An asteroid will hit Earth at some point. What can we do about it?

Dinosaurs look on as an asteroid plumets through the sky.

Any day now, it could happen: We could detect an asteroid on an Earth-crossing trajectory. It might be a small one; it might be a big one. It might be tomorrow; it might be a thousand years from now. So what can we do about it?

We know that rocks of all shapes and sizes have struck Earth pretty much since it formed. We see evidence of it on the moon: All those craters preserve a record of the damage. On our planet, craters are rarer, but that's only because wind and water erode the evidence of recent craters, and plate tectonics takes care of the older ones.