In this photo released by the U.S. National Aeronautics and Space Administration (NASA) on May 27, 2011, astronaut Michael Fincke works outside the Space station during a spacewalk of the STS-134 mission. Two U.S. astronauts conducted the fourth and final scheduled spacewalks for space shuttle Endeavour's STS-134 mission on Friday morning, NASA announced. [Xinhua/NASA]