A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors

Abstract

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 damage 1 than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects 2 . 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 wave 3 , but owing to lack of observations this is uncertain 4,5 . 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 (6100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT 54.185310 12 joules). We show that a widely referenced technique 4-6 of estimating airburst damage does not reproduce the observations, and that the mathematical relations 7 based on the effects of nuclear weapons-almost always used with this technique-overestimate blast damage. This suggests that earlier damage estimates 5,6 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 techniques 8,9 . 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. for the Chelyabinsk airburst, based on indirect illumination measured from video records. The brightness is an average derived from indirect scattered sky brightness from six videos proximal to the airburst, corrected for the sensor gamma setting, autogain, range and airmass extinction, following the procedure used for other airburst light curves generated from video 24,25 . The light curve has been normalized using the US government sensor data peak brightness value of 2.7 3 10 13 W sr 21, corresponding to an absolute astronomical magnitude of 228 in the silicon bandpass. The individual video light curves deviate by less than one magnitude between times 22 and 11.5 with larger deviations outside this interval. Time zero corresponds to 03:20:32.2 UTC on 15 February 2013. b, The energy deposition per unit height for the Chelyabinsk airburst, based on video data. The conversion to absolute energy deposition per unit path length assumes a blackbody emission of 6,000 K and bolometric efficiency of 17%, the same as the assumptions used to convert earlier US government sensor information to energy 26 . The heights are computed us...

The Effect of Parallax and Cadence on Asteroid Impact Probabilities and Warning Times

“…7, right) with the single-survey system using the Brown et al (2013) PHO sizefrequency distribution appropriate for objects in this size range, our calculated close-approacher survey efficiency (Fig. 7, left), and the fraction of them that retain a nonzero impact probability on the (false) impact date (Fig.…”

“…11). Our calculation of the discovery rates used the Brown et al (2013) impactor size-frequency distribution and impact rate. Note that the close-approacher statistics only includes objects before closest approach for direct comparison to the impactors.…”

“…The surveys are further limited by the simple fact that groundbased facilities cannot survey during the day, and about 40% of all impactors will approach from the direction of the Sun. These problems were spectacularly highlighted by the Chelyabinsk impact on the morning of 2013 February 15 (e.g., Brown et al 2013;Borovička et al 2013)-with absolutely no warning, a ∼17 m diameter object blew up in the atmosphere with an energy equivalent to about 500 kilotons of TNT, damaging buildings 50 km away in the city of Chelyabinsk and injuring about 1500 people.…”

“…The new balance in the impact risk, along with the realization that smaller impacts may be more numerous but less destructive than anticipated a decade ago (e.g., Brown et al 2013), has contributed to an increased interest and funding for the NEO survey programs in recent years. For example, NASA's NEO Observations (NEOO) program office now solicits 6 proposals for surveys that "provide capability to detect the subset of 90% of PHOs down to 140 meters in size.…”

A catalog of video records of the 2013 Chelyabinsk superbolide

“…The skies were mostly clear in that region and the superbolide was widely observed from the Russian regions of Kurganskaya, Tyumenskaya, Sverdlovskaya, Chelyabinskaya, Republic of Bashkortostan, Orenburskaya, Republic of Tatarstan, Samarskaya, and from the Kostanay and Aktobe regions of Kazakhstan. The blast wave produced by the superbolide caused serious damage -mostly broken windows and doors -in the city of Chelyabinsk and its surroundings and injured more than 1500 people (Popova et al 2013;Brown et al 2013). This unexpected and unusual event attracted worldwide attention.…”