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This day in tech: First untethered space walk using the MMU - February 7, 1984


The first use of the Manned Maneuvering Unit (MMU) occurred on the tenth flight of a Space Shuttle, mission 41-B, during which the MMU was flown on demonstration flights. These MMU flights demonstrated capabilities deemed appropriate for use in the planned retrieval of the Solar Max satellite on a later shuttle mission. Courtesy of the manned maneuvering unit, Bruce McCandless II, then a Navy captain, became the first person to fly free, untethered in space; the date was February 7, 1984. While orbiting around the Earth at a speed of 17,500 miles per hour, McCandless floated from the cargo bay into outer space, 150 nautical miles above Earth, an experience he described as "a heck of a big leap". Mission specialist Robert L. Stewart, an Army lieutenant colonel, also flew the MMU on shuttle mission 41-B. While flying the MMU, these men were in a journalistic phrase of the time "human satellites". They checked out the equipment, maneuvered within the cargo bay, flew away from and back to the orbiter, performed docking exercises, recharged the MMU nitrogen tanks, and collected engineering data. The MMU, according to the post mission report, "performed as expected and no anomalies were reported".

The main purpose of flight 41-B, the fourth using the orbiter Challenger, was the deployment of two commercial communication satellites, Western Union's Westar VI and the Indonesian Palapa-B2. These satellites were released, but failed to reach geostationary orbit due to problems with the commercial upper-stage technology designed to lift the satellites from the low orbit of the Space Shuttle to the higher geosynchronous orbit ?justifying a later rescue mission using MMUs.

The final MMU mission was STS-51-A, which flew in November 1984. The propulsion unit was used to retrieve two communication satellites, Westar VI and Palapa B2, that did not reach their proper orbits because of faulty propulsion modules. Astronauts Joseph P. Allen and Dale Gardner captured the two satellites and brought them into the Orbiter payload bay for stowage and return to Earth.

After the Space Shuttle Challenger disaster, the MMU was judged too risky for further use. NASA also discontinued using the Shuttle for commercial satellite contracts, and the military discontinued the use of the Shuttle, eliminating the main potential uses. Although the MMU was envisioned as a natural aid for constructing the International Space Station, with its retirement, NASA developed different tethered spacewalk approaches.


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