On manned space missions before the shuttle era, the astronauts who went on EVA (spacewalking) were always attached to their spacecraft by a safety tether. Because it could have proved disastrous if the astronauts had become separated and floated away in space. But it was planned from the start that shuttle astronauts would take part in more adventurous EVAs, which would require them to become more mobile. It was with this in mind that a jet-propelled backpack called the MMU, or manned manoeuvring unit, was designed. A prototype of the MMU was test-flown in the cavernous upper compartment of the experimental US space station Skylab in 1973. But an MMU was not flown on EVA until February 7, 1984, on the STS-41B shuttle mission. The first 'pilot' was Bruce McCandless, who eventually flew 90 metres from the shuttle. The MMU was subsequently flown on spectacular satellite capture, repair and recovery missions on STS-41C and 51A. The MMU has been referred to as a 'flying armchair', and their is a similarity. The astronaut backs into the MMU, attaching it over the spacesuit. It carries twin tanks of compressed nitrogen gas, which feed a total of 24 jet thruster units dotted around the structure. They are sufficient for a six-hour EVA. Batteries supply electricity for the controls and the winking locator lights on the MMU. The astronaut fires different sets of thrusters to propel himself in different directions or rotate around any axis. The controls for the thrusters are located at the ends of the MMU's arms. Following the standard layout for spacecraft crews, the left-hand controller handles fore-and-aft, right-and-left, and up-and-down movements. The right-hand controller handles rotational movements of roll, pitch and yaw.
