One of the main goals of his groundbreaking mission is to study how well humans can endure — mind, body and spirit — on a long-duration spaceflight.

After living for nearly a year aboard the International Space Station, NASA astronaut Scott Kelly is two inches taller than his identical twin brother Mark.

Kelly, who has spent more time in space than any other American astronaut, reports that overall he "feels pretty good" and now begins what may be a yearlong project to monitor his health. One unique advantage he provides to NASA's doctors is his identical twin brother, Mark, a former NASA astronaut who spent last year with his feet planted on terra firma.



Comparing the twins will help researchers spot any genetic changes that might have occurred in Scott in space.

Meanwhile — here's what National Space Biomedical Research Institute and NASA tells us they'll be on the lookout for: