> Our eggs are currently all in one celestial basket. We need to try, desperately, to ensure that the human race does not go extinct (by fault of our own or otherwise). This means an alternative habitat.

Killing off humanity requires killing off 99.9999% of the population in a short amount of time. There is no plausible scenario that can do that: nuclear weapons won't do it, biological warfare won't do it, bolide impacts even at the scale of the Sudbury or Chicxulub probably can't do it.