Okay, let’s think this out a little bit.
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.
An impending catastrophe – asteroid strike – threatens to kill everyone in the society. That doesn’t violate anyone’s “rights” because you don’t have a “right to life” but rather a right not to have your life taken away by somebody else against your will. Therefore, the government has no right to tax you to protect you – and everybody else – from the asteroid.
So how is the asteroid to be stopped?
Presumably, everyone in society would agree voluntarily to cooperate to stop the asteroid. That is to say: we could still have collective action, but it would have to be voluntary, not coerced.
But would everyone participate?
The government goes around, passing the hat for contributions to stop the asteroid. A certain percentage of people, though, don’t believe in asteroids. Another percentage believe that the asteroid will bring the Rapture and so must not be stopped. These people are crazy, though, and crazy people are not interesting to talk about. Let’s hope there aren’t too many and ignore them.
Some people, though, notice that there are wealthier people than them in the society, and figure those other people should shoulder the burden of saving society. These are the “free-riders.”
Now, so long as this group is relatively small, no problem. Enough people will still put up enough money to stop the collective catastrophe. But so long as that is the case, free-riding is the economically rational thing to do. Indeed, in any large enough society, free-riding is always the rational thing to do: in a society with enough people putting up enough money voluntarily to stop the asteroid, free-riding is costless; in a society without enough such people, contributing is pointless.
The salvation of this ultra-libertarian society, then, depends upon the existence of a sufficient number of irrationally self-sacrificing people, people who ignore their rational self-interest in order to procure a social good for the group, without regard for the amount of “free riding” going on around them.
On the assumption – which I don’t think is pushing it at all – that there are a whole lot of communal problems that require collective action to address, libertarianism is only practical in highly communitarian societies.
I don’t know that that’s a knock-down argument against libertarianism. Wikipedia is a highly communitarian activity that grew up in a highly libertarian environment (the Internet), and most of the world is free-riding.
But it’s worth stressing nonetheless, because libertarians tend to talk as if rationality will lead to the necessary level of cooperation. But it won’t. In any case of communal threat where attempted free-riders cannot independently exposed to the threat, while contributors are protected, the rational thing to do is free-ride.