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

toggle caption Bill Ingalls/NASA/Getty Images
'Everybody Stretches' Without Gravity: Mark Kelly Talks About NASA's Twins Study
  • Download
  • <iframe src="https://www.npr.org/player/embed/469163874/469180943" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript

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

toggle caption NASA
Scott Kelly Reflects On His Year Off The Planet
  • Download
  • <iframe src="https://www.npr.org/player/embed/468628180/468751778" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">

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

toggle caption NASA/Bill Ingalls
NASA To Study A Twin In Space And His Brother On Earth
  • Download
  • <iframe src="https://www.npr.org/player/embed/395536140/395698534" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript

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

toggle caption 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

toggle caption Americans for Responsible Solutions
From 'Weekend Edition Saturday': Mark Kelly talks with NPR's Linda Wertheimer
  • Download
  • <iframe src="https://www.npr.org/player/embed/199364601/199362710" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">

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

toggle caption 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

toggle caption Chip Somodevilla/Getty Images

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

toggle caption P.K. Weis/Facebook.com/GabrielleGiffords
Melissa Block speaking with Mark Kelly
  • <iframe src="https://www.npr.org/player/embed/142342203/142342242" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">