[pvg]

Beside all the redundancy built into the device itself, the shuttle carried a second one so someone else could go out and retrieve the stranded astronaut. There's a detailed write-up on the development and use of the thing on NASA's history site:

https://history.nasa.gov/SP-4219/Chapter13.html

[ProAm]

> the shuttle carried a second one

Nothing like taking the second one out for a spin after the first one failed... lol.

[pvg]

Most space gear (excluding the stuff that gets you to and back from space) has been almost implausibly reliable. Voyager 1's thrusters still work far past their intended best-before date and they were not designed to have someone's life depend on them.

If a Manned Maneuvering Unit somehow failed, it would have been a highly extraordinary event. The crew on the shuttle along with their mission controllers could have made an assessment of the risk of failure of the second MMU.

This isn't like your computer kernel-panicking and you rebooting it and hoping for the best.

[limaoscarjuliet]

Correct, but serious issues do arise, e.g.: https://en.wikipedia.org/wiki/Apollo_Guidance_Computer#PGNCS...

[sitkack]

And it had load shedding built in, so even in the mis configured state, it still worked.

[ProAm]

Definitely, but it doesn't stop the carnival from playing in your mind as you strap in to go and rescue MMU-01 with MMU-02

[greenhouse_gas]

I guess that the second one could have gone tethered.