Astronaut Bruce McCandless II, one of two 41-B mission specialists, 'spacewalking' a few metres from the Earth-orbiting space shuttle Challenger. The shuttle's cabin and open payload bay may be seen in the reflection from his gold-tinted visor. This historic extravehicular activity was the first to make use of a nitrogen-propelled hand- controlled device called the manned manoeuvring unit (MMU). The unit allows for much greater mobility than that afforded to previous spacewalkers who had to use restrictive tethers.