Surely, the interest in space colonization isn't merely because of the threat of an earth-destroying asteroid?
And comets and gamma-ray bursts and a half dozen other big bad boogeymen hanging out there. Some extinction level event on this planet is not a matter of if but when. When you look at the timing of such events in the past we are overdue.
We gotta get off this rock like quick if this species, and a whole bunch more we can take with us, are going to survive.
"merely because of a threat"? It damn well is the most important reason.
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