earth (because of the Station's speed and time dilation due to special relativity) BUT the effect is very small—after spending six months on the ISS, astronauts have aged about 0.005 seconds less than the rest of us,” ESA stated on X (formerly Twitter). This might remind you of Interstellar, but it's real — astronauts do age more slowly in space.
Beyond aging, space travel triggers other biological changes. NASA’s famous twins study compared astronaut Scott Kelly, who spent a year in space, to his identical twin, Mark Kelly, who remained on Earth. The study revealed that Scott experienced shifts in gene expression, thickening of his carotid artery, and changes in telomere length — structures that protect chromosomes and are linked to aging. While some of these changes reversed after returning to Earth, a few persisted for months.
Microgravity also leads to muscle atrophy, with astronauts losing up to 20% of their muscle mass in missions as short as five days. Long-duration space travel has been linked to vision issues, caused by increased intracranial pressure, and immune system changes due to the sterile space environment.
To track these effects, astronauts undergo extensive medical tests upon return. These include:
As Williams and Wilmore prepare for reentry, their journey back to normalcy will extend well beyond their landing. Space may slow aging, but the body’s adjustment to Earth’s gravity is a challenge of its own.