Astronaut Bruce McCandless II, STS 41-B mission specialist, participates in a historical spacewalk. He is pictured a few meters away from the cabin of the Earth-orbiting Space Shuttle Challenger. This spacewalk represented the first use of a nitrogen-propelled, hand-controlled device called the Manned Maneuvering Unit (MMU), which allows for much greater mobility than that afforded previous space walkers who had to use restrictive tethers.