NASA astronaut Scott Kelly is returning to Earth tonight, wrapping up a 340-day-long stay aboard the International Space Station. It's the longest amount of time an American astronaut has lived in space, says NASA. The extended stay is part of the space agency's One-Year Mission, an experiment to see how long-term spaceflight changes the human body.

Living for months in microgravity can alter many of the body's systems

Living for months in microgravity can alter many of the body's systems, since humans evolved on Earth. Astronauts who have stayed in space for long periods have problems with their circulation and eyesight. That’s in addition to losses in bone and muscle tissue. Kelly has collected fluid samples and undergone rigorous medical testing to document these changes over the course of his trip. "When you put a person's cells in a different environment, the cells in the body are going to get a bit of a shock, and there are going to be some changes," said Graham Scott, the vice president of the National Space Biomedical Research Institute, which has been working with NASA on the One-Year Mission.

Kelly’s various samples will help NASA better understand the health effects of space. That’s crucial for preparing crewed trips to deep space.NASA is eager to send astronauts to Mars someday, and a round-trip mission to the Red Planet is going to be at least two to three years, according to the space agency. Astronauts on these deep-space voyages need to know what kinds of health changes they can expect along the way, as well as how to determine whether — for instance — dizziness is being caused by poor circulation or a disturbed sense of balance. Correct diagnosis is key for managing the effects of space.

Mark (L) and Scott (R) are the only twins to have traveled to space. (NASA)

Some people also experience more bone loss or eyesight changes than others when in space, and NASA isn't exactly sure why. Scott Kelly’s identical twin brother, Mark (a former astronaut himself), is serving as a "control" subject for the One-Year Mission by remaining on Earth. Since there are very few differences between the brothers’ genes, comparing samples from the twins will help researchers determine if there is a genetic basis for some of the health effects in space.

The body in space

Kelly has likely lost a significant amount of muscle mass and bone density while on board the ISS, NASA says. Just to stand up or walk around on Earth, the body has to work against gravity, which is constantly pulling down. In space, gravity is no longer a factor, so people don't get that same level of exercise just from going about their daily lives. Astronauts must perform up to 2.5 hours of exercises every day, such as running on a treadmill and weightlifting, to counteract this effect.

Without gravity, fluids shift toward the upper body

Without gravity to pull fluids — like blood — down the legs, fluids shift toward the upper body, says Jennifer Fogarty, the head of NASA's Space Life Sciences Innovation. The heart still pumps enough blood to the lower limbs so that they can function, but the legs don't get nearly the same amount of blood as they do on Earth. That means blood vessels in the legs don't have to work as hard to send blood back to the heart. "They essentially get 'lazy,'" said Fogarty. "They don't have to contract as much, so they kind of lose the ability to do so a little bit."

That poses a problem for return. When astronauts experience gravity again, their blood rushes to the legs — but the weak, lazy blood vessels have trouble sending it back to the upper body. This can cause extreme dizziness, and even fainting. Normally, returning astronauts are met by a team of experts that help keep the crew members from passing out. But astronauts landing on Mars won't have a welcoming committee; they’ll have to adjust to the Red Planet's gravity all on their own.

Kelly wears the Chibis device to balance out his fluids. (NASA)

One of the experiments Kelly participated in space addresses the fluid problem directly. Kelly and his fellow crew members have been using a Russian-built device known as the Chibis on board the ISS. The machine works like a large pair of rubber pants that sucks blood and other fluids from the upper part of the body down towards the legs. NASA hopes to find out if the Chibis has been effective at balancing out Kelly's bodily fluids while in space.

Astronauts returning from space often have swelling in the back of the eye

That fluid shift may also play a role in another side-effect of space: altered vision. Astronauts returning from space often have swelling in the back of the eye, which leads to poorer eyesight. These changes are usually temporary after week-long trips to space, but the problems have shown to be much more persistent after months in microgravity. NASA isn't sure about the exact cause of the eye swelling, but it's possible that fluid is shifting and accumulating in the head behind the eye, according to Fogarty. Kelly noted in a recent press call that he has noticed changes to his vision during his year on the ISS.

There may be other changes too, to Kelly’s immune response and digestive system. Even the types of bacteria in his gut will probably be completely different when he lands. Kelly isn't eating the same types of foods he would normally eat on Earth, which could alter his gut microbiome. And research has also shown that space may change how these bacteria express RNA — the genetic messengers that tell cells how to behave. "Some bacteria may become more potent when they’re in space," says the National Space Biomedical Research Institute’s Scott.

Collecting data

Kelly's involvement in the One-Year Mission started long before he launched to the ISS. He and his brother Mark took body samples and underwent various medical tests, such as MRIs and ultrasounds, for a year before the spaceflight, according to NASA. The idea was to create a comprehensive picture of how healthy the two brothers were before launch.

Kelly continued taking samples about once a month during his time on board the ISS, according to Scott. He also monitored his heart rate and blood pressure; he even used ultrasound technology on the station to scan his heart and eyes. Now, his blood, fecal, urine, and saliva samples are currently stored in the station's freezer, and will eventually return to Earth on a SpaceX Dragon capsule sometime this year, according to NASA.

NASA warns the results may not be published for up to six years

But NASA warns the results of the One-Year Mission may not be published for up to six years once Kelly lands. Kelly and his brother will be followed by doctors for the next year at least; then the researchers want to measure all the samples at once, to ensure that the fluids are introduced to the exact same testing environment. Scientists will be comparing all of Kelly's bodily fluids to the fluids from his brother, hoping to tease out which of Kelly's health changes — from his gut bacteria to his cardiovascular system — were induced by space and which were the result of a genetic predisposition. The data could tell NASA a great deal about what 340 days in space does to the body.

NASA will also be looking for changes in Kelly’s mental state, but so far he’s been a great participant in the study, says Fogarty. Kelly’s said he’s enjoyed his time in space, and compared it to camping in the woods for a year. But he’s eager to get home to Earth where there are showers — and also, people he loves. Though Kelly is slated to undergo testing for the next year, he will still have some free time. Once his initial round of medical tests are complete, Kelly says he plans to go home and jump in his pool.