With talk about NASA exploring human colonies in space, some people wonder why we don't spend more energy and resources on saving the planet we live on now. Saving planet Earth from our own mistakes is definitely something we need to do in order to become a Level I civilization (a civilization that can manage the resources of its planet).
The problem is, saving planet Earth is only an intermediate step for human survival. The long term problem is that our star, the Sun, is already middle aged and has lived out half its life. In another 5 billion years our Sun will run out of fuel. Long before that however, perhaps only 1 billion years, Earth will be uninhabitable by humans. We have a billion years to get, not to Mars (our next door neighbor), but out of our dying solar system altogether in order to survive. In order to do that we will have to become a Level III civilization (a civilization that can manage the resources of its galaxy.)
The Universe is 13.8 billion years old, Earth is 4.5 billion years old and we have only 1 billion years remaining to find a new home outside of our solar system. That is the supreme design challenge and the reason we are making baby steps to other planets now. We have a long way to go.