Famous for teasing out the effects of nature and nurture, studies comparing twins are a hallmark of research in many fields, including psychology, biology and medicine. Now NASA is preparing to run the first twin study in space, comparing how identical twin astronauts fare while one spends a year in orbit and the other remains on the ground. Scott Kelly, along with Russian cosmonaut Mikhail Kornienko, is due to spend a full year living on the International Space Station from spring 2015 to spring 2016. Meanwhile Mark Kelly, Scott’s six-minutes-older brother and a retired astronaut, will live his normal life on the ground, undergoing periodic medical tests that will match those being performed on Scott in space. Identical twins share essentially all their DNA, so any differences between them, the thinking goes, must be attributable to environmental disparities (that is, nurture, not nature). Spaceflight is known to cause bone-density loss, muscle atrophy, eyesight damage, a weakened immune system, sleep disturbance and a host of other problems. But why these symptoms differ among individuals is less well understood and may be partly attributable to genetics. By comparing Scott with Mark, as well as with Kornienko, NASA scientists hope to explore how DNA affects these changes. "It offers an opportunity for an extra control to see if that can help us better understand this long-duration spaceflight process," says International Space Station program scientist Julie Robinson. NASA scientists admit the study is less than ideal because of the extremely small sample size—two participants—and because Scott and Mark will experience other environmental differences in addition to gravity and radiation. "Mark Kelly and Scott Kelly will not be on the same diet during the period of the one-year mission," NASA documents state, "nor will they be experiencing the same levels of carbon dioxide. Nor will they be sleeping on the same schedule.… We acknowledge that this schema is not ideal and we would like to better control all these potentially confounding variables—but at least for this particular twin study, that will not be possible." "Obviously the sample groups have to have a certain number to be statically significant, so some of the science we're doing is more anecdotal," Scott Kelly says. "But you can learn something and maybe learn about certain phenomena that you would then decide to investigate further." The response from the scientific community has been mixed. "My first thought was, this is so exciting, this is wonderful," says Nancy Segal of California State University, Fullerton, who directs the Twin Studies Center there. "I really applaud NASA for going ahead with it." But others say having just a single twin in space will be practically useless. "This strikes me as a bit silly," says Tim Spector of Kings College London, who founded the U.K. Twins Registry. "You can compare the same person before and after [spaceflight] better than you can compare the person and their twin. If they’d taken both of them on the mission, they could have explored how similarly they behave." And with the ground-based twin experiencing so many environmental differences from his brother beyond simply the space environment, it will be hard to know which factors contributed to any divergent outcomes. "I think this study will provide some insights that will have a lot of caveats," says twin researcher Jeanette Taylor of Florida State University. These caveats have not stopped researchers from submitting numerous ideas to NASA for studies that can be carried out on the Kelly brothers during the year-long mission. The space agency has called for studies on the bodily effects of radiation in space, on how gravity transitions affect RNA, protein expression and metabolic changes in the body, and on how the space environment affects thinking, decision making, alertness, stress and emotions, among other topics. Proposals were due in September, and NASA will make a final selection at the end of January. The Kelly brothers were born in Orange, N.J., and both grew up to be captains and test pilots in the U.S. Navy before joining NASA in 1996. That both of them turned out to have “the right stuff," when so many do not, seems in part to be traceable to genetics. But of course, they have their differences. "I'm the smarter of the two and the better-looking, with the more charming personality," Scott jokes. In reality, he says, "we're two individuals who were born on the same day from the same egg, but other than that we are two different people." Mark Kelly flew on four space shuttle missions before retiring in October 2011 to spend more time with his wife, Gabrielle Giffords, the Arizona congresswoman who was shot in front of a grocery store earlier that year while meeting with constituents. Scott is a veteran of two space shuttle missions and one previous long-duration stay on the space station, where he lived for five months from 2010 to 2011. NASA credits Scott and Mark themselves with the idea for the twin study, but Scott demurs, saying he merely raised the topic as a possible subject that might come up during a press conference announcing the mission. Once NASA scientists decided the study had some merit, the brothers were onboard. Both have agreed to provide blood samples, saliva, cheek swabs and stool, and to undergo psychological and physical tests. "It's not necessarily a comfortable experience," Scott says, "but you recognize how important it is, and you recognize how privileged you are to be in this position and serve your country as an astronaut." Scott Kelly and Kornienko's mission will be the first time anyone has spent a year on the International Space Station, although four Soviet cosmonauts spent roughly a year or more on the space station Mir in the 1990s. A main motivation for the upcoming extended mission is to study how the bodily effects of spaceflight differ over long periods compared with the usual five- or sixth-month stints on the orbiting laboratory. Knowing how people fare for a year in space—and how best to mitigate the negative effects—is critical if humans are to venture farther out into the solar system. "If we're ever going to go to Mars or do missions of longer duration," Scott says, "we're going to have to expand the envelope of how long we've flown people in space." 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