FILE - In this July 25, 2012 file photo, Mark Kelly, retired NASA astronaut and commander of mission STS-134, briefs the media at the European Organization for Nuclear Research in Meyrin near Geneva, Switzerland. Mark and his twin Scott Kelly are taking part in an unprecedented study of identical twins looking into the effects of prolonged weightlessness. Mark will undergo tests on Earth while his brother Scott will embark on a one-year space station stint. (AP Photo/Keystone/Martial Trezzini, File)
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