Well, you're kind of assuming that the Hypothetical Space Disaster is just going to eradicate all of us earthlings in one fell swoop, like an attack of the Deathstar. In actuality, this hypothetical disaster would merely render the earth
less habitable, and gradually reduce the population. Suppose something happens which knocks out a large chunk of the earth's arable land, like a super volcano eruption or asteroid impact. This doesn't wipe out the human race, be it does condemn us to a few hundred years of starvation, war and misery and a drastic downward adjustment in the population and a regression to a medieval standard of living. However if we have, for example, space farms, it's the off world colonies that are supporting continued life on earth.