THE FRAGMENTATION OF SMALL ASTEROIDS IN THE ATMOSPHERE

The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D(M) < 220 m and iron meteoroids with D(M) < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D(M) < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to ''explode'' or ''flare'' at the end of their visible paths. The dissipation of energy in the atmosphere protects the Earth from direct impact damage (e.g., craters), but the blast wave produced by the dissipation in the atmosphere can cause considerable damage to structures on the ground. The area of destruction around the impact point in which the over pressure in the blast wave exceeds 4 pounds/inch2 = 2.8 X 10(5) dynes/cm3, which is enough to knock over trees and destroy buildings, increases rapidly from zero for chondritic meteoroids less than 56 m in diameter (15 megatons) to about 2000 square km for those 80 m in diameter (48 megatons). (The minimum diameter of the Tunguska impactor of 1908 is about 80 m.) The area of destruction produced by stony asteroids between 70 and 200 m in diameter is up to twice as great as it would be without fragmentation. Crater formation and earthquakes are not significant in land impacts by stony asteroids less than about 200 m in diameter because of the air protection. The situation is similar for the production of water waves and tsunami for ocean impacts. Tsunami is probably the most devastating type of damage for asteroids that are 200 m to 1 km in diameter. An impact by an asteroid this size anywhere in the Atlantic would devastate coastal areas on both sides of ocean. The atmosphere plume produced by asteroids with diameters exceeding about 120 m cannot be contained by the atmosphere, so this bubble of high-entropy gas forms a new layer on top of the atmosphere. The dust entrapped in this hot gas is likely to have optical depths exceeding tau = 10 for asteroids with diameters exceeding about 0.5-1 km. The optical flux from asteroids 60 m or more in diameter is enough to ignite pine forests. However, the blast wave from an impacting asteroid goes beyond the radius in which the fire starts. The blast wave tends to blow out the fire, so it is likely that the impact will char the forest (as at Tunguska), but the impact will not produce a sustained fire. Because comets dissipate their energy much higher in the atmosphere than asteroids, they illuminate a much larger region and their blast wave is weaker, so they are much more effective in producing large fires. This suggests that the Cretaceous-Tertiary impactor was a comet rather than an asteroid.