Scott Kelly spent his 216th consecutive day in space Thursday. That's the single-longest mission ever completed by an American astronaut. Michael Lopez-Alegria spent 215 days in space back in 2007. Kelly and a Russian astronaut are spending a full year on the International Space Station to conduct research on long-duration spaceflight. Kelly will return to earth next March, falling just short of the world record for most continuous days in space. A Russian astronaut spent more than 400 days in space two decades ago.