Explosions of small Spacewatch objects in the Earth's atmosphere

RECENT observations with the Spacewatch telescope indicate that the flux of Earth-crossing objects with diameters below about 50 m is some 10-100 times higher than predicted by simple extrapolation from the known main-belt asteroid population^1,2. This might seem to imply³ a significantly greater terrestrial hazard from atmospheric explosions such as those that occurred over Revelstoke or Tunguska^4,5. Here I show that explosions due to Spacewatch objects with diameters less than 50 m (having kinetic energies below about 10 megatonnes high-explosive equivalent) typically occur too high in the atmosphere to cause substantial surface damage. Exclusive of relatively rare iron objects, no comet or asteroid with an energy below ∼2 megatonnes threatens the Earth's surface. The high flux of small Earth-crossing objects identified by Spacewatch therefore does not imply a greater terrestrial hazard.