You actually have to go to Mars or there is no impetus to invent the technology required to go to Mars.

Without that you have two problems.

The first is that there is no goal, so nobody knows what to work towards, so you get undirected research that never goes anywhere. This is the problem the NIH and NSF have -- they fund a lot of research and 5% of it is brilliant but 95% of it is not.

The second is that there are other goals, but they're pedestrian. So there is money and somebody figures out how to divert it to their own business, and it ends up funding research that would have otherwise been done by the private sector if those companies' lobbyists hadn't figured out how to get the government to foot the bill instead. Which doesn't grow the pie at all, it just transfers tax money to well-heeled companies with good lobbyists.

Plus, I mean, we get to go to Mars. How is that not awesome enough to justify <1% of the federal budget? Spend a different <1% of the federal budget on the thing you wanted.