This by Tom Murphy is a must read. Read it right to the end, though, or you’ll miss his point.
tl;dr?
He’s saying: getting humans into space is not a way to save Earth from its resources and overpopulation crisis. It will take technology and resources well beyond what we can marshall at the moment to reach some sort of interstellar future. Don’t drop the ball on the making this planet a good place project, because space won’t save us. But, even so, space exploration is a wonderful scientific necessity.
I agree, pretty much. We’re not going to travel a few tens of light years and find another Earth. We may find potentially terraformable planets which with hundreds of years of robotic resource gathering (much of which would be spent with mining robots building more robots, probably in the asteroid belt) we could, eventually, turn into nice places to live. That’s a 500 to a thousand year project. I really hope we get to do it. But we never will if we render this Earth uninhabitable first. We have to explore, learn, fix this planet, and then, (maybe) colonise.