The flux of small near-Earth objects colliding with the Earth


Asteroids with diameters smaller than 50–100 m that collide with the Earth usually do not hit the ground as a single body; rather, they detonate in the atmosphere1. These small objects can still cause considerable damage, such as occurred near Tunguska2, Siberia, in 1908. The flux of small bodies is poorly constrained, however, in part because ground-based observational searches pursue strategies that lead them preferentially to find larger objects3. A Tunguska-class event—the energy of which we take to be equivalent to 10 megatons of TNT—was previously estimated to occur every 200–300 years, with the largest annual airburst calculated to be 20 kilotons (kton) TNT equivalent (ref. 4). Here we report satellite records of bolide detonations in the atmosphere over the past 8.5 years. We find that the flux of objects in the 1–10-m size range has the same power-law distribution as bodies with diameters >50 m. From this we estimate that the Earth is hit on average annually by an object with 5 kton equivalent energy, and that Tunguska-like events occur about once every 1,000 years.

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Figure 1: Optical light curve from the 6 June 2002 bolide over the Mediterranean Sea.
Figure 2: Bolide energy calibrations.
Figure 3: The flux of small near-Earth objects colliding with the Earth, for diameters less than 200 m.
Figure 4: The flux of small near-Earth objects colliding with the Earth.


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We thank the US Department of Defense for making these data available. We also thank A.W. Harris, J. Borovicka and Z. Ceplecha for discussions; A.W. Harris for making available to us LINEAR debiased data before publication; and W. Bottke and R. Jedicke for comments and suggestions that improved an earlier version of this Letter. This work was supported in part by the Canada Research Chair program, and the Natural Sciences and Engineering Research Council of Canada. There is no implied endorsement by the US Department of Defense of factual accuracy or opinion in this paper.

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Brown, P., Spalding, R., ReVelle, D. et al. The flux of small near-Earth objects colliding with the Earth. Nature 420, 294–296 (2002).

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