NEO News (03/26/04) Damage from small impacts

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.

David Morrison

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