Our bodies evolved on Earth, so they're not built for weightlessness — which is exactly why NASA plans to use Kelly to study the long-term effects of spaceflight the human body.
Scott's identical twin brother Mark Kelly has spent the past year on Earth, so NASA will compare Mark's changes to those that Scott experienced.
NASA's ultimate goal is to use the lessons learned from Scott Kelly's Year in Space mission to send astronauts to Mars — and soon. That journey will take about 2.5 years round-trip, so understanding spaceflight's effects on the human body will be even more important.
Here are eight biological oddities that happen to your body if you're in space for a year.