One twin went to space, and his telomeres grew

Ten research teams from twelve universities, coordinated by NASA and including Weill Cornell Medicine, tracked both brothers' bodies before, during and after the mission to see what spaceflight does to human biology when genetics are held constant.

Scott's telomeres, the protective caps on chromosome ends that usually shorten with age, lengthened during the mission, the opposite of what the team expected. His gene expression, gut microbiome and DNA methylation all shifted in space. Most of these changes reversed within six months of landing, but some gene expression changes and extra short telomeres persisted.

The experiment could not separate microgravity, radiation and isolation from each other, since all three happened at once aboard the station, and it involved one pair of twins, so none of it generalizes statistically on its own. It still stands as the most detailed single-subject map of what a year away from Earth does to a human body, shaping health monitoring on NASA's later missions.

One year in orbit, written into one body.