Dave Morrison in his NEO News has some comments on the Easterbrook article entitled "The Sky is Falling." Here are Mr. Morrison's comments...
NEO News (05/22/08) Is the Sky Falling?
The June issue of The Atlantic features a long article on the impact threat and how to defend against asteroids, called "The Sky is Falling". The author is Gregg Easterbrook, a contributing editor to both The Atlantic and The New Republic. This is one of the most visible and potentially influential articles on NEO impact issues to be published in several years.
Easterbrook has written an advocacy article, not an objective discussion of this topic. His primary thesis (which I will not discuss further here) is that NASA should make asteroid defense a major part of its mission, perhaps replacing current emphasis on human flight to the Moon. To support this thesis, Easterbrook has painted the impact hazard as severe and has chosen examples that suggest that that the risk is much greater, by factors of up to a thousand, than in the "conventional wisdom". I will comment briefly on some of this evidence. I thank Bill Bottke, Mark Boslough, Clark Chapman, Al Harris, Michael Paine, Rusty Schweickart, and other colleagues for their email comments on this article.
Here are some of the rhetorical techniques used by Easterbrook to dramatize the threat.
The article repeatedly states that perception of the threat is increasing, but it does not indicate the baseline against which to measure this supposed increase. While it is certainly true that we are much more aware of the impact threat than we were a century ago, or even 50 years, it is not true that the scientific perception of the threat has increased in the past decade. In fact, current risk estimates are a factor or 2-3 lower than the frequently-quoted values published by Chapman and Morrison in 1994 and described in the NASA Spaceguard Report. The science threat estimate, which is primarily what Easterbrook is discussing, has thus been stable and even decreased over the past 15 years. However, it is true that the public perception has been increasing.
Of the various ways that the population and impact rate of NEAs are calculated, Easterbrook mentions only estimates based on the number of craters on land areas of Earth. There is no reference to the astronomical surveys that today provide the best population estimates. He asserts that until very recently the estimates of impact frequency, based on land craters, neglected impacts into the ocean or airbursts like Tunguska; when ocean impacts and airbursts are considered, the impact frequency jumps at least a factor of ten. This is wrong. Gene Shoemaker and others, who began in the 1980s to use terrestrial craters to estimate impact rates, counted craters per unit area (thus correcting for unobserved areas such as the ocean) and also allowed for airbursts that do not produce craters. Since the early 1990s, impact rates derived from terrestrial craters, lunar craters, and astronomical surveys have been consistent.
There is one reference to survey results: Easterbrook writes "in 1980, only 86 [NEOs] were known to exist. By 1990, the figure had risen to 170; by 2000, it was 921, as of this writing, it was 5388." He apparently takes these numbers as an indication that "near-Earth space rocks are more numerous than once was thought." An analogy illustrates the flaws in this logic: You move to a town of 50,000 and initially know only a handful of your neighbors. After a decade the number of your acquaintances has increased to several thousand. But this does not mean that the town has grown; indeed, its population may have fallen from 50,000 to 40,000.
Here is another paragraph that raises questions. He writes "A generation ago, the standard assumption was that a dangerous object would strike the Earth perhaps once in million years. By the mid-1990s, researchers began to say that the threat was greater: perhaps a strike every 300,000 years. This winter I asked William Ailor Š what he thought the risk was. Ailor's answer: a one-in-ten chance per century of a dangerous space-object-strike." Most readers of "NEO News" will recognize one-in-a-million-years as the estimate for a civilization-threatening impact, by a NEA 1-2 km across (energy a million megatons). The one-in-a-thousand-year number quoted by Ailor is for a Tunguska class airburst of 10 megatons, ten thousand times smaller. If we compare impacts of the same size, then the current estimate for the million-megaton impact is once in 2 million years, and the earlier (1994) estimate for Tunguska is once in a couple of centuries. Both calculated frequencies are consistent with each other, and both have gone down with our increasing knowledge, not up.
One could make many more criticisms, but these give the flavor of Easterbrook's rhetorical style. You can judge for yourself in reading his article, below.
David Morrison
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