Human heart adapts to space by shrinking 27% Updated El Broide Share this: A scientific study reveals that Scott Kelly’s heart shrunk by one third during the year he spent on the International Space Station. Astronaut Scott Kelly jetted off on the adventure of a lifetime on March 27, 2015, where he spent an historic 340-day mission aboard the International Space Station. Kelly made sure to stay active by exercising six days a week with the help of a treadmill, a stationary bicycle, and a resistance machine. Upon his return, however, a scientific study reveals that his heart shrunk by a third, with the official results published in the journal Circulation on Monday, March 29 2021.