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The lack of gravity, radiation exposure, Kelly's diet, and other facts of life in orbit affected his body in significant ways - including, as NASA is learning now, even his genetic blueprint.
The Twin Study, which is still in progress, uses Scott Kelly's identical twin brother and fellow former astronaut, Mark Kelly, to unmask the subtle but important effects of long-duration space travel on the human body.
Business Insider's Dina Spector reported on some of the most fascinating preliminary results of the study, which NASA released at the end of January, including the possible existence of a "space gene."
Here are eight other biological oddities that happen to your body if you're in space for a year.