Scott Kelly is the first American to spend almost a year in space. The NASA astronaut lived for a record 340 days onboard the International Space Station (ISS) from 2015 to 2016. Like other astronauts, he endured the stresses of microgravity, cosmic radiation and “headward fluid shift,” in which blood and tissue fluid collect in the head. But Kelly’s experience was unique in that researchers painstakingly documented his physiology and cognitive performance while in orbit—and simultaneously monitored his identical twin brother, Mark Kelly, as an earthbound control.