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Two crew commanders greet and reunite onboard the International Space Station (ISS) following the docking of the space shuttle Endeavour with the ISS on May 18, 2011. Russian cosmonaut Dmitry Kondratyev, Expedition 27 commander, at right, welcomes NASA astronaut Mark Kelly (frame center), with whose crew he and half of his current crewmates will spend almost two weeks at work on the space station.

European Space Agency astronaut Roberto Vittori, STS-134 mission specialist, is partially obscured at left. Three members of the six person Expedition 27 crew will be excusing themselves to return to Earth in a Soyuz spacecraft on May 23. In March 2011, Kondratyev succeeded Kelly's twin -- NASA astronaut Scott Kelly -- as station commander.