Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage1 than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects2. Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave3, but owing to lack of observations this is uncertain4,5. Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×1012 joules). We show that a widely referenced technique4,5,6 of estimating airburst damage does not reproduce the observations, and that the mathematical relations7 based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates5,6 near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques8,9. This suggests a non-equilibrium (if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.
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Funding was provided by the NASA co-operative agreement NNX11AB76A and the Czech institutional project RVO:67985815. D.P.D. acknowledges support from the Office of Naval Research. We appreciate discussions with F. Gilbert (of UCSD), J. Stevens (of SAIC), P. Earle and J. Bellini (of USGS). D. Dearborn provided assistance with video reductions.
The authors declare no competing financial interests.
This file contains Supplementary Tables 1-5, Supplementary Figures 1-7 and Supplementary Text and Data 1-7 comprising: 1. Infrasonic measurements and analysis procedures used to measure airburst yield; 2. Seismic measurements and analysis procedures used to estimate airburst yield; 3. - Analysis procedures for US Government sensor data and a discussion of the choice of radiative efficiencies; 4. Observational information, analysis and interpretation related to airblast window damage used to estimate overpressure; 5. Procedures used to calibrate the video-derived lightcurve; 6. Details of a fragmentation model; 7. Details of the CTH Hydrocode model. (PDF 2740 kb)
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Brown, P., Assink, J., Astiz, L. et al. A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors. Nature 503, 238–241 (2013). https://doi.org/10.1038/nature12741
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