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.

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Abstract-We present the basic differential equations of meteor physics (the single body equations). We solve them numerically including two possible types of fragmentation: into large pieces and into a cluster of small fragments. We have written a Fortran code that computes the motion, ablation and light intensity of a meteoroid at chosen heights, and allows for the ablation and shape density coefficients σ and K, as well as the luminous efficiency τ, to be variable with height/time. We calibrated our fragmentation model (FM) by the best fit to observational values for the motion, ablation, radiation, fragmentation and the terminal masses (recovered meteorites) for the Lost City bolide. The FM can also handle multiple and overlapping meteor flares. We separately define both the apparent and intrinsic values of σ, K, and τ. We present in this paper values of the intrinsic luminous efficiency as function of velocity, mass, and normalized air density. Detailed results from the successful application of the FM to the Lost City, Innisfree, and Benešov bolides are also presented.Results of applying the FM to 15 bolides with very precise observational data are presented in a survey mode (Table 7). Standard deviations of applying our FM to all these events correspond to the precision of the observed values. Typical values of the intrinsic ablation coefficient are low, mostly in the range from 0.004 to 0.008 s 2 km −2 , and do not depend on the bolide type. The apparent ablation coefficients reflect the process of fragmentation. The bolide types indicate severity of the fragmentation process. The large differences of the "dynamic" and "photometric" mass from numerous earlier studies are completely explained by our FM. The fragmentation processes cannot be modeled simply by large values of the apparent ablation coefficient and of the apparent luminous efficiency. Moreover, our new FM can also well explain the radiation and full dynamics of very fast meteoroids at heights from 200 km to 130 km.

An analysis of the generation and propagation characteristics of infrasonic pressure waves excited during meteor entry into the earth's atmosphere is presented. Possible line source sound producing regions are determined for an assumed range of meteor entry parameters, gross fragmentation phenomena being neglected. A pressure wave model of a line source cylindrical blast wave produced by a high‐velocity meteoroid in a continuum gas is then formulated by using similarity theory. It is found that the strong shock behavior of the blast wave is confined to a cylindrical region whose radius R0 is proportional to the product of the meteor's Mach number and its diameter. By using the numerical blast wave solutions of Plooster as initial conditions a description of the wave form far from the source is obtained. Both refraction and attenuation of the airwaves are then calculated separately in an approximate manner. For meteors with an associated R0 ≲ 10 m for source altitude regions determined earlier, predicted attenuation is very severe. Dominant wave periods predicted for arrivals at the ground are 0.4–2.5 s for sources with 10 ≤ R0 ≤ 100 m. Finally, infrasonic data from Goerke, from Shoemaker, and from Johnson and Wilson for four recent events are analyzed. Kinetic energy estimates which are obtained range from 1017 to 1022 ergs, each with an uncertainty of about 2 orders of magnitude.

Abstract-We present instrumental observations ofthe Tagish Lake fireball and interpret the observed characteristics in the context of two different models of ablation. From these models we estimate the pre-atmospheric mass of the Tagish Lake meteoroid to be -56 tonnes and its porosity to be between 37 and 58%, with the lowest part of this range most probable. These models further suggest that some 1300 kg of gram-sized or larger Tagish Lake material survived ablation to reach the Earth's surface, representing an ablation loss of97% for the fireball. Satellite recordings of the Tagish Lake fireball indicate that 1.1 x 10 12 J of optical energy were emitted by the fireball during the last 4 s of its flight. The fraction of the total kinetic energy converted to light in the satellite pass band is found to be 16%. Infrasonic observations ofthe airwave associated with the fireball establish a total energy for the event of 1.66 ± 0.70 kT TNT equivalent energy. The fraction ofthis total energy converted to acoustic signal energy is found to be between 0.10 and 0.23%. Examination ofthe seismic recordings ofthe airwave from Tagish Lake have established that the acoustic energy near the sub-terminal point is converted to seismic body waves in the upper-most portion of the Earth's crust. The acoustic energy to seismic energy coupling efficiency is found to be near 10-6 for the Tagish Lake fireball. The resulting energy estimate is near 1.7 kT, corresponding to a meteoroid 4 m in diameter. The seismic record indicates extensive, nearly continuous fragmentation ofthe body over the height intervals from 50 to 32 km. Seismic and infrasound energy estimates are in close agreement with the preatmospheric mass of 56 tonnes established from the modeling. The observed flight characteristics of the Tagish Lake fireball indicate that the bulk compressive strength of the pre-atmospheric Tagish Lake meteoroid was near 0.25 MPa, while the material compressive strength (most appropriate to the recovered meteorites) was closer to 0.7 MPa. These are much lower than values found for fireballs of ordinary chondritic composition. The behavior of the Tagish Lake fireball suggests that it represents the lowest end of the strength spectrum of carbonaceous chondrites or the high end of cometary meteoroids. The bulk density and porosity results for the Tagish Lake meteoroid suggest that the low bulk densities measured for some small primitive bodies in the solar system may reflect physical structure dominated by microporosity rather than macroporosity and rubble-pile assemblages.