Asteroids with diameters smaller than approximately 50-100 m that collide with the Earth usually do not hit the ground as a single body; rather, they detonate in the atmosphere. These small objects can still cause considerable damage, such as occurred near Tunguska, 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 objects. 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 approximately 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 approximately 5 kton equivalent energy, and that Tunguska-like events occur about once every 1,000 years.
In the absence of a firm link between individual meteorites and their asteroidal parent bodies, asteroids are typically characterized only by their light reflection properties, and grouped accordingly into classes. On 6 October 2008, a small asteroid was discovered with a flat reflectance spectrum in the 554-995 nm wavelength range, and designated 2008 TC(3) (refs 4-6). It subsequently hit the Earth. Because it exploded at 37 km altitude, no macroscopic fragments were expected to survive. Here we report that a dedicated search along the approach trajectory recovered 47 meteorites, fragments of a single body named Almahata Sitta, with a total mass of 3.95 kg. Analysis of one of these meteorites shows it to be an achondrite, a polymict ureilite, anomalous in its class: ultra-fine-grained and porous, with large carbonaceous grains. The combined asteroid and meteorite reflectance spectra identify the asteroid as F class, now firmly linked to dark carbon-rich anomalous ureilites, a material so fragile it was not previously represented in meteorite collections.
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...
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