As David Weintraub, Lee Clarke, and Micheal Hickerson have shown, Donald Yeomans makes a persuasive case not only for the scientific interest of near-Earth objects and what they have to tell us about the cosmos but also for prudent attention to the threat they pose. For this we owe Yeomans a vote of thanks. But his book can be helpful in another way as well, provoking thought about the nature of the highly risk-aware society in which we live.

The recent collapse of an interstate highway bridge over the Skagit River in Mount Vernon, Washington, prompted the Daily Herald, a newspaper in the western suburbs of Chicago, to run a story on the condition of bridges in our area. The bridge in Washington, the article reported, had been "rated 'functionally obsolete,' a term also applied to 1,175 bridges [of a total of 3,759] in the six-county Chicago metropolitan area." This term is used for bridges that do not "meet current design standards for things like lane width or safety shoulders" but are nevertheless regarded as safe. A smaller number of bridges, by contrast—317 in the Chicago metro area—are classified as "structurally deficient," of which a much smaller number—15 in the Chicago metro area—are deemed to have "critical structural deficiencies," including five bridges in DuPage County, where my wife and I and our youngest daughter live.

Stop and think for a moment about the prodigies of risk-assessment implied by this Daily Herald article. More or less every bridge in the U.S., we can assume, has been assessed. Such judgments will always be fallible—witness the collapse of the bridge in Washington—but they represent a much larger (almost unimaginably larger) attempt to measure "risk" across the length and breadth of our nation—anywhere where Something Can Go Wrong.

Then consider what happens once these assessments are made. At first glance it is hard to understand how the various overlapping jurisdictions have allowed so many bridges to become "structurally deficient," let alone those bridges with "critical structural deficiencies." But getting these bridges up to snuff would be very expensive. There are a lot of competing risks, not to mention countless other competing priorities. (Our national expenditure in Afghanistan, say, would pay for a lot of new bridges.) And, after all, even the "structurally deficient" bridges are not assessed as "unsafe." This results in angry finger-pointing and litigation when, as occasionally happens, a bridge does collapse. Meanwhile, we muddle on, assessing the risks.

John Wilson is the editor of Books & Culture.

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