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One of the comments on this recent question touched on the use of the Manned Maneuvering Unit. It seemed like a good idea at the time: a system that would allow astronauts to do EVA without being tethered.
The wikipedia page on the MMU suggests that use of the MMU was stopped after a few flights because it was deemed too risky, but it doesn't expand on this. Were there incidents on the missions that they were used, or other technical issues? Or was it purely due to the fear of being marooned?

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They didn't have an application that couldn't be done better by maneuvering the shuttle itself and using the shuttle arm and tethered astronauts (sometimes with astronauts serving as the end effector on the arm). There was little to no benefit to the MMU, so even a small risk outweighs a non-existent benefit. Also the things are big and heavy. In space, if you don't need it, you don't bring it.

They were cool though.

From an upcoming Smithsonian article by Andy Chaikin:

But after the Challenger disaster in 1986, NASA re-evaluated shuttle missions, including spacewalks, and the MMU was deemed unnecessary. “It became pretty obvious that you didn’t need it,” explains Nelson. “The shuttle had such an amazing capability to fly right up to something, and it made more sense to just reach out and grab it, either with the [robotic] arm or just with a person, that the MMU became a really cool piece of technology that didn’t quite have a purpose.”

“Too bad,” Nelson adds, “because it’s a very cool machine.”

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