alt Hacker NewsA high enough threshold for "accident" is a death. In the space shuttle the "accident" rate was 2/135 that is somewhat close to 1/100, but it shows that 1/100000 is too optimistic.
You can pick a more strict safety criteria, but the result would be (something)/135 >= 2/135 > 1/100 >> 1/1000000
Yes… for the space shuttle. But where is what on this for Artemis II? Especially if it has more heat shield failure modes that are not immediately deathly, the whole consideration becomes non-obvious. Is it possible the astronauts will arrive back "well toasted" but alive and OK other than heatstroke? (Probably not, but that's the kind of question involved here...)
(Also, technically [you may begin your eyeroll] even death isn't a simple boolean condition. I guess brain death should do it.)
Anyway all I'm saying is that all of these considerations need more qualifiers to be really useful.