Absence of Evidence

by Joel Marks

Sky and Telescope is an excellent magazine, and I have been a loyal subscriber for decades. However, in the introductory Spectrum column about his own fine article on planetary defense elsewhere in the June 2018 issue, Editor in Chief Peter Tyson overstates the case in his understandable eagerness to impress upon the reader the tremendous strides planetary defense has taken in mere decades since its inception.

For he writes, "NASA-funded efforts have determined that no mountain-size asteroid or comet is on a trajectory to hit Earth for at least the next 100 years." This assertion is simply false. It is not claimed in the article, and it is even contradicted by Tyson himself at the end of the very column in which it appears when he writes, "it's always possible that one of our surveys could pick up a previously unknown comet ... that might be making a beeline for Earth." One fervently hopes it does not suffer the same fate as President George W. Bush's so-called Mission Accomplished speech. 

This is a classic case of the fallacy of equating absence of evidence with conclusive evidence of absence. It is true that the odds of a large asteroid heading our way have been impressively reduced, and the odds are always low of a comet doing so. But as the article makes clear, we have at present and for the foreseeable future no way of "determining" whether or not such an object will be at our cosmic doorstep more than months or at most a few years ahead of time.

I am afraid that, next to such an impactor itself, the greatest threat to human survival today is complacency. It will be small consolation on the day one of them wipes us out because we failed to prepare for it to know that, in statistical terms, even then the probability of such an event will remain vanishingly small.