David,

I don't think your arguments salvage the moral ambiguity you seek. The fact is that the missions will be financed by money taken from people who have earned it, often without consent. That this theft is a small part of a larger regime of theft making it difficult to trace a particular project to particular victimizations does not change its character. It's still wrong to steal the funding for these projects. Yes, canceling one mission will probably not force a tax reduction; but it's a start. And cancelling many such projects will result in less of a tax burden (as you seem to recognize with "almost independently", and "the inexorable laws of arithmetic"). Moral people should be calling for such cancellations.

And it's not true that my point depends on what would otherwise be done with the resources. I'm not responsible for what other harms the government does with the loot, and the threat of such harms does not justify this harm. If the question is "Would you prefer that the money be spent on space missions to being spent on school canes?" then my answer is "Yes, I do." But if the question is: "Should the government use taxes to fund non-security-related space missions?" then my answer is "No, it shouldn't." These are different questions, with different answers.

It's also wrong to propose that my point depends on the second assumption that the Mars mission is not economically viable. It would be wrong for me to invest your money (even with a greater monetary return than you would have) against your will, wouldn't it? The person who should control your resources and your life is you. Not me. And not your neighbors.

And while it's impossible to be sure how this will affect private contributions and investments in private space development, it seems very plausible to me that many will conclude that they're already paying for such development with their taxes and be less inclined to contribute any more to similar and, perhaps, redundant projects.

Gil