Kelly's identical twin, Scott Kelly, is also an astronaut. The Kelly brothers are the only known siblings to have both traveled in space. In 2015, Scott Kelly began a mission spending a year in space on the International Space Station. He returned to Earth on March 1, 2016, after 340 days in space. During and after Scott's year-long mission, the brothers were studied to find physical differences caused by living in space versus a baseline on Earth.