Space isn’t a friendly place—just ask astronaut Scott Kelly. He spent 340 days aboard the International Space Station (ISS), orbiting high above Earth. During that time, his body experienced significant changes scientists are just starting to understand.
Before, during, and after his spaceflight, Scott underwent a series of tests. So did his identical twin brother, Mark Kelly, back on Earth. The pair share the exact same DNA—molecules that carry hereditary information. That allowed scientists to run tests on the brothers’ bodies to determine how living in space affected Scott.
Researchers found that the activity level of about 1,400 of Scott’s genes, or units of hereditary material, changed. That was likely because of exposure to greater levels of high-energy radiation in space, says Christopher Mason. He’s a geneticist at Weill Cornell Medicine in New York City. Thankfully, says Mason, “more than 90 percent of Scott’s changes returned to baseline back on Earth.”