Stuhlinger's letter expresses a basic optimism toward science that was characteristic of the modern era in general and the middle of the 20th century in particular, and which has tarnished a great deal since then. Their faith held that science has a sort of generalized power to produce good; more specifically, that given a relatively small cost investment (in terms of money, time, effort, attention, education, etc.) in scientific endeavor, a very large and concrete benefit is likely to eventually result. His story of the lens grinder captures that expectation.

Without arguing the case, I think it's fair to say that a much smaller proportion of the people in Western societies (at least) now hold this faith as strongly. The costs of modern "progress" have become more apparent over time: cures often harbor severe side effects; machines have their wastes and their dangers; knowledge that seems sure at first often proves less sure with further study. Our confidence that the costs of science are as low as we once thought they were has weakened; our confidence that the benefits are as great or as inevitable has also declined.

His argument that reapportioning money from science to humanitarian causes is not as simple, in a democratic, bureaucratic country, as it seems it should be is still as strong as it was when he gave it, if not more so.

But the underlying assumption that, "If we just do something ambitiously scientific, vastly great goods for humanity will result," is harder to sell than it was. We no longer accept that a manned voyage to Mars (for example) will result in concrete benefits for humanity in a way that is remotely analogous to the support of a lens grinder at the dawn of optics.

Proponents of space exploration must continue to both show the rest of us the concrete benefits of investment and, at the same time, limit their expectations. It will never again be like it was in the 60s and early 70s.