The peculiar circumstances of life aboard the International Space Station both advanced and retarded astronaut Scott Kelly’s age relative to that of his identical twin brother Mark, who remained on the ground.

Radiation, weightlessness, and changes in diet shortened Scott’s telomeres more quickly than his brother’s, effectively causing him to age more quickly.

At the same time, due to relativistic effects, Scott aged about 8.6 milliseconds less than Mark during his year in space.