Asteroid Airbursts: Risk Assessment and Reduction

Abstract:

Airbursts are events in which small (meters to tens-of-meters in diameter) asteroids deposit most of their energy in the atmosphere with a total energy greater than small nuclear explosions (>0.1 kilotons of TNT). The airburst risk is higher than previous assessments for two reasons. First, they are more frequent than previously thought. The Tunguska-class (~40 meters) population estimate has doubled, and Chelyabinsk-class (~20 meters) has increased by a factor of 2.6. Second, asteroid airbursts are significantly more damaging than previously assumed. In most cases, they more efficiently couple energy to the surface than nuclear explosions of the same yield.

Past Near-Earth Object (NEO) risk assessments concluded that the largest asteroids (> 1 km) dominated the hazard. Large NEOs represent only a tiny fraction of the population but the potential for global catastrophe means that the contribution from low-probability, high-consequence events is large. Nearly 90% of these objects, none of which is on a collision course, have been catalogued. This has reduced their assessed near-term statistical risk by more than an order of magnitude because completion is highest for the largest and most dangerous. The relative risk from small objects would therefore be increasing even if their absolute assessed risk were not.

Uncertainty in the number of small NEOs remains large and can only be reduced by expanded surveys. One strategy would be to count small NEOs making close passes in statistically significant numbers. For example, there are about 25 times as many objects of a given size that pass within the distance of geosynchronous orbit than collide with the earth, and 2000 times as many pass within a lunar distance (accounting for gravitational focusing). An asteroid the size of the Chelyabinsk impactor (~20 m) could potentially be observed within geosynchronous orbit every two years and within lunar orbit nearly once a week. A Tunguska-sized asteroid (~40 m) passes within a lunar distance several times a year. A survey optimized to discover and count these objects would rapidly reduce the uncertainty in their populations. An additional benefit would be early warning of an imminent impact to give authorities time to issue evacuation or take-cover instructions in circumstances for which there would be no time the prevent an impact.