UNITED STATES—Scott Kelly returned to Earth on Tuesday, March 1 after spending 11 months at the International Space Station. He holds the record for spending the longest consecutive amount of time in space by an American.

Scientists from NASA’s Human Research Program and many other groups did health tests on Mark Kelly, Scott’s twin, while Scott was in space. By comparing these tests to those done on Scott, they will learn more about what prolonged space travel does to the human body. Never before has science been given the opportunity to learn about the physical and mental effects of long stays in orbit by comparing two genetically identical individuals.

Urine, blood, fecal, and cheek cell samples were sent back to Earth by Scott Kelly when he was still at the International Space Station to be compared with the samples by his brother. Cardiovascular tests were done, as well as visual tests. Since Scott’s return to  Earth, he has already been observed to be 2-inches taller than when he left.

More tests will be done on Scott, including physical exams as well as mental health evaluations to observe the effects of living in space for 340 days. The study was done in preparation for a prolonged mission to Mars in the future.