Lifestyle Health Astronaut Scott Kelly's DNA No Longer Matches Identical Twin's After a Year in Space Astronaut Scott Kelly spent a year in space and his DNA changed 7 percent — no longer matching that of his identical twin, Mark By Julie Mazziotta Julie Mazziotta Twitter Associate Editor, PEOPLE Health People Editorial Guidelines Published on March 15, 2018 10:15 AM Share Tweet Pin Email After a year spent in space, former astronaut Scott Kelly’s DNA no longer matches that of his identical twin, Mark. Scott, 54, returned in March 2016 from a year aboard the International Space Station — but 7 percent of his genes had changed. In the two years since, they have not returned to normal, NASA says. Scott’s 340-day stay was longer than NASA’s typical six-month deployments to space, and while it is known that a person’s genes change away from Earth, they did not expect his genes to stay altered. NASA says the 7 percent DNA changes, which they call “space genes,” bring up “possible longer term changes in genes related to his immune system, DNA repair, bone formation networks, hypoxia, and hypercapnia.” KIRILL KUDRYAVTSEV/AFP/Getty Scott Kelly As identical twins, Scott and Mark, who is also a former astronaut, were purposefully studied before, during and after Scott’s mission to research the changes that occur in space. Steve Jennings/WireImage (L-R) Scott and Mark Kelly Scott tweeted on Saturday that he wasn’t aware of his “space genes” until reading about it in a Newsweek article. “What? My DNA changed by 7%! Who knew? I just learned about it in this article,” he tweeted, joking: “This could be good news! I no longer have to call @ShuttleCDRKelly my identical twin brother anymore.” NASA says these findings will aide in their preparations for a three-year mission to Mars, as they figure out the body changes that will occur when humans spend over 1,000 days in space.