Earlier quoted context omitted.
Isn't the small quantity of humans exactly what makes it less survivable? The smaller the group, the more severe a single fault in the system. This is the way habitats ecosystems work anyway. A trivial example might be aquariums: the bigger the easier to maintain a healty equilibrium.
If you have 1000000 people, there's a very high risk at least one is crazy or illogical with respect to survival. If you have a couple hundred really well examined people who all really wanted to be somewhere the odds are much better, and all the more so, that problems can be contained when identified.
Furthermore, some 500 healthy adults do not form a stable society. Being in a high risk environment will flare tempers, 500 is on the low side of genetic diversity, there are no children, there will be pregnancies...