Expedition 43 NASA astronaut Scott Kelly (left) and his identical twin, Mark Kelly, pose for a photograph in 2015 at the Cosmonaut Hotel in Baikonur, Kazakhstan. Bill Ingalls/NASA/Getty Images hide caption

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Bill Ingalls/NASA/Getty Images

'Everybody Stretches' Without Gravity: Mark Kelly Talks About NASA's Twins Study

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Kelly posted this photo of an aurora taken from the International Space Station to Twitter on Aug. 15, 2015. NASA hide caption

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NASA

Scott Kelly Reflects On His Year Off The Planet

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NASA astronaut Scott Kelly is seen inside a Soyuz simulator at the Gagarin Cosmonaut Training Center on March 4 in Star City, Russia. Kelly, along with Russian cosmonaut Mikhail Kornienko of the Russian Federal Space Agency, are scheduled for launch Friday aboard a Soyuz TMA-16M spacecraft from the Baikonur Cosmodrome in Kazakhstan. NASA/Bill Ingalls hide caption

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NASA/Bill Ingalls

NASA To Study A Twin In Space And His Brother On Earth

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Former Arizona Rep. Gabrielle Giffords, accompanied by her husband, retired astronaut Mark Kelly, speaks during a news conference in Manchester, N.H., in July. They were there to encourage state political leaders to have courage in the fight to expand background checks on gun purchases. Mary Schwalm/AP hide caption

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Mary Schwalm/AP

Former Rep. Gabrielle Giffords, D-Ariz., at a firing range in Nevada earlier this week. Her husband, retired astronaut Mark Kelly, was behind her. Americans for Responsible Solutions hide caption

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Americans for Responsible Solutions

From 'Weekend Edition Saturday': Mark Kelly talks with NPR's Linda Wertheimer

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Former Arizona Rep. Gabrielle Giffords and her husband, Mark Kelly, at an April 16 ceremony naming a Capitol Hill conference room for her aide Gabe Zimmerman. Zimmerman died in the same Tucson, Ariz., shootings that Giffords wounded. Saul Loeb/AFP/Getty Images hide caption

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Saul Loeb/AFP/Getty Images

Former Rep. Gabby Giffords, D-Ariz., and her husband Mark Kelly at Wednesday's Senate hearing about gun violence. Chip Somodevilla/Getty Images hide caption

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Chip Somodevilla/Getty Images

Rep. Gabrielle Giffords last May. P.K. Weis/Facebook.com/GabrielleGiffords hide caption

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P.K. Weis/Facebook.com/GabrielleGiffords

Melissa Block speaking with Mark Kelly

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