Nearly a year in space put astronaut Scott Kelly's immune system on high alert and changed the activity of some of his genes compared to his Earth-bound identical twin, researchers have found.
Scientists don't know if the changes were good or bad but results from a unique Nasa twins study are raising new questions for doctors as the space agency aims to send people to Mars.
Tests of the genetic doubles gave scientists the first opportunity to track details of human biology, such as how an astronaut's genes turn on and off in space differently. One puzzling change that was announced at a science conference: Kelly's immune system was hyperactivated.
"It's as if the body is reacting to this alien environment sort of like you would a mysterious organism being inside you," said geneticist Christopher Mason of New York's Weill Cornell Medicine, who helped lead the study. He said doctors were now looking for that in other astronauts.
Since the beginning of space exploration, Nasa has studied the toll on astronauts' bodies, such as bone loss that requires exercise to counter. Typically they're in space about six months at a time. Kelly, who lived on the International Space Station, spent 340 days in space and set a US record.