If you could choose from a list of projects a la Kickstarter where YOU choose where your taxes go to, I'm sure people would not only feel better about taxes in general but would allow them to get involved in said projects.
I want to say that that's not nothing, but the Rosetta equipment alone cost €220 million. I'm not even sure how much of an impact $20m would have if you spread it across, e.g., the New York public school system.
This is why taxes cannot be donations. Indeed, they are not donations.
I remember the people at institutional giving at my old college say that nobody wants their name on a parking lot.
http://www.fms.treas.gov/faq/moretopics_gifts.html
They obviously don't take in anything more than a trivial amount of money. If you think that people would donate an equivalent amount of money if their taxes were eliminated - then you must have an awful lot of faith in people's inherent generosity in the face of their own self-interest. The economically optimal decision for any one person is to not give, but in the aggregate that means we don't have funding for infrastructure, research, etc.
The one exception to all of this is war where people will effectively donate by buying things like war bonds or agreeing to charge non-market rates for goods, but as a solution for taxation war is hardly ideal. As I said I hate taxes, but I hate no taxes more.
> The United States is the world’s single largest financial supporter of humanitarian mine action, which includes not only clearance of landmines, but also medical rehabilitation and vocational training for those injured by landmines and other explosive remnants of war. Since the United States Humanitarian Mine Action Program was established in 1993, the United States has provided over $2.3 billion in aid in over 90 countries for conventional weapons destruction programs.[0]
While I am quite cynical and definitively against a lot of the ways our military has been employed, the degree of safety and the diplomatic security I receive as a result of it, provides more utility to me than a parking lot.
More broadly, roads and localized infrastructure are quite important, as is the military. I agree that personally I would allocate more money for infrastructure than to the military, and agree if this is the point you were making. I do want to illustrate the counterpoints that many people (once again, not me personally) would "Rather have their names on the bombs that kill terrorists"[1], than on a highway.
[0] http://www.state.gov/t/pm/wra/c11735.htm
[1] This is a dramatic characterization and hyperbole. However, I do suspect a large swathe of the country believes this.
We should move carefully though because I am sure hard earmarking can be disastrous if taken all the way.
If we could choose how our taxes were used (a libertarian dream of mine for awhile), we'd have really lumpy funding. Hopefully it'd cause our military to be defunded, but the VA would probably end up taking the brunt of the hit.
Along those lines, I bet the funding of NASA would be even lumpier than it usually is. Don't they have difficulty planning engineering efforts because they're not sure if congress is going to continue funding next year?
Here's a thought: why don't NASA and other gov research institutes have an endowment instead? The government can add in a one-time lump sum and then periodically add the typical budget to the endowment? I imagine this would be pretty hairy though. It could be just another fund to get ransacked by congress on a rainy day, and there'd easily be controversy about how the fund was invested.