S66-28636 (1 April 1966) --- Astronaut Clifton C. Williams Jr., backup crew pilot of the Gemini-10 spaceflight, undergoes zero-gravity egress training. A KC-135 Air Force plane, flying a parabolic curve, creates a weightless environment as a training exercise in preparation for spaceflight. The Hand-Held Maneuvering Unit in his right hand produces controlled thrust for moving about. Photo credit: NASA