How are humans affected by spending a long time in space? To find out, NASA turned to twin astronauts Scott and Mark Kelly. While Mark stayed on Earth, Scott spent a year aboard the International Space Station, returning in March 2016. He and Mark underwent a series of medical tests before and after the mission. The result? A year in space affected the composition of Scott’s DNA, slowed his reflexes, and even temporarily made him two inches taller. And the ends of Scott’s chromosomes—which typically shorten with age—actually lengthened, before returning to normal back on Earth. The study offers important clues about how humans would handle a Mars mission, which would take a minimum of 18 months and could happen by the 2030s. “We just don’t have [much data on] human beings in space for really long periods of time,” says John O’Meara, a physics professor at St. Michael’s College in Vermont. “Clearly Mars is the next horizon for NASA.”