LETTER doi:10.1038/nature12741 A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors P. G. Brown1,2, J. D. Assink3, L. Astiz4, R. Blaauw5, M. B. Boslough6, J. Borovicˇka7, N. Brachet3, D. Brown8, M. Campbell-Brown1, L. Ceranna9, W. Cooke10, C. de Groot-Hedlin4,D.P.Drob11, W. Edwards12, L. G. Evers13,14, M. Garces15, J. Gill1, M. Hedlin4, A. Kingery16, G. Laske4, A. Le Pichon3, P. Mialle8, D. E. Moser5, A. Saffer10, E. Silber1, P. Smets13,14, R. E. Spalding6, P. Spurny´7, E. Tagliaferri17,D.Uren1, R. J. Weryk1, R. Whitaker18 & Z. Krzeminski1 Most large (over a kilometre in diameter) near-Earth asteroids are referenced technique4–6 of estimating airburst damage does not now known, but recognition that airbursts (or fireballs result- reproduce the observations, and that the mathematical relations7 ing from nuclear-weapon-sized detonations of meteoroids in the based on the effects of nuclear weapons—almost always used with atmosphere) have the potential to do greater damage1 than prev- this technique—overestimate blast damage. This suggests that earl- iously thought has shifted an increasing portion of the residual ier damage estimates5,6 near the threshold impactor size are too impact risk (the risk of impact from an unknown object) to smaller high. We performed a global survey of airbursts of a kiloton or more objects2. Above the threshold size of impactor at which the atmo- (including Chelyabinsk), and find that the number of impactors sphere absorbs sufficient energy to prevent a ground impact, most of with diameters of tens of metres may be an order of magnitude the damage is thought to be caused by the airburst shock wave3, but higher than estimates based on other techniques8,9. This suggests owing to lack of observations this is uncertain4,5. Here we report an a non-equilibrium (if the population were in a long-term collisional analysis of the damage from the airburst of an asteroid about steady state the size-frequency distribution would either follow a 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, single power law or there must be a size-dependent bias in other Russia, on 15 February 2013, estimated to have an energy equivalent surveys) in the near-Earth asteroid population for objects 10 to of approximately 500 (6100) kilotons of trinitrotoluene (TNT, 50 metres in diameter, and shifts more of the residual impact risk where 1 kiloton of TNT 54.18531012 joules). We show that a widely to these sizes. a b 100 –28 80 –26 ) 60 –24 –1 (kt km 40 –22 (magnitude) 20 Absolute brightness –20 –18 deposition per unit height Energy 0 –3.0 –2.0 –1.0 0.0 1.0 2.0 3.0 253035404550 20 Time (s) Height (km) Figure 1 | Light curve of the Chelyabinsk airburst. a, The brightness profile for the Chelyabinsk airburst, based on video data. The conversion to absolute for the Chelyabinsk airburst, based on indirect illumination measured from energy deposition per unit path length assumes a blackbody emission of video records. The brightness is an average derived from indirect scattered sky 6,000 K and bolometric efficiency of 17%, the same as the assumptions used brightness from six videos proximal to the airburst, corrected for the sensor to convert earlier US government sensor information to energy26. The heights gamma setting, autogain, range and airmass extinction, following the are computed using the calibrated trajectory10 and features of the light procedure used for other airburst light curves generated from video24,25.The curves common to different video sites, resulting in a height accuracy of light curve has been normalized using the US government sensor data peak about 1 km. The total energy of the airburst found by integrating under the brightness value of 2.7 3 1013 Wsr21, corresponding to an absolute curve exceeds 470 kt. The half-energy-deposition height range is 33–27 km; astronomical magnitude of 228 in the silicon bandpass. The individual video these are the heights at which energy deposition falls below half the light curves deviate by less than one magnitude between times 22 and 11.5 peak value of approximately 80 kt per kilometre of height, which is reached with larger deviations outside this interval. Time zero corresponds to at an altitude near 29.5 km. 03:20:32.2 UTC on 15 February 2013. b, The energy deposition per unit height 1Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada. 2Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario N6A 5B7, Canada. 3Commissariat a` l’Energie Atomique, De´partement Analyse Surveillance Environnement (CEA/DAM/DIF), Bruye`res-le-Chaˆtel, 91297 Arpajon Cedex, France. 4Laboratory for Atmospheric Acoustics, Institute of Geophysics and Planetary Physics, University of California, San Diego, La Jolla, California 92093-0225, USA. 5Marshall Information Technology Services (MITS)/Dynetics Technical Services, NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA. 6Sandia National Laboratories, PO Box 5800, Albuquerque, New Mexico 87185, USA. 7Astronomical Institute, Academy of Sciences of the Czech Republic, CZ 251 65 Ondrejov, Czech Republic. 8International Data Center, Provisional Technical Secretariat, Comprehensive Test Ban Treaty Organization, PO Box 1200, A-1400 Vienna, Austria. 9Bundesanstalt fu¨ r Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany. 10Meteoroid Environments Office, EV44, Space Environment Team, Marshall Space Flight Center, Huntsville, Alabama 35812, USA. 11Space Science Division, Naval Research Laboratory, 4555 Overlook Avenue, Washington DC 20375, USA. 12Natural Resources Canada, Canadian Hazard Information Service, 7 Observatory Crescent, Ottawa, Ontario K1A 0Y3, Canada. 13Seismology Division, Royal Netherlands Meteorological Institute, Wilhelminalaan 10, 3732 GK De Bilt, The Netherlands. 14Department of Geoscience and Engineering, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands. 15Infrasound Laboratory, University of Hawaii, Manoa 73-4460 Queen Kaahumanu Highway, 119 Kailua-Kona, Hawaii 96740-2638, USA. 16ERC Incorporated/Jacobs ESSSA Group, NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA. 17ET Space Systems, 5990 Worth Way, Camarillo, California 93012, USA. 18Los Alamos National Laboratory, EES-17 MS F665, PO Box 1663 Los Alamos, New Mexico 87545, USA. 238 | NATURE | VOL 503 | 14 NOVEMBER 2013 ©2013 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH The Chelyabinsk airburst10 was observed globally by multiple Table 1 | Energy estimates for the Chelyabinsk airburst instruments—including infrasound, seismic, US government sensors Technique Best estimate (kt) Range (kt) and more than 400 video cameras—at ranges up to 700 km away. The Seismic 430 220–630 resulting airblast (shock wave travelling through the air from an explo- Infrasound (mean period) 600 350–990 sion) shattered thousands of windows in urban Chelyabinsk, with US government sensor 530 450–640 flying glass injuring many residents. Video-derived lightcurve .470 Data from US government sensors timed the peak brightness to Here ‘kiloton’ refers to the energy equivalent to a kiloton of TNT. To estimate the energy from infrasonic airwaves, all 42 infrasound stations of the International Monitoring System23 were examined. Of these, 03:20:32.2 UTC (coordinated universal time) on 15 February 2013 with 20 stations showed clear signals from the airburst. Our infrasound energy estimates are based on the 14 an integrated radiated energy of 3.75 3 10 J and a peak brightness of average observed dominant infrasound period from 12 stations that have stratospheric returns 13 21 showing the highest signal-to-noise ratio. Seismic Rayleigh waves generated by the airburst 2.7 3 10 Wsr . These values correspond to an estimated energy shock wave impinging on the Earth’s surface just south of Chelyabinsk were detected by about 70 equivalent of about 530 kt of TNT. The peak brightness was equivalent seismic stations at ranges in excess of 4,000 km. The amplitude of these waves in specific to an absolute astronomical magnitude of 228 (referenced to a range passbands as calibrated to nuclear airbursts19 were used as an independent estimate of source energy. of 100 km) in the silicon bandpass, making the airburst appear 30 times brighter than the Sun to an observer directly under this point. The airburst’s light curve has been reconstructed by considering the mea- as that of Chelyabinsk, which is less than one-third of the observed sured light production from several video records (see Supplementary value (more than 6 km). (We note that any object striking the Earth or Information for details) as shown in Fig. 1. We note that point-like its atmosphere is an impactor; a ground impactor creates a crater, but models4–6 of airburst energy deposition, which treat the impactor as a most burn up before that, releasing a large amount of energy into the strengthless, liquid-like material, predict that the height range in which atmosphere as an airburst.) Airburst energy estimates from four dif- the energy deposition per unit path length falls to half its maximum ferent techniques are summarized in Table 1. Our preferred mean value is less than 2 km for impacts as shallow (17u from the horizontal)10 energy estimate is in the range of 400–600 kt. Details of the analysis a b 40 0 0.0 Time residual Cloud width –1 35 0.5 –2 30 –3 1.0 Source height (km) Source –4 25 1.5 Cloud width from video (km) Cloud width from –5 Time residual (observed – expected) Time residual 20 –6 2.0 1 10 15 20 25 30 35 40 Overpressure (kPa) Originating height (km) c d 50.02 seconds T (K) 550 500 450 400 350 20 km Figure 2 | Observed and predicted shock characteristics for the Chelyabinsk motion) minus the expected time, calculated assuming propagation at the local airburst.