Sir

An important, technically valid analysis of meteoric fireball data (P. Brown et al., Nature 420, 294; 2002) has been transformed into media stories asserting (falsely) that the threat from dangerous asteroids is less than once thought. Nature joined the media hyperbole with words on its cover about “reassessing the hazard”, although the data published by Brown et al. have nothing to do with the hazard.

Brown et al. studied satellite and infrasound data over an 8.5-year period pertaining to impacts of meteoroids 1–10 metres in diameter. Except for the few per cent that are strong nickel-iron, such objects burn up harmlessly high in the Earth's atmosphere. Thus they constitute no direct danger. Only objects perhaps 1,000 times more massive than the largest studied by Brown et al. actually reach the ground with much cosmic velocity intact.

Yet many of the media reports about the paper asserted that this implies that the frequency of dangerous near-Earth asteroid (NEA) impacts is decreased, and therefore that the hazard has also declined significantly. In just one of many examples, the Los Angeles Times editorialized on 30 November 2002 that “if Nature is to be believed” then asteroids capable of causing catastrophic damage “can recede from the worry list”.

But the danger from NEAs is not less than once thought. Nature, in its own press release, mentioned the extrapolation by Brown et al. of their data to objects tens of metres in size, like the small NEA that caused the roughly 10 megaton 'Tunguska' explosion over Siberia in 1908. But the lower, once-a-millennium estimate for Tunguskas derives from telescopic Spaceguard Survey data reported during the past two years (see, for example, D. Rabinowitz et al., Nature 403, 165–166; 2000), not from the study by Brown et al. of much smaller objects. And even Tunguskas contribute very little to the hazard, which is dominated by rare impacts of NEAs several thousand times more massive than Tunguska (with diameters 1–3 km).

The estimated frequency of such larger NEAs (and hence of the total impact hazard) is not changed by the work that Brown et al. have done. Indeed, data analyses during the past year suggest that NEAs of civilization-threatening sizes may be more numerous than previous estimates by tens of per cent.

For such low-probability events as NEA impacts, the important issue is not the statistical frequency but the determination of whether there is (or is not) an asteroid that will collide with Earth in the next few decades. That is the purpose of the Spaceguard Survey — to find these NEAs one at a time, and determine from their orbits whether or not they pose a danger in the near future.