On the other hand -- and at the risk of confirming Mark Kleiman in his belief that libertarians are loopy -- I don't speak for all libertarians, but I think there's a good case to be made that taxing people to protect the Earth from an asteroid, while within Congress's powers, is an illegitimate function of government from a moral perspective. I think it's O.K. to violate people's rights (e.g. through taxation) if the result is that you protect people's rights to some greater extent (e.g. through police, courts, the military). But it's not obvious to me that the Earth being hit by an asteroid (or, say, someone being hit by lightning or a falling tree) violates anyone's rights; if that's so, then I'm not sure I can justify preventing it through taxation.