aboard the International Space Station. Kelly’s extended stay on the orbital lab was for NASA’s

year-long mission. It’s 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 into 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 mitigate any negative health effects along the way and when the astronauts get

there. 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.

As for what those body changes will be, NASA will find out once Kelly gets back to Earth.

But the agency has a pretty good idea of what 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, since 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.

[KELLY] You know, I have a little bit of, i think, effect on my vision that is 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 as gravity pulls us down. 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 Earth 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 won’t be released until at least 2017. But Kelly will still

get to enjoy the comforts of Earth as the experiment continues.

[KELLY] 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,

then i’m going to go home and jump in my pool.