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.
Not very. We know that no gravity is lethal in the long term for human beings (for a variety of reasons), but we really have no sense of where the cutoff is -- moon gravity is probably too little, but we've no clue if Martian gravity might be enough. Human biology is definitely tuned to Earthly norms for obvious reasons, but it might benefit from greater or smaller g-effects; there simply isn't enough research on the subject, as for obvious reasons it's inordinately difficult to do the relevant studies.