Damage from the impacts of small asteroids

The fragmentation of a small asteroid in the atmosphere greatly increases its aerodynamic drag and rate of energy dissipation. The differential atmospheric pressure across it disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with asteroid density. Extending our previous work, we use a spherical atmosphere and a fitted curve to its density profile to find the damage done by an asteroid entering the atmosphere at various zenith angles. In previous work we estimated the blast damage by scaling from data on nuclear explosions in the atmosphere during the 1940s, 1950s and early 1960s. This underestimated the blast from asteroid impacts because nuclear fireballs radiate away a larger fraction of their energy than do meteors, so less of their energy goes into the blast wave. We have redone the calculations to allow for this effect. We have found the area of destruction around the impact point in which the over pressure in the blast wave exceeds 4 pounds/inch{sup 2} = 2.8 X 10{sup 5} dynes/cm{sup 3}, which is enough to knock over trees and destroy buildings. About every 100 years an impactor should blast an area of 300 km{sup 2} or more somewhere on the land area of Earth. The optical flux from asteroids 60 meters or more in diameter is enough to ignite pine forests. However, the blast from an impacting asteroid goes beyond the radius within which the fire starts. It 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. Because of the atmosphere, asteroids less than about 200 m in diameter are not effective in producing craters and earthquakes. They are also not effective in producing water waves and tsunami in ocean impacts. Tsunami is probably the most devastating type of damage for asteroids that are between 200 meters and 1 km in diameter