NASA astronauts Rex Walheim (center) and Sandy Magnus, both STS-135 mission specialists, participate in an extravehicular activity (EVA) hardware training session in the Neutral Buoyancy Laboratory (NBL) near NASA's Johnson Space Center. EVA instructor Art Thomason assisted Walheim and Magnus. STS-135 is planned to be the final mission of the space shuttle program. Photo credit: NASA or National Aeronautics and Space Administration