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In this photo provided by NASA, International Space Station (ISS) crew member Scott Kelly of the U.S. gestures after landing near the town of Dzhezkazgan, Kazakhstan, on Wednesday, March 2, 2016. The Soyuz TMA-18M spacecraft landed with Expedition 46 Commander Kelly of NASA and Russian cosmonauts Mikhail Kornienko and Sergey Volkov of Roscosmos. Kelly and Kornienko are completing an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov is returning after six months on the station (Bill Ingalls/NASA via AP)

In this photo provided by NASA, International Space Station (ISS) crew member Scott Kelly of the U.S. gestures after landing near the town of Dzhezkazgan, Kazakhstan, on Wednesday, March 2, 2016. The Soyuz TMA-18M spacecraft landed with Expedition 46 Commander Kelly of NASA and Russian cosmonauts Mikhail Kornienko and Sergey Volkov of Roscosmos. Kelly and Kornienko are completing an International Space Station record year-long mission to collect valuable data on the effect of long duration weightlessness on the human body that will be used to formulate a human mission to Mars. Volkov is returning after six months on the station (Bill Ingalls/NASA via AP)