Their successful work increased the lifespan of the satellite. 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. Retirement After a safety review following the Space Shuttle Challenger disaster, the MMU was judged too risky for further use and it was found many activities planned for the MMU could be done effectively with manipulator arms or traditional tethered EVAs. 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. The two operational, flown flight units MMU No. 2 and No. 3 were stored by NASA in a clean room at Lockheed Martin in Denver through 1998. NASA transferred flight article No. 3 to the National Air and Space Museum in 1998, which now hangs suspended in the hall above Space Shuttle Discovery in the Udvar-Hazy Center annex. Flight article No. 2 is on display at the U.S. Space & Rocket Center in Huntsville, Alabama. As of 2017, MMU No. 1 is on display in the Space Vehicle Mockup Facility at Johnson Space Center. See also Single-person spacecraft Soviet SPK References External links Human spaceflight Extravehicular activity 1984 introductions