We like to fantasize about it but, when it comes to the crunch, most people are horrified at the mere thought. The point is that, in the final instance, what we want won’t matter. If we eventually board our spaceships and go, it won’t be a matter of wanting to — but having to. And the process will change us — starting on the purely biological level. Any new world, no matter how earthlike, will initially present difficult conditions. If the planet is a bit bigger, its gravity will be greater. If the oxygen content is not quite the same, we won’t cope without respirators. Over generations, our bodies will gradually acclimatize. That’s the way it works. No life form willingly leaves its natural habitat. But when it has to, evolution kicks in and it adapts — physically and mentally.