FILE - This undated file photo provided by NASA shows astronauts Mark Kelly, right, and Scott Kelly in the check-out facility at Ellington Field near NASA's Johnson Space Center in Houston. Scott and his twin Mark 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/NASA, File)
Featured Photo Galleries
