Nick,

Your statistics are correct. Having the human race spread between two planets is obviously our civilization's best defense against asteroid/comet collision. However I disagree with you on one point:

>If the Human race were to colonise Mars, it is equally
>likely that an impact on Mars could destroy civilisation
>there as for the folks "back home". No planet is safer than
>any other.

Clearly there is nothing magic about Mars that keeps it from getting whacked by asteroids or comets (the Hellas depression is clear evidence of that). However assuming the asteroid collision didn't smash the planets to bits, the big problem is atmospheric and oceanic effects, i.e. powerful shockwaves blowing everything to bits, firey matter raining back on the planet causing forest fires that lead to a "nuclear winter" and most important, horrific tidal waves. Now obviously for a Mars colony there's no need to worry about tidal waves or forest fires. With a 6 millibar atmosphere, the initial blast wave would attenuate pretty quick. I don't believe the prompt terraforming of Mars is feasible. Therefore the colonists would live in sealed habitats. Consequently a really nasty asteroid collision that could destroy civilization on Earth might go unnoticed if it were to occur on Mars.

I should also point out that the best defense for the Earth against asteroid collision is to colonize Mars. A Mars colony would eventually lead to commerce between the civilizations of Earth and Mars. Such commerce would create a significant interplanetary transportation infrastructure. That transportation infrastructure would provide the enabling technology for clearing away or deflecting dangerous asteroids and comets (right now we can do almost nothing about them).

Gary