From: Ames Research Center
Posted: Friday, March 26, 2004
What would have been the consequences if 2004 AH had actually hit the Earth instead of passing by at a distance of 50,000 km?
Stimulated by recent media attention on asteroids AL00667 and 2004 FH, there has been a lot of speculation about the damage a small asteroid (of roughly 30 m diameter, kinetic energy about 1 megaton) might cause if it hit the Earth. This is an important question, since such "mini-Tunguska" impacts are the most frequent, happening once every few decades. There has been a lot of confusion on this point, however. Some have assumed that if it "disintegrates in the atmosphere" it will do no harm, neglecting the fact that air-bursts near the surface are exceedingly destructive, a fact well known to the designers of nuclear bombs. At the opposite extreme, some assume that an explosion of 1 megaton energy taking place anywhere in the atmosphere is a major hazard, but this is true only if the explosion occurs within about 10 km of the surface. Either conclusion - that there is no danger or that such an impact would be equivalent to a major nuclear strike - misrepresents the real situation.
It is very difficult to answer the seemingly simple question of what would have been the consequence if 2004 AG had hit. There are many sources of uncertainty, including (1) imprecision in the 30 m estimate of diameter, (2) unknown physical and chemical nature of he asteroid, and (3) dependence of damage on the target location. In addition, there are uncertainties in the calculations, since the nominal size of 30 m lies near the lower limit for penetration of the atmosphere.
Let's look first at uncertainties in the calculations of atmospheric penetration. A variety of models have been calculated for the effects of the atmosphere on incoming projectiles, and these were reviewed for the recent NASA Science Definition Team (SDT) report on sub-kilometer asteroids. As an example, the 1908 Tunguska impactor was a stony (asteroidal) object about 60 m in diameter. While it may have been significantly decelerated in the upper atmosphere, the Tunguska impactor still produced a very destructive explosion of about 10-15 megatons energy when it disintegrated at an altitude of 6-8 km. In contrast, the energy of a 30 m stony asteroid at the same speed is about 1 megaton, and it does not penetrate within 10 km of the surface.
The studies reviewed by the SDT indicated that, for fixed composition and entry conditions, the damage from a stony asteroid falls off very rapidly for sizes smaller than Tunguska, going to zero for energies below about 2 megatons. The drop-off is so fast toward smaller sizes because two effects add: the energy is smaller and the explosion takes place at higher altitude. If severe ground damage is defined by a blast overpressure of 4 psi (pounds per square inch) as has been standard in discussing the effects of nuclear explosions, then Al Harris notes: "What is happening is that at 3 MT, the overpressure falls under 4 psi at a distance of around 15 km from the blast center, and the blast center is around 10 km or so up, so the intersection with the ground is around 100 sq. km. If you drop the size to 1 MT, the numbers are reversed: the blast is around 15 km up and the radius out to 4 psi overpressure is around 10 km, so nothing happens on the ground, even right below it."
One of the uncertainties is in estimating the damage for peak pressures below 4 psi. My impression is that in the case of the 1 MT explosion discussed above, we would expect a large sonic boom with breakage of some windows near ground zero, but that buildings would not generally suffer structural damage and trees would not be knocked down. Flying debris from short-lived gale-force winds might cause some injuries. The diameter for this 2-3 megaton limiting case is near 40-45 m. The definitions of the Torino Scale for impact hazards are more conservative, including any asteroid larger than 25 m as a possibly harmful.
For purposes of estimating casualties, the NASA SDT concluded that there is a significant atmospheric attenuation of the hazard for impactors below about 70 m diameter (see their Figure 3-2), and that there would be no deaths from an impact smaller than about 45 m diameter. SDT member Al Harris stated this more colorfully by musing that for a 30 m impact "I am about equally divided as to whether I would run away from the impact site, or toward it. I seriously doubt that any harm would come to anyone."
Damage from an airburst even as large as Tunguska (10-15 MT) depends on the target. For the majority of the Earth's surface, which is water, there would be no damage (the lower limit for tsunami generation is about 10 times the Tunguska energy). Most of the land is also still sparsely populated.
The larger source of uncertainty is the nature of the projectile itself. Penetration and explosive energy will differ significantly depending on the density and composition of the asteroid. At one extreme, the rare iron object of this size makes it to the surface and forms a crater - somewhere in the range between Meteor Crater in Arizona and the cluster of small craters formed in the 1947 Sikhote-Alin strike in Siberia. At the other extreme, an ice-rich impactor would detonate harmlessly at very high altitude. This discussion has focused on the middle ground of stony objects because they are by far the most numerous.
Also note that the astronomers do not measure the actual diameter of the asteroid, but rather its apparent brightness. The diameter is inferred from the brightness based on an assumed reflectivity or albedo. Thus, even if we felt confident that a 30 m diameter stony asteroid would do no harm, it does not follow that we can be sure that asteroid 2004 FH would not be dangerous, since it could easily be a factor of 3 larger or smaller than nominal in mass and energy. Statistically, such asteroids do not constitute a significant threat, as indicated in the NASA SDT report, where objects of this size are concluded to contribute nothing to the hazard of sub-km asteroids. However, that does not mean that we can ignore the possible danger posed by a specific, poorly characterized object for which we have measured only its brightness. Nevertheless, I would argue that it is usually not appropriate to assume the worst case. By far the most likely expectation for 2004 FH is negligible damage and zero casualties if it had hit the Earth.
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