Scott Kelly Is Already Showing Us the Effects of Long-Term Spaceflight on the Human Body
First, it has been experimentally proven that astronauts on the ISS experience the effects of time dilation, which causes them to age a little bit slower than they would have in the same amount of time on Earth. As Neil DeGrasse Tyson confirmed, Scott Kelly would have gained approximately .01 seconds in his record-breaking 340 days in space:
Welcome back to Earth, Scott Kelly. After a year in orbit, Relativity says you're 1/100 sec younger than you'd otherwise be.— Neil deGrasse Tyson (@neiltyson) March 2, 2016
According to a tenet of general relativity, when an object is moving at an increased velocity relative to an outside observer, time appears to move slower for the object moving at a greater speed. (A similar concept provides the basis for sci-fi films like Clockstoppers or the Quicksilver sequences in X-Men, in which the characters appear to be "stopping" or "slowing" time because they are moving very quickly relative to everyone else.) The effect is especially pronounced when moving at near-relativistic speeds, but has also been observed to be true on the ISS, which moves at a velocity of about 7,700 m/s. As a result, less time has passed for Kelly than has passed for an observer on Earth, and he has technically "gained" a fraction of a second compared to the rest of us.
But an effect that is more readily observable is Kelly's height, as the astronaut has grown a whopping two inches in his time on the ISS. Although this seems crazy to us, it's actually a perfectly normal effect of the lack of a gravitational force, and will normalize now that he's under the influence of gravity once again.
But before Kelly shrinks again, he'll likely get the chance to have a surreal moment with his identical twin brother, former astronaut Mark Kelly, who is now two inches shorter than him. Mark, who has also spent time in space but has been on Earth for the last year, will serve as the control group for further experiments on Scott in order to determine the physiological effects of long-term spaceflight.