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J.G. Hills, M.P. Goda (Los Alamos National Lab)
The fragmentation and dispersal of an asteroid in the atmosphere help determine the damage it can cause (Hills and Goda, 1993, Astronomical J. 105, 1114-1144). Large asteroids are suspected to be rubble piles with little overall strength. This lack of strength causes them to break up higher in the atmosphere than would be the case if they had the same material strength as normal meteorites. The higher elevation breakup causes them to spread apart more at a given elevation in the atmosphere, so less of their energy is available for ground impact.
We made computer simulations of such dispersal using asteroids of normal strength and those with much reduced strength to see if the more fragmented asteroids produce less damage. We find that these differences are much greater for irons than for stones, which is not surprising given the greater material strength of the irons. Irons with radii less than about 20 meters lose most of their energy before they reach sea level if they are of normal strength. If they are rubble piles, they produce little ground impact damage unless their radii exceed 70 meters. Iron asteroids have to have radii above these critical values to allow them to produce significant craters on land and tsunami in water. If the radius of an iron asteroids exceeds 200 meters, the size of the crater it produces is nearly independent of its material strength. Solid-stone asteroids with radii greater than about 100 meters produce significant craters. This critical limit is only about 20% larger for rubble-pile stone asteroids.
Blast damage from stony asteroids is not very sensitive to their strength. Small iron asteroids, with radii less than about 20 meters, produce more blast damage if they are solid, because their energy is dissipated lower in the atmosphere. If their radii exceed this value, the weaker asteroid produces more blast damage than the stronger one because the stronger one loses less of its energy in the atmosphere and more of it on ground impact.