What good is astronomy? How can astronomers justify the support they receive from taxpayers? What benefit does society derive from astronomical research? What is the rate of return on our investment?
These questions are difficult, but not impossible, to answer. The economic benefits of basic scientific research are often realized over the long-term, and it's hard to anticipate what spin-offs may develop as a result of any specific research program.
A purist might refuse even to respond to these questions. What justification does basic research need other than the pursuit of knowledge? What higher achievement can civilization hope to accomplish than the luxury of seeking answers to some of the oldest questions: where did we come from, and what is our place in the universe?
The proper response is probably somewhere in between. The advances in scientific knowledge made possible through basic research have had a definite impact on the standard of living of the average taxpayer. The magnitude of this impact isn't easily quantified, but it probably doesn't need to be. The money that taxpayers contribute to scientific research in some sense obligates the researchers to make their work accessible to the public. A combination of teaching and public outreach provides an adequate return on the investment.
If this doesn't seem reasonable, put this in perspective by looking at exactly how much it costs taxpayers to fund scientific research.