One year ago, NASA embarked on its very own astronaut twin experiment. Scott Kelly shot up to the International Space Station while his identical-except-for-the-mustache twin, retired astronaut Mark Kelly1, stayed firmly planted on Earth.

Now on March 1, after 340 days in orbit, Kelly-the-spacebound is coming home. Riding inside the snug Soyuz back with him will be fellow year-in-space resident, cosmonaut Mikhail Korniyenko.

But back to the twins (sorry, Mikhail), because that’s one of the most exciting parts of this year-in-space adventure. While Kelly-the-Earthbound wasn’t spending time munching space lettuce or drinking space coffee or doing important astronaut stuff, he was collecting blood, urine, saliva, and poop—for science! At the same time, NASA has been taking samples from his identical twin. By comparing the data, scientists hope to get a better handle on what happens to human bodies in space.

One year is twice as long as the typical ISS trip—but it is how long astronauts will have to spend in interplanetary space to get to Mars and back. That’s one year for the body to slowly break down in space thanks too little gravity and too much radiation.

Some of the effects of microgravity are obvious. When bones and muscles no longer have to bear the weight of walking, they start to weaken. Bones thin, muscles atrophy. To counteract that, ISS astronauts spend an average of two hours a day exercising, strapped onto a treadmill with elastic bands or doing weight resistance training. Supplements like vitamin D also help. But still, astronauts lose on average 1.5 percent of their bone mass per month in space.

Less obvious are the effects of gravity on fluids. Yes, your internal fluids! Like your blood and urine and the all the interstitial fluid that bathes the cells of your body.

Without gravity, for example, the heart shrinks because it no longer has to work as hard to pump blood to the legs and back. It also accounts for the Charlie Brown effect, where fluids in the face make astronauts more round-cheeked than they typically are on Earth. This makes it harder to smell food, but more seriously, also harder to see. Swollen tissues in the head put pressure on the optic nerve, leading to decreased vision. To counteract some of this, NASA has a device called a chabis, basically an uncomfortable-looking pair of pants that pull fluids back down to the legs.

Cosmic radiation is another very present danger. The ISS sits low enough in Earth’s orbit that it still enjoys some protection from the Earth’s mass and magnetic field; out in space en route to Mars, radiation becomes an even more pressing problem. NASA needs to find out exactly how big of a dose of space radiation is dangerous.

When Scott Kelly touches down on Earth, most of the data collecting part of the twin study will be over. But analyzing the data will have only just begun.

1UPDATE 1:15PM ET 2/29/16: This story has been updated with the correct name for Scott Kelly's brother, Mark Kelly.