The Manned Maneuvering Unit (MMU) is a backpack propulsion device that gave astronauts mobility for extravehicular activities outside the Space Shuttle. It enabled them to maneuver within the payload bay or fly some distance away without needing safety tethers anchored to the vehicle. The MMU had 24 small gaseous nitrogen thrusters and was operated with hand controllers on the arms of the unit.
On February 7, 1984, on Space Shuttle mission STS 41-B, astronaut Bruce McCandless tested this MMU, serial number 3. He made the first untethered spacewalk as he flew some 300 feet from the Shuttle. This MMU also flew on missions STS 41-C as the backup unit for the Solar Max satellite retrieval and STS 51-A as the prime unit for retrieving the Palapa communications satellite. Astronauts Bruce McCandless, Robert Stewart, James van Hoften, and Joseph Allen flew MMU #3 a total of 6 hours 29 minutes during these three 1984 missions. The MMUs were not used again.
NASA transferred this one to the Museum in 2001.