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FILE - In this March 4, 2016 file photo, NASA astronaut Scott Kelly, left, and his identical twin, Mark, stand together before a news conference in Houston. From his eyes to his immune system, Scott's body sometimes reacted strangely to nearly a year in orbit, at least compared to his Earth-bound brother _ but  research published on Thursday, April 11, 2019 shows nothing that would cancel even longer space treks, like to Mars. (AP Photo/Pat Sullivan, File)

FILE - In this March 4, 2016 file photo, NASA astronaut Scott Kelly, left, and his identical twin, Mark, stand together before a news conference in Houston. From his eyes to his immune system, Scott's body sometimes reacted strangely to nearly a year in orbit, at least compared to his Earth-bound brother _ but research published on Thursday, April 11, 2019 shows nothing that would cancel even longer space treks, like to Mars. (AP Photo/Pat Sullivan, File)

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