HOUSTON – Scott Kelly is the only American to ever live in space for almost a year. He's also the only astronaut with an identical twin brother, former astronaut Mark Kelly.
As Scott Kelly returns to Earth, his body becomes a scientific phenomenon while NASA compares him to his twin and figures out how being out of this world changes a human body.
Scott Kelly's year in space has already changed him.
"Astronauts are up to 2 inches taller while they're in space. The spine elongates. The increased height is short-lived. Shortly after coming back to Earth, they return to their normal height," Kelly said.
But scientists believe the twin study will reveal something more extraordinary.
Human metabolic and cardiovascular health in space could now be more closely studied than in previous astronauts, because while Kelly was orbiting Earth, his genetic makeup was simultaneously living here in his twin.
Researchers said their goal is to understand what it takes to send astronauts beyond the Earth orbit, to the moon, to asteroids — and possibly to Mars.
Astronauts have to be highly functional during their missions. Sending someone to Mars is 2.5 years round-trip. So this is the closest we've gotten to seeing the toll these lengthy trips take on major organs and mental functions: reasoning, alertness, responsiveness.
"Exactly how he's doing with the exercise, so how strong is he when he comes back? How long is it going to take him to recover? These are things that interest me," Blake Chamberlain, NASA medical officer, said.
The Kelly brothers did not stick to the same diet, exercise or lifestyle, but NASA's chief scientist of the Human Research Program, John Charles, said that's not a realistic expectation.
"We took it as a given that Mark would continue to live his normal, earthly life and experience all the stressors and all the pleasures of a normal earthly life and those would be reflected in his genetic information that we are collecting," Charles said. "...those areas of difference will tell us where the important differences are at the genetic level for future investigations."
Future investigations, like exploring Mars, depend on being able to shield astronauts from the challenges of the outer space environment and could one day help every human on Earth.
"It's going to be applicable to life here on Earth, it's going to be applicable to all of us that have radiation exposure on Earth and it's also going to be helpful in our understanding of how that radiation affects biological processes," Charles said.
Experiments are still ongoing and some samples will remain on the International Space Station until it returns in May, so as of right now there are no final results in the twin study.