 NASA astronaut Scott Kelly is returning to Earth today after spending nearly a year aboard the International Space Station. Kelly's extended stay on the orbital lab was for NASA's year-long mission, as an experiment to see how long-term spaceflight affects the human body. The idea behind the mission is to help NASA better prepare for trips in a deep space. A crewed mission to Mars, for instance, is going to take upwards of several months. NASA wants to know how astronauts will fare during these missions. They also want to figure out how to lessen any negative health effects when the astronauts get to their final destination. Kelly collected several body samples prior to and during his flight to document how his body has been changing. NASA also had a unique opportunity with Kelly since he has an identical twin, Mark Kelly, who is a former astronaut himself. Mark is serving as a control subject in this experiment by remaining on Earth. Since the twins are genetically identical, samples from the brothers can be compared to show just how much Scott's body has adapted to living in microgravity for so long. Researchers can then try to tease out if there is a genetic basis for some of the health changes people experience in space. NASA has a pretty good idea of what changes they'll see based on the experiences of other astronauts. Many people who live in space experience problems with their cardiovascular system. Microgravity causes bodily fluids like blood to float up into the torso region, so veins in the legs weaken and can't contract blood as easily. Astronauts also have a hard time with their spatial orientation and balance, so they don't have the same cues in space to tell them which way is up. And people also report vision changes, potentially due to fluid building up in the brain behind the eyes. It's something that's affected Kelly during his time in space. You know, I have a little bit of, I think, you know, effect on my vision that was very consistent with what I experienced last time. There are a whole host of other physical changes that Scott may experience too, including changes in his gut microbiome and how his RNA is expressed. His muscles and bones are likely a lot weaker too. On Earth, we work out these systems just by standing and walking around as gravity pulls us down. But without gravity, astronauts have to compensate as much as possible by doing specific exercises. Once Kelly is back on solid ground, NASA will try to figure out how space has really affected him. First, Kelly will do a field test as soon as he gets back to see how well he adjusts to a gravity environment again. He'll then undergo months of sampling and medical tests, the results of which probably won't be released until at least 2017. But Kelly will still get to enjoy the comforts of Earth as the experiment continues. When I get back to Houston, I'll go to the astronaut crew quarters at the Johnson Space Center for several hours of medical tests that end at about 3 in the morning. And then I'm going to go home and jump in my pool.