re: A Slight Misunderstanding
OK, but then the point Tom first raised and I echoed applies -- in the long term as Mars gains self-sufficiency, we'll also be improving technology relevant to impactor detection and deflection. My bet is that we have the impactor problem solved for the vast majority of cases before we have a civilization established on Mars that would be able to sustain itself forever in the face of extinction of the Earth (and by sustain I mean with enough redundancy and robustness to survive political and psychological upheaval as well as the loss of supplies from Earth).