Damage from the impacts of small asteroids

Abstract

Previous work is extended by using a model spherical atmosphere with a fitted density profile to find the damage done by an asteroid entering it at various zenith angles. At zenith angle 0° and a typical impact: velocity at the top of the atmosphere of V = 17.5 km s −1, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M<230 m and iron meteoroids with D M<50 m. At zenith angle 45° the corresponding figures are 360 and 70 m while at 60° they are 500 and 100 m. For comets with V = 50 km s −1 the values are D M<1900 and 3000 m for 45 and 60°, respectively, using typical values of ablation, but they are much smaller if ablation is reduced. Only impactors with D M above these critical values are effective in producing ground impact damage: craters, earthquakes, and tsunami. Smaller impactors can still produce atmospheric blast waves. It is found that the area of destruction around the impact point in which the overpressure in the blast wave exceeds 4p.s.i. = 2.8 × 10 5 dyn cm −2, which is enough to knock over trees and destroy buildings. It is found that for chondritic asteroids entering at zenith angle 45° and an impact velocity at the top of the atmosphere of 17.5 km s −1 that it increases rapidly from zero for those less than 50 m in diameter (13.5 megatons) to about 2000 km 2 for those 76 m in diameter (31 megatons). If we assume that a stony asteroid 100 m in diameter hits land about every 1000 years, we find that a 50 m diameter one (causing some blast damage) hits land every 125 years while a Tunguska size impactor occurs about every 400 years. If iron asteroids are about 3.5 per cent of the frequency of stony ones of the same size, they constitute most of the impactors that produce areas of blast damage of less than 300 km 2. While the optical flux from a small asteroid such as Tunguska is enough to ignite pine forests, the blast from it goes beyond the radius within which the fire starts. The blast tends to blow out the fire, so it is likely that the impact will char the forest (as at Tunguska), but it will not produce a sustained fire.