On the recent missions to build the space station, I noticed that none of the astronauts wear the Manned Maneuvering Unit (the jet pack that allows for untethered spacewalks). In fact, as far as I know the MMU was only used on a handful of flights in the mid-80’s. Whatever happened to the MMU? Did NASA decide untethered spacewalks were too dangerous, or is there something wrong with the MMU (too bulky, impractical)? I know about the SAFER emergency recovery unit, was the MMU just a step in it’s development, perhaps?
On a brief search around the internet I saw nothing to suggest the MMU is no longer used.
My WAG is they never do anything risky (relatively speaking considering just being there is risky) if they don’t have to. Floating untethered in an MMU is definitely more risky then being tied to the spaceshuttle or spacestation.
Remember, jetting around in the MMU is a lot harder than you might suspect. You don’t just point yourself at an object and fly there. When using the MMU you will effectively be changing your orbital velocity. As a result you will achieve a lower orbit the faster you go and a higher orbit the slower you go. If you pointed yourself at the shuttle from some distance behind it and sped up to catch it you’d likely find yourself flying ‘below’ the shuttle when you arrived.
Of course, astronauts are trained to to the whole thing properly but it’s still tricky and probably not something they’d do just to have a ‘walk’ around the neighborhood. I imagine missions are designed to avoid untethered spacewalks if whenever possible.
Other things I’ve read about the shuttle suggest that the maneuvering arm has proven so accurately controllable, and the shuttle itself so accurately positionable, that it’s just as effective, if not more so, as well as safer, to use it as a cherry-picker than to use the MMU.
Found the answer to my own question at Encyclopedia Astronautica. To quote that site: “After the shuttle disaster, use of the unit was discontinued on safety grounds.”
I read that and it makes no sense to me. Did the MMU use the same O rings? What exactly is/was the “safety” issue?
I found a thread on Usenet that addresses this question. It’s quite in depth.
Relevant bit:
I think the distances and velocities the MMU was designed for are too small for this to come into play. But since this is GQ, I figured it was important to correct some of your description of orbital mechanics. Actually, if you use a rocket to increase your velocity, you’ll go into a higher orbit, or at least one with a higher average altitude. Assuming you’re in a circular orbit to start, and you fire one burst of the rocket, you’ll go halfway around the Earth gradually climbing; then on the second half of the orbit you’ll descent and come right back to where you started (orbitally speaking, at least, since the Earth will have rotated underneath you during the course of your orbit). And to further confuse things, even though you’re going faster, the new, higher orbit will take longer to complete than the original, lower orbit.