The MMU Manned Maneuvering Unit was designed for maneuvering by astronauts untethered from the shuttle. It was used on several satellite retrieval missions in the early 1980's. After the shuttle disaster, use of the unit was discontinued on safety grounds.

The MMU was a one-man, nitrogen-propelled backpack that latched to the Shuttle EMU spacesuit's PLSS life support system. Using rotational and translational hand controllers, the crewmember could fly with precision in or around the orbiter cargo bay or to nearby free-flying payloads or structures, and could reach many otherwise inaccessible areas outside the orbiter. Astronauts wearing MMU's deployed, serviced, repaired, and retrieved satellite payloads.

The MMU propellant - non-contaminating gaseous nitrogen stored under high pressure - could be recharged from the orbiter. The reliability of the unit was guaranteed with a dual parallel system rather than a backup redundant system. In the event of a failure in one parallel system, the system would be shut down and the remaining system would be used to return the MMU to the orbiter cargo bay. The MMU, which weighed 140 kg, included a 35-mm still photo camera that was operated by the astronaut while working in space.

Mass : 140 kg (300 lb).

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STS-41-B Credit - NASA
STS-41-B View of Astronaut Bruce McCandless during EVA... Credit- NASA View of Astronaut Bruce McCandless during EVA STS-41-B Close-up view of Astronaut McCandless during his EVA... Credit- NASA Close-up view of Astronaut McCandless during his EVA STS-41-C View of Astronaut Nelson using MMU to examine Solar Maximum Mission Satellite... Credit- NASA View of Astronaut Nelson using MMU to examine Solar Maximum Mission Satellite STS-51-A Astronaut Dale Gardner using MMU to travel to Westar VI satellite... Credit- NASA Astronaut Dale Gardner using MMU to travel to Westar VI satellite STS-51-A Astronaut Dale Gardner using MMU to travel to Westar VI satellite... Credit- NASA Astronaut Dale Gardner using MMU to travel to Westar VI satellite STS-51-A Astronaut Dale Gardner using MMU to travel to Westar VI satellite... Credit- NASA Astronaut Dale Gardner using MMU to travel to Westar VI satellite