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If a suit suffers damage creating a rupture significantly larger than a 1/8-inch hole, the crewmember has very little chance of making it back to the habitat alive. On the other hand, we could repair a rupture much larger than that. So it's likely we could repair any such damage if the incident occurred while the crewmember was EVA.
However, the damage might occur while working inside the habitat; perhaps while servicing or stowing the suit. In this case, we have two bright hopes: (a) the crew will be trained as best we can not to kill themselves, and (b) they will have an extra suit aboard which they can cannibalize.
PLSS is the Portable Life Support System, or EVA backpack. This is the most likely source of failure since all the moving parts are in the PLSS. Some of these parts are rather delicate, especially the oxygen flow rate sensor. Recovery from a PLSS failure scenario is plausible because the crewmembers might make it back to the habitat by using one PLSS via a buddy umbilical. They also have a secondary oxygen pack (which actually contains more oxygen than the primary system) which will keep the crewmember alive if the primary system fails.
However, when they're getting ready for ascent, the crewmembers leave their backpacks in the lunar habitat. The entire ascent scenario is done on umbilicals. First the crew gets to the Ascent Stage on an umbilical connected to the habitat, and then they connect their suit umbilicals to the Ascent Stage. Upon arrival at the Lunar Transfer Vehicle, the final umbilical swap connects them to the LTV's internal oxygen supply until the LTV is repressurized.
An oxygen connector might be damaged during suit operation or maintenance. Tolerances are very tight and sometimes hard to operate, so it's possible that a crewmember might wreck a balky connector.
These connectors are on the upper torso, so that's the part which would need replacement. If the backup suit is also damaged, we're in a dual-failure scenario. I believe the probability of such a dual failure occurring is so remote that we can count on crew selection and training to avoid this happening twice.
The connectors are redundant, but it's not inconceivable that one of the two could fail open. In that case, there are several options for plugging the failed connector.
If an umbilical connector or hose were to fail, we have enough redundancy among the many umbilicals and three space-suited crewmembers to recover from any failure. The worst-case single-failure scenario just has one person translating back and forth a lot.
If it's impossible to ship all three crewmembers back up to the LTV, then someone will have to stay at the habitat. In that case, we're in a rescue scenario best left to science fiction stories.
Yes, a pressurized cover over the ascent vehicle would add a lot of weight. We have considered adding a transparent cover to avoid getting the Ascent Stage dusty, but pressurizing it is unacceptably more complex.
Well, there's the old "rescue ball" concept. Actually, although these got a lot of play in some books about the Space Shuttle and in Popular Mechanics magazine, they were never developed because to date there is no scenario where they might be used. However, it's certainly feasible in concept.
The rescue ball amounts to sealing the unlucky crewmember into a trash bag with an oxygen bottle and a teddy bear. This scenario, again is best left to science fiction stories.
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