Rob Navias / NASA From left: Expedition 26 Commander Scott Kelly is reunited with his twin brother, Mark Kelly on March 17, 2010, following a flight back to Ellington Field, Houston from Kustanai, Kazakhstan.

No one ever pretended outer space is a hospitable place. There’s the hard vacuum, the lethal cold and the ever-present risk of even a small meteor hit. And then there’s the way the human body seems to revolt at the very idea of being there.

No sooner do astronauts arrive in space than up to 40% of them begin throwing up. Most of them adjust soon enough, but they may continue to feel dizzy and fatigued all the same. There’s also back pain, calcium loss, muscle atrophy, nasal congestion, accelerated heart rate and increased blood pressure—all a result of the skeletal and circulatory systems trying to adjust to zero gravity. Longer term damage may include distorted cell growth—also a result of zero-g—and DNA changes caused by cosmic radiation, leading to an increased risk of cancer and other diseases.

But how much of all that can you blame exclusively on space? Some people just have weaker bones than others; some are predisposed to hypertension or cancer. Separating heredity from environment isn’t easy even when the environment you’re talking about is a decidedly extreme one. Serendipity, however, has now provided a perfect way to begin investigating the mystery, in the form of Scott and Mark Kelly, identical twins who just happen to be astronauts.

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The Kellys have already achieved national and even global renown—Mark for his four shuttle missions and his cumulative 54 days in space, not to mention his high-profile devotion to his wife, former Congresswoman Gabrielle Giffords, as she continues to recover from the 2011 assassination attempt that nearly took her life; and Scott for his two shuttle flights, including a six-month stay aboard the International Space Station (ISS). Mark has since retired from NASA, but Scott is still flying, set for a full-year aboard the ISS, beginning in March 2015. And that has provided NASA with an unprecedented opportunity to run a nifty experiment.

In anticipation of the time Scott will spend aloft, NASA’s Human Research Program—a division of the space agency that studies space biology and safety—has begun requesting ideas from biologists and other researchers for comparison tests that can be conducted on the two men, both during and after the year-long mission. The name of the project—Differential Effects on Homozygous Twin Astronauts Associated with Differences in Exposure to Spaceflight Factors—is just the kind of scientific mouthful you’d expect, but it also frames perfectly just how valuable twins can be to scientists trying to understand the human body. The genetic software of identical twins matches perfectly, which means that most differences in how they age and the illnesses they do or don’t develop are attributable to environmental factors or life experiences. That makes the Kellys priceless lab subjects.

“This is a once-in-a-space-program opportunity,” said John Charles, chief of the Human Research Program’s International Science Office, in a statement that accompanied the announcement of the project. “The mission of the HRP is to reduce the risk to astronauts during long-duration space flight. In typical investigations, we usually have a specific outcome in mind and are goal-oriented. In this case, the slate is essentially blank. I am anxious to see what proposals we receive from the scientific community.”

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Some of the experiments are obvious. Blood samples will be taken of both men at the same intervals during the course of the year. Saliva samples and cheek swabs may be collected too and psychological and fitness tests may be conducted—though exactly what those last two will involve has yet to be determined. Follow-up tests after Scott comes home will be carried out as well—for years and perhaps even for the remainder of the brothers’ lives.

The Kellys aren’t perfect subjects: the time Mark has already spent in space muddies the study up a little since he’s been exposed to the same exotic physical environment Scott has. The ideal subjects would be a twin who is flying in space and one who has never left the planet. But after Scott’s return he will have amassed 540 days in space, or precisely 10 times Mark’s total, which ought to be a big enough difference to give the findings real credibility.

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Whatever results the study yields will have more than theoretical implications. A year in orbit seems like a lot, but it’s only a fraction of the time an eventual Mars trip would take—with its outbound and inbound legs of at least eight months each, not to mention a long stay on the Martian surface. That mission, in turn, would be nothing compared to a long-imagined trip to Jupiter’s moon Europa, with its globe-girdling ocean that could be home to life. On the same day NASA announced the Kelly experiment, it also released a paper in the Journal Astrobiology, outlining the goals for a Europa mission.

Neither trip is coming in the near future; at the moment, NASA doesn’t even have a way to launch its own astronauts to low-Earth orbit. But for the U.S., China and other countries, deep space colonization does remain a long-term—if distant—goal. Making sure we can survive the trip is a vital first step.

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