Elon Musk, the billionaire founder and CEO of the private spaceflight company SpaceX, wants to help establish a Mars colony of up to 80,000 people by ferrying explorers to the Red Planet for perhaps $500,000 a trip.
(...) we've put all our eggs in one basket. If we were on many worlds and were to mess up down here, there's a way for the human species to continue. I don't for a moment propose that the Earth is a disposable planet, and we have to put enormous efforts into making sure we don't muss up down here. But there is a chance.
By the same logic,
Is not a good addition. The Mars-hardened facilities will be hardened only for Mars conditions (unless it's extremely easy to harden against any level of radiation?) in order to cut colonization costs from 'mindbogglingly expensive and equivalent to decades of world GDP' to something more reasonable like 'decade of world GDP'. So given a supernova, they will have to upgrade their facilities anyway and they are worse positioned than anyone on Earth: no ozone layer, no atmosphere in general, small resource & industrial base, etc. Any defense against supernova on Mars could be better done on Earth.
Good point. Mars would only be better off if the colonies over-engineered their radiation protection. Otherwise anything that gets through Earth's natural protection would probably get through Martian settlements designed to give the same level of protection. It might be relatively cheap to over-engineer (e.g. digging in an extra meter), but it might not.