Before discussing these results, it may be worth noting that the exact statistical frequency, while of interest to asteroid scientists, is not directly relevant to the present impact hazard.
Impacts by NEAs of any size are exceedingly rare, from the 5-megaton limit

of atmospheric shielding up to the hundreds of millions of megatons associated with mass extinctions. Statistically, no impact is to be expected within a human lifetime. The real issue, therefore, is whether against the odds there is an NEA on an impact trajectory. If there is such a threat, we want to identify it and deal with it. That is the purpose of the Spaceguard Survey. The important issue is not the average frequency of impacts but whether we can expect an impact within our lifetime, or the lifetime of our grandchildren.

Still, the total population of NEAs and their average impact frequency are often discussed. Sometimes these issue are used as a "hook" to attract attention to work that might otherwise be overlooked by the media and the general public. This week there has been considerable press interest in the conclusion that the new Sloan Survey results have reduced the estimated frequency of impacts and therefore that we were safer than had been previously thought. It is true that the estimated frequency for impacts by 1 km and larger NEAs has dropped in recent years, but not by more than the probable uncertainties, which have been given by most scientists studying the problem as about a factor of 2 or 3. Since the mid-1990s, the most common estimate has been that the Earth is hit by a "civilization threatening" impact (by a 1.5-km-diameter asteroid) about twice per million years, which is equivalent to a 1-in-5000 chance per century. But it is hard to tie down such estimates, in part because there is also a range of uncertainty as to what constitutes a civilization threatening impact, spreading over at least a factor of two in asteroid size (from 1 km to 2 km diameter).

Alan Harris of JPL has provided several estimates of the impact frequency for NEAs with absolute magnitude H less than 18 (approximately corresponding to a 1-km asteroid with average reflectivity or albedo). For these estimates he used various recent values of both the population of NEAs and their dynamical lifetime (how long they orbit the Sun, on average, before they hit the Earth). These estimates for the chances of an impact in the next century are: 1 in 8600, 1 in 7100, 1 in 4800, and 1 in 4000. Expressed in terms of NEAs 1.5 km in diameter, the estimated impact frequencies are 1 in 12000, 1 in 14000, 1 in 10500, and 1 in 8800. The range in these values, which us about a factor 3, is probably representative of our uncertainty in the impact frequency.
It should be noted that all of Harris's numbers are based on the directly observed population of NEAs, of which nearly 1500 are now known. In contrast, the Sloan Survey measured main belt asteroids and assumed that the main belt population had the same size dependence as the NEAs. It is by no means clear that this assumption is correct, or that main belt asteroids are a satisfactory surrogate for NEAs.

I would also like to comment on the statement in the Sloan news release that "the new impact risk estimate, like most previous ones, relies on assumptions about a single event 65 million years ago when a 10-kilometer asteroid collided with earth and killed the dinosaurs. The researchers assumed that such impacts occur on roughly 100 million-year intervals and used that statistic to calculate the impact odds for the more common asteroids of smaller sizes." As one of the first to publish NEA impact frequencies (with Clark Chapman, see our Nature paper in 1994), I note that we made no such assumption. Nor, to my knowledge, have any previous estimates involved any assumption about the frequency of KT-size impacts.
Indeed, we have no way from a single example 65 million years ago of estimating the average frequency of such impacts. To my knowledge, this Sloan impact frequency estimate is the first to depend on such an assumption.

The Sloan Survey is an exciting astrophysics project with great promise in a number of fields of astronomy. It has already contributed to our knowledge of the main belt asteroids. But it has not measured NEAs, and it has not made an independent estimate of the impact frequency. It is therefore probably premature to conclude from these observations of the main belt asteroids that we should revise our estimates of the impact frequency of NEAs or of the magnitude of the impact hazard.

NEO News is an informal compilation of news and opinion dealing with Near Earth Objects (NEOs) and their impacts. These opinions are the responsibility of the individual authors and do not represent the positions of NASA, the International Astronomical Union, or any other organization. To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov .
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