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.