Astronaut Jeffrey A. Hoffman onboard the Space Shuttle Discovery on mission 51-D. He is wearing a special apparatus on his right leg which measures the variations in its volume. The purpose of this experiment is to study the shift of body fluids, from the lower limbs to the upper part of the body, that occurs in microgravity conditions. During the mission two communications satellites, ANIK C-1 and LEASAT 4, were released. LEASAT 4, commissioned by the US Naval Space Command, failed to operate after the deployment. Discovery was launched on mission 51-D from the Kennedy Space Centre, Florida, on April 12th 1991 and stayed in orbit for seven days.