Previously on here I've promoted a practical and hopefully widely appealing way to address externalities: ditch ag subsidies and double NASA's budget. Food is a product where virtually all the costs and benefits one can think of are internalized - particularly in today's day and age where it doesn't contribute all that much to macroeconomic volatility. Alternatively, there are lots of positive externalities associated with space exploration. The two budgets are roughly comparable, which makes this deficit-neutral budget plan convenient to think about.
Oddly enough, then, this post at the Space Politics blog directly ties NASA to agriculture subsidies!
In a lot of ways, they're absolutely right - and they hit on one of the many interesting externality/collective action questions swirling around NASA. I've talked about the positive externalities (the diffuse benefits and concentrated costs) of space exploration here before. But there are also concentrated benefits and diffuse costs to think about: namely that NASA is a boon to Florida, Texas, and Alabama. That's part of the reason why you have a Tea Party candidate from Florida backing manned space flight.
I think this poses more of a risk in terms of tying NASA to old ways of doing things that are associated with Cape Canaveral, Houston, and Huntsville. It doesn't seem to be presenting a huge risk of over-investment in space exploration.
Still, it's interesting. There are negative externalities wrapped within positive externalities. And the sorts of things NASA spends most of its time talking about in externalityish lingo - their "spin-offs" - aren't really externalities at all!