Kelly and his Russian counterpart, Mikhail Kornienko, have spent nearly a year living in the International Space Station. Scott now holds the U.S. record for longest time in space (he's logged over 500 days in total), and NASA's hoping his long-term stay could shed light on what will happen when they send astronauts to Mars--a journey that could take 2 or 3 years, round-trip. Specifically, they're hoping to learn how space affects a person's eyesight, immune system, stomach bacteria, bone and muscle loss, mental health, and more.