DAMAGE FROM THE IMPACTS OF SMALL ASTEROIDS

Previous work is extended by using a model spherical atmosphere with a fitted density,profile to find the damage done by an asteroid enterin g it at various zenith angles. At zenith angle 0 degrees 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 meteoro ids with diameters, D-M < 230 m and iron meteoroids with D-M < 50 m. A t zenith angle 45 degrees the corresponding figures are 360 and 70m wh ile at 60 degrees they are 500 and 100m. Far comets with V = 50 km s(- 1) the Values are D-M<1900 and 3000 m for 45 and 60 degrees, respectiv ely, using typical values of ablation, but they are much smaller if ab lation 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 w aves. It is found that the area of destruction around the impact point in which the overpressure in the blast wave exceeds 4 p.s.i. = 2.8 x 10(5) dyn cm(-2), which is enough to knock over trees and destroy buil dings. It is found that for chondritic asteroids entering at zenith an gle 45 degrees 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 50m in diameter (13.5 megatons) to about 2000 km(2) for those 76m in d iameter (31 megatons). If we assume that a stony asteroid 100m in diam eter hits land about every 1000 years, we find that a 50 m diameter on e (causing some blast damage) hits land every 125 years while a Tungus ka size impactor occurs about every 400 years. If iron asteroids are a bout 3.5 per cent of the frequency of stony ones of the same size, the y 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 s uch as Tunguska is enough to ignite pine forests, the blast from it go es 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 fores t las at Tunguska), but it will not produce a sustained fire. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.