On Friday, half a pair of twins leaves for a year in orbit. It’s a chance to measure the effects on long-term space travel.

On Friday, a Soyuz rocket brings Russian Mikhail Kornienko and American Scott Kelly to the International Space Station for its longest expedition ever.

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The first and last time astronauts spent such a stretch in space was decades ago on the now-defunct Russian Mir space station. This time NASA is going in with its science guns fully loaded.

Mark’s Earth-side participation in research will make the historic mission all the more valuable. Scientists can compare the way their bodies change over the course of a year, using them as experimental controls against each other — one in an isolated box where all variables can be fixed, but where radiation and lack of gravity pose health concerns, and the other going about a normal life in Houston.

Andrew Feinberg has known he’d be part of the twin study for about a year. But in some ways, he’s been waiting his whole life for it. As a young child, the Johns Hopkins Medical School researcher watched NASA’s first manned space missions take off on television. Now his lab is participating in an unprecedented experiment on epigenetics, or the way our DNA expresses itself in different environments.

The space-bound Kelly will take blood samples just before each time a shuttle returns to Earth during his tenure, allowing scientists to study fresh, unfrozen cells just hours after they’re drawn. Meanwhile, Mark will donate countless hours of the next year to providing samples of his own, as well as undergoing the same psychological and cognitive tests his brother completes in space.

The applications in space travel are obvious: Man has never traveled farther than the moon, and NASA wants to take astronauts much, much farther. To do that, scientists have to ensure that the isolation, radiation and zero-gravity environment won’t send astronauts off the deep end after a year or two. For an astronaut like Scott Kelly, whose three missions have brought him closer and closer to his long-term stay — eight-and 12-day shuttle missions followed by a 159-day stay on the space station — the hope is that things will go smoothly.

But these experiments are important for Earth-based science, too. For scientists such as Feinberg, the space station represents a perfectly controlled environment the likes of which he’s hardly dreamed of.

Epigeneticists try to study how environmental changes, including in diet and exercise, affect the way DNA expresses itself, which can lead to tangible changes in the human body. But it’s hard to control a person’s environment enough to do a long-term study on how their epigenome might change — unless that person is locked in a box orbiting the Earth.

“Obviously, this is a tiny sample size, so we’re not really looking at how Scott and Mark are different during the year, exactly,” Feinberg told The Washington Post. “It’s not statistically valid to say that differences between them must be due to the spaceflight.”

Instead, Feinberg and his research group will be looking at how each man changes over the course of the year.

“If something happens after Scott departs, increases during his trip, and then goes back to normal after he comes back to Earth — if we don’t see that kind of sequential change in his twin, well, it’s not proof of anything, but it certainly suggests something interesting is going on,” Feinberg said.

He’s also excited to tie in the other nine projects with his own. It goes without saying that the researchers studying Kelly’s microbiome (the bacteria that live in and on his body) will see it change during the year he spends isolated in space eating a regimented menu of space food. But by comparing microbiome changes to epigenetic ones, the researchers may be able to draw points of connection between diet and DNA that we’ve never seen before.

NASA scientists are hoping the new data will help them optimize future space missions.

“It’s our first real organized foray into these deep genetic changes, and that brings us closer to using this idea of personalized medicine that’s gaining popularity,” said Mark Shelhamer, chief scientist of NASA’s Human Research Program. “We love the idea of using someone’s unique genomic structure to support their health in space.”

For example, he said, in the future some astronauts may get to use their DNA to shirk off daily exercise requirements. Right now all astronauts work out for two hours a day because it seems to generally be a good standard for maintaining health. But in addition to taking up precious research and leisure time, those workouts put a strain on the station: The excess movement is taxing on equipment, and systems have to work hard to compensate for the heat and moisture that comes with exercise.

“Maybe one day we’ll say, ‘Hey, you need two hours to stay healthy, but you only need 20 minutes to keep your bone and muscle health in a good place,” Shelhamer said.

But they’ll be keeping an especially close watch on how Kelly and Kornienko fare mentally.

“A year away from home in a small space without a lot of other people — that’s pretty stressful,” Shelhamer said. “We’ve done a lot of six-month missions, and we don’t anticipate a lot of surprises in the space between six months and one year, but we won’t know until we do it.”

(Optional add end)

America’s previous record-holder for the longest spaceflight, Michael E. Lopez-Alegria, spent about 215 days on the International Space Station starting in September 2006. In an interview with The Washington Post, he wished Kelly the best of luck and was optimistic that the five months or so that the new expedition will tack on won’t make a huge difference.

“I’ll be surprised if he comes back materially different in a year,” Alegria said. But all space flight is taxing. “You feel maybe a little lethargic,” he said, referring to the months after his record-breaking mission. “When you stand up from a chair, it’s harder than normal.” But he remembers how well he adapted to life in space given that much time to get his sea legs. When shuttles would bring up crews on short visits, he said, it was like watching bulls in a china shop.

But the longer we’re away from home, he said, the worse the isolation will feel. And unlike the space station, missions to Mars won’t be able to maintain constant, virtually delay-free communication with Earth. Kelly may be anxious about leaving school-age children behind — the divorced Kelly has two daughters, ages 20 and 11 — but he’s still capable of speaking to them throughout his trip. Should any problems arise on Earth, however, he won’t be able to come home early. That was all too clear during his previous trip to the space station: In January 2011, with over two months left of his command of Expedition 26, Kelly’s sister-in-law, former congresswoman Gabrielle Giffords was shot in an assassination attempt. Her long recovery led Mark Kelly to his retirement, but Scott Kelly was unable to see the couple until he landed in March. For a Mars mission to be successful, astronauts will have to be willing to put years of distance between them and their loved ones.

These are exactly the kinds of emotional hurdles that NASA has been preparing to clear for generations now, right along with the physical.

“NASA is working on this science project that’s the greatest in the history of civilization,” Feinberg said. “They’re turning humankind from an Earth-dwelling species into a space-exploring species. One day, humankind will be a species that can settle on other planets. It might be a hundred years before we have humans living on Mars, but this is a whole new kind of science. It’s a multi-generational effort.”

Joel Achenbach contributed to this report.

NASA’s Human Research Program is examining the effects of spaceflight on the human body by studying identical twin astronauts, Scott and Mark Kelly

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