Determination of the height of the “meteoric explosion”

When cosmic bodies of asteroidal and cometary origin, with a size from 20 to approximately 100 m, enter dense atmospheric layers, they are destroyed with a large probability under the action of aerodynamic forces and decelerated with the transfer of their energy to the air at heights from 20–30 to several kilometers. The forming shock wave reaches the Earth’s surface and can cause considerable damage at great distances from the entry path similar to the action of a high-altitude explosion. We have performed a numerical simulation of the disruption (with allowance for evaporation of fragments) and deceleration of meteoroids having the aforesaid dimensions and entering the Earth’s atmosphere at different angles and determined the height of the equivalent explosion point generating the same shock wave as the fall of a cosmic body with the given parameters. It turns out that this height does not depend on the velocity of the body and is approximately equal to the height at which this velocity is reduced by half. The obtained results were successfully approximated by a simple analytical formula allowing one to easily determine the height of an equivalent explosion depending on the dimensions of the body, its density, and angle of entry into the atmosphere. A comparison of the obtained results with well-known approximate analytical (pancake) models is presented and an application of the obtained formula to specific events, in particular, to the fall of the Chelyabinsk meteorite on February 15, 2013, and Tunguska event of 1908, is discussed.     

Platinum group element abundances in a peat layer associated with the Tunguska event, further evidence for a cosmic origin

We have measured excesses of Pd, Rh, Ru, REE, Co, Sr, and Y in a peat column from the Northern peat bog of the 1908 Tunguska explosion site. Earlier, in this peat column the presence of an Ir anomaly at the event layers (30- depth) has been found (Planet Space Sci. 48 (1998) 179). In these layers, Pd, Rh, Ru, Co, Sr, and Y show pronounced anomalies of a factor 4-7 higher than the background value. In the event layers there are also good correlations between the siderophile platinum group elements (Pd, Rh, Ru) and Co, indicators of cosmic material, which imply they might have the same source, i.e. the Tunguska explosive body. The patterns of CI-chondrite-normalized REE in the event layers are much flatter than those in normal peat layers and different from those in the nearby traps. Furthermore, in these layers the patterns of CI-chondrite-normalized PGEs and the element ratios (e.g. C/Pd, C/Rh, and between some siderophile elements) give evidence that the Tunguska explosive body was more likely a comet, although we cannot exclude the possibility that the impactor could be a carbonaceous asteroid. We have estimated the total mass of a solid component of the explosive body up to 10³-10⁶ tons.     

On the problem of the Tunguska meteorite material

The approximate composition of the Tunguska meteorite remnants obtained by averaging the results of several measurements is presented. It is pointed out that the matter of the cosmic-body remnants was enriched with alkaline and alkaline-earth elements. The composition of the meteorite matter was extremely heterogeneous. The upper limit of the density of the Tunguska cosmic body has been estimated at 2.8 g/cm³. It is suggested that, due to interaction with the Earth’s atmosphere, the cosmic body disintegrated into fragments from 10^?7 to 10^?3 m in size, with the majority of the matter being ejected to the upper atmospheric layers. Calculations of the rate and the time of the sedimentation of particles in the atmosphere have shown that the change in atmosphere transparency is controlled by particles larger than 10^?5 m in radius.     

Ionospheric and magnetospheric disturbances caused by impacts of small comets and asteriods

Disturbances in the Earths’s ionosphere and magnetosphere caused by impacts of small comets and asteroids (with diameters from 50–60 m to 1–2 km) are analyzed. Two-dimensional hydrodynamical computations of the passage of a cosmic body through the atmosphere with allowance for deceleration and destruction due to aerodynamic loading and formation of the wake behind the body are performed. The tenuous wake facilitates an upward ejection of the plume (heated air and ablation products of the cosmic body). Numerical simulations of the motion of the plume and of its interaction with the geomagnetic field are performed. It is shown that part of the plume moves at higher than escape velocity. The rising plume operates as an MHD generator. Field-aligned currents heat the ionosphere and change its conductivity. The estimated magnetic variations are on the order of those of typical magnetic storms (for bodies with sizes comparable to the Tunguska meteorite) and are even higher for cosmic bodies with diameters of 200–400 m. Excitation of MHD waves is demonstrated. These disturbances are capable of triggering precipitation of particles from radiation belts and exciting intense electromagnetic noise. Strong oscillations of conducting ionospheric layers propagate radially from the place of impact of the low-velocity part of the plume to large distances from the impact point. For a 1-km body the energy of the high-velocity plume is comparable to that of the Earths’s magnetic field. This causes extremely intense magnetospheric disturbances. However, even 200-to 400-m bodies whose high-velocity part of the plume has energies exceeding 0.4–3 Mt TNT—i.e., much lower than the initial kinetic energy of the intruding body—produce global ionospheric and magnetospheric disturbances.     

Air penetration enhances fragmentation of entering meteoroids

The entry and subsequent breakup of the ~17–20 m diameter Chelyabinsk meteoroid deposited approximately 500 kT of TNT equivalent energy to the atmosphere, causing extensive damage that underscored the hazard from small asteroid impacts. The breakup of the meteoroid was characterized by intense fragmentation that dispersed most of the original mass. In models of the entry process, the apparent mechanical strength of the meteoroid during fragmentation, ~1–5 MPa, is two orders of magnitude lower than the mechanical strength of the surviving meteorites, ~330 MPa. We implement a two-material computer code that allows us to fully simulate the exchange of energy and momentum between the entering meteoroid and the interacting atmospheric air. Our simulations reveal a previously unrecognized process in which the penetration of high-pressure air into the body of the meteoroid greatly enhances the deformation and facilitates the breakup of meteoroids similar to the size of Chelyabinsk. We discuss the mechanism of air penetration that accounts for the bulk fragmentation of an entering meteoroid under conditions similar to those at Chelyabinsk, to explain the surprisingly low values of the apparent strength of the meteoroid during breakup.     

The nature of the Tunguska meteorite

Abstract— Arguments in favor of the cometary origin of the Tunguska meteorite are adduced along with reasons against the asteroidal hypothesis. A critical analysis is given for the hypotheses by Sekanina (1983) and Chyba et al. (1993). On the basis of the azimuth and inclination of the trajectory of the Tunguska body with plausible values of the geocentric velocity, the semimajor axis of the orbit and its inclination to the ecliptic plane are calculated for this body. It is noted that the theory of the disintegration of large bodies in the atmosphere put forward by Chyba et al. (1993) is crude. Applying more accurate theories (Grigoryan, 1979; Hills and Goda, 1993) as well as taking into account the realistic shape of the body yield for the cometary body lower disruption heights than obtained by Chyba et al. Numerical simulations carried out by Svettsov et al. agree well with the cometary hypothesis and the analytical calculations based on Grigoryan's theory. The asteroidal hypothesis is shown not to be tenable: the complete lack of stony fragments in the region of the catastrophe, cosmochemical data (in particular, the results of an isotope analysis), and some other information contradict this hypothesis. It is shown that stony fragments that would have originated in the explosive disruption of the Tunguska body would not be vaporized by the radiation of the vapor cloud nor as a result of their fall to the Earth's surface.     

Ionosphere disturbances accompanying the flight of the Chelyabinsk body

Data received from a network of ionosondes located at distances of 1500–3100 km from the Chelyabinsk meteorite site are used to analyze ionospheric disturbances at a height of approximately 300 km following the flight and explosion of the space body. The fall of the meteoroid is believed to be accompanied by the generation of gravitational waves in the neutral atmosphere and traveling ionospheric disturbances. The velocity and period of the latter are 600–700 m/s and 70–135 min, respectively; the amplitude of relative electron concentration disturbances is 10–20%. There is evidence of the 6–7 h ionospheric presence of wave electron concentration disturbances with relative amplitude of 10–20%, which could have been caused by long-living whirlwinds in the upper atmosphere.     

Physical mechanism of Chelyabinsk superbolide explosion

The work presents modern ideas on the physical mechanism of explosion of large meteoroids (superbolides) in the Earth’s atmosphere at the end of their trajectories. As a result of our work, the values of following parameters were obtained: the altitude of the beginning of the aerodynamic destruction of a meteoroid like the Chelyabinsk superbolide; the altitude of a relatively very thin layer, characterized by sharp aerodynamic deceleration of a fragmenting and laterally expanding space object, accompanied by an impulse transformation of kinetic energy into thermal energy with plasma generation which results in intense electromagnetic radiation and an explosive shock wave; and, the initial temperature of such a plasma.     

Asteroid Apophis: Evaluating the impact hazards of such bodies

Soon after the discovery of asteroid 99942 Apophis, it was classified as a potentially hazardous object with a high probability of an impact on the Earth in 2029. Although subsequent observations have substantially reduced the probability of a collision, it has not been ruled out; moreover, similar-sized asteroids in orbits intersecting the Earth’s orbit may well be discovered in the near future. We conduct a numerical simulation of an atmospheric passage and an impact on the Earth’s surface of a stony cosmic body with a diameter of 300 m and kinetic energy of about 1000 Mt, which roughly corresponds to the parameters of the asteroid Apophis, at atmospheric entry angles of 90° (vertical stroke), 45°, and 30°. The simulation is performed by solving three-dimensional equations of hydrodynamics and radiative transfer equations in the approximations of radiative heat conduction and volume emission. The following hazards are considered: an air shock wave, ejecta from the crater, thermal radiation, and ionospheric disturbances. Our calculations of the overpressure and wind speed on the Earth’s surface show that the zone of destruction of the weakest structures can be as large as 700–1000 km in diameter; a decrease in the flight path angle to the surface leads to a marked increase in the area affected by the shock wave. The ionospheric disturbances are global in nature and continue for hours: at distances of several thousand kilometers at altitudes of more than 100 km, air density disturbances are tens of percent and the vertical and horizontal velocity components reach hundreds of meters per second. The impact of radiation on objects on the Earth’s surface is estimated by solving the equation of radiative transfer along rays passing through a luminous area. In clear weather, the size of the zone where thermal heating may ignite wood can be as large as 200 km, and the zone of individual fire outbreaks associated with the ignition of flammable materials can be twice as large. In the 100-km central area, which is characterized by very strong thermal damage, there is ignition of structures, roofs, clothes, etc. The human hazardous area increases with the decrease in the trajectory angle, and people may experience thermal effects at distances of up to 250–400 km from the crater.     

Discovery of probable Tunguska cosmic body material: anomalies of platinum group elements and rare-earth elements in peat near the Explosion Site (1908)

Ten Sphagnum fuscum peat samples collected from different depths of a core including the layer affected by the 1908 Tunguska explosion in the Tunguska area of Central Siberia, Russia, were analyzed by ICP-MS to determine the concentrations of Pd, Rh, Ru, Co, REE, Y, Sr, and Sc. The analytical results indicate that the Pd and Rh concentrations in the event- and lower layers were 14.0–19.9, and 1.23–1.56 ppb, respectively, about 3–9 times and 3 times higher than the background values in the normal layers. In addition, the patterns of CI-chondrite-normalized REE in the event layers were much flatter than in the normal layers, and differed from those in the nearby traps. Hence, it can be inferred from the characteristics of the elemental geochemistry that the explosion was probably associated with extraterrestrial material, and which, most probably, was a small comet core the dust fraction of which was chemically similar to carbonaceous chondrites (CI). In terms of the Pd and REE excess fluxes in the explosion area, it can be estimated that the celestial body that exploded over Tunguska in 1908 weighed more than 10⁶ t, corresponding to a radius of >60 m. If the celestial body was a comet, then its total mass was more than 2×10⁷ t, and it had >160 m radius, and released an energy of >10⁷ t TNT.     

Astronomical and physical aspects of the Chelyabinsk event (February 15, 2013)

Various observational data including infrasound, seismic, optical (onboard) monitoring, ground video and photo records, and evidence from witnesses of the Chelyabinsk event on February 15, 2013, have been analyzed. The extensive material gathered has provided a base for investigations of the physical properties of the object, the results of which are discussed. A bolide light curve is constructed, which shows a multiplicity of flashes. Estimations of the energy of the meteoroid explosion, which took place in the atmosphere at an altitude of about 23 km, show evidence of the formation of a high-power shock wave equivalent to 300–500 kilotons of TNT. The object diameter corresponding to this energy falls within the range 16–19 m. The trajectory of the meteor is outlined. It is preliminarily concluded that the Chelyabinsk meteorite was a representative the Apollo asteroid family.     

Cosmogenic radionuclides and mineralogical properties of the Chelyabinsk (LL5) meteorite: What do we learn about the meteoroid?

On February 15, 2013, after the observation of a brilliant fireball and a spectacular airburst over the southern Ural region (Russia), thousands of stones fell and were rapidly recovered, bringing some extremely fresh material for scientific investigations. We undertook a multidisciplinary study of a dozen stones of the Chelyabinsk meteorite, including petrographic and microprobe investigations to unravel intrinsic characteristics of this meteorite. We also study the short and long‐lived cosmogenic radionuclides to characterize the initial meteoroid size and exposure age. Petrographic observations, as well as the mineral compositions obtained by electron microprobe analyses, allow us to confirm the classification of the Chelyabinsk meteorite as an LL5 chondrite. The fragments studied, a few of which are impact melt rocks, contain abundant shock melt veins and melt pockets. It is likely that the catastrophic explosion and fragmentation of the Chelyabinsk meteoroid into thousands of stones was in part determined by the initial state of the meteoroid. The radionuclide results obtained show a wide range of concentrations of ¹⁴C, ²²Na, ²⁶Al, ⁵⁴Mn, ⁵⁷Co, ⁵⁸Co, and ⁶⁰Co, which indicate that the pre‐atmospheric object had a radius >5 m, consistent with other size estimates based on the magnitude of the airburst caused by the atmospheric entry and breakup of the Chelyabinsk meteoroid. Considering the observed ²⁶Al activities of the investigated samples, Monte Carlo simulations, and taking into account the ²⁶Al half‐life (0.717 Myr), the cosmic‐ray exposure age of the Chelyabinsk meteorite is estimated to be 1.2 ± 0.2 Myr. In contrast to the other radionuclides, ¹⁴C showed a very large range only consistent with most samples having been exposed to anthropogenic sources of ¹⁴C, which we associate with radioactive contamination of the Chelyabinsk region by past nuclear accidents and waste disposal, which has also been confirmed by elevated levels of anthropogenic ¹³⁷Cs and primordial ⁴⁰K in some of the Chelyabinsk fragments.     

Nitrate in the Greenland ice sheet in the years following the 1908 tunguska event

The Tunguska event on 30 June 1908 has been subjected to much speculation within different fields of research. Publication of the results of the 1961 expedition to the Tunguska area (Florensky, 1963) supports that a cometary impact caused the event. Based on this interpretation, calculations of the impactor energy release and explosion height have been reported by Ben-Menahem (1975), and velocity, mass, and density of the impactor by Petrov and Stulov (1975). Park (1978) and Turco et al., 1981, Turco et al., 1982, used these numbers to calculate a production of ca. 30 × 10⁶ tons of NO during atmospheric transit. This paper presents a high-resolution study of nitrate concentration in the Greenland ice sheet in ca. 10 years covering the Tunguska event. No signs of excess nitrate are found in three ice cores from two different sites in Greenland in the years following the Tunguska event. By comparing these results with results for other aerosols generally found in the ice, the lack of excess NO₃^? following the Tunguska event can be interpreted as indicating that the impactor nitrate production calculated by Park (1978) and Turco et al., 1981, Turco et al., 1982 are 1–2 orders of magnitude too high. To explain this it is suggested, from other lines of reasoning, that the impactor density determined by Petrov and Stulov (1975) probably is too low.     

THE TUNGUSKA EVENT AND THE METEORS WITH TERMINAL FLARES

The comparison of the Tunguska body explosion with the effect of terminal flares of meteors and fireballs leads us to the conclusion that these events are of a similar nature but differ only by their scale. We consider that the dynamics of progressive breaking and evaporation of meteoric bodies during their entry into the terrestrial atmosphere could explain the terminal burst. An extremely porous body model for the Tunguska meteorite was analysed and rejected as unsatisfactory. The realistic values of the initial velocity (～30 km/sec) and of the inclination angle for the Tunguska's trajectory (5–15°) give orbital elements not in contradiction with the cometary origin of the Tunguska body.     

On the possibility of a secondary explosion in the antipode of a large meteorite impact point

At large distances, due to atmospheric absorption and the dispersion of high-frequency components, the airwaves from the fall of large meteorites or heavy-yield explosions are transformed into an infrasonic wave train propagating over large distances via atmospheric sound channels. In approaching the antipode, the amplitude of infrasonic oscillations increases significantly and the nonlinear effects may trigger the formation of a blast wave, that is, another explosion. The condition which allows such a phenomenon to happen was obtained in this study. Infrasonic waves from the Tunguska fall event and waves generated by the largest nuclear explosions were considered in this study.     

A numerical assessment of simple airblast models of impact airbursts

Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy‐based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving‐source airburst model where the meteoroid's energy is partitioned as two‐thirds internal energy and one‐third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving‐source model predicts overpressures two times greater than the static‐source model, whereas the cylindrical line‐source model based on the pancake model predicts overpressures two times lower than the static‐source model. Given other uncertainties associated with airblast damage predictions, the static‐source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.     

The Chelyabinsk meteorite: Thermal history and variable shock effects recorded by the 40Ar-39Ar system

We studied three lithologies (light and dark chondritic and impact melt rock) differing in shock stage from the LL5 chondrite Chelyabinsk. Using the ⁴⁰Ar-³⁹Ar dating technique, we identified low- and high-temperature reservoirs within all samples, ascribed to K-bearing oligoclase feldspar and shock-induced jadeite–feldspar glass assemblages in melt veins, respectively. Trapped argon components had variable ⁴⁰Ar/³⁶Ar ratios even within low- and high-temperature reservoirs of individual samples. Correcting for trapped argon revealed a lithology-specific response of the K-Ar system to shock metamorphism, thereby defining two distinct impact events affecting the Chelyabinsk parent asteroid (1) an intense impact event ~1.7 ± 0.1 Ga ago formed the light–dark-structured and impact-veined Chelyabinsk breccia. Such a one-stage breccia formation is consistent with petrological observations and was recorded by the strongly shocked lithologies (dark and impact melt) where a significant fraction of oligoclase feldspar was transformed into jadeite and feldspathic glass; and (2) a young reset event ~30 Ma ago particularly affected the light lithology due to its low argon retentivity, while the more retentive shock-induced phases were more resistant against thermal reset. Trapped argon with ⁴⁰Ar/³⁶Ar ratios up to 1900 was likely incorporated during impact-induced events on the parent body, and mixed with terrestrial atmospheric argon contamination. Had it not been identified via isochrons based on high-resolution argon extraction, several geochronologically meaningless ages would have been deduced.     

Mineralogy,petrology, chronology,and exposure history of the Chelyabinsk meteorite and parent body

Three masses of the Chelyabinsk meteorite have been studied with a wide range of analytical techniques to understand the mineralogical variation and thermal history of the Chelyabinsk parent body. The samples exhibit little to no postentry oxidation via Mössbauer and Raman spectroscopy indicating their fresh character, but despite the rapid collection and care of handling some low levels of terrestrial contamination did nonetheless result. Detailed studies show three distinct lithologies, indicative of a genomict breccia. A light‐colored lithology is LL5 material that has experienced thermal metamorphism and subsequent shock at levels near S4. The second lithology is a shock‐darkened LL5 material in which the darkening is caused by melt and metal‐troilite veins along grain boundaries. The third lithology is an impact melt breccia that formed at high temperatures (~1600 °C), and it experienced rapid cooling and degassing of S₂ gas. Portions of light and dark lithologies from Chel‐101, and the impact melt breccias (Chel‐102 and Chel‐103) were prepared and analyzed for Rb‐Sr, Sm‐Nd, and Ar‐Ar dating. When combined with results from other studies and chronometers, at least eight impact events (e.g., ~4.53 Ga, ~4.45 Ga, ~3.73 Ga, ~2.81 Ga, ~1.46 Ga, ~852 Ma, ~312 Ma, and ~27 Ma) are clearly identified for Chelyabinsk, indicating a complex history of impacts and heating events. Finally, noble gases yield young cosmic ray exposure ages, near 1 Ma. These young ages, together with the absence of measurable cosmogenic derived Sm and Cr, indicate that Chelyabinsk may have been derived from a recent breakup event on an NEO of LL chondrite composition.     

The orbit and dynamical evolution of the Chelyabinsk object

The orbit of the Chelyabinsk object is calculated, applying the least‐squares method directly to astrometric positions. The dynamical evolution of this object in the past is studied by integrating equations of motion for particles with orbits from the confidence region. It is found that the majority of the Chelyabinsk clones reach the near‐Sun state. Sixty‐seven percent of these objects have collisions with the Sun for 15 Myr in our numerical simulations. The distribution of minimum solar distances shows that the most probable time for the encounters of the Chelyabinsk object with the Sun lies in the interval from ?0.8 Myr to ?2 Myr. This is consistent with the estimate of a cosmic ray exposure age of 1.2 Myr (Popova et al. 2013). A parent body of the Chelyabinsk object should experience strong tidal and thermal effects at this time. The possible association of the Chelyabinsk object with 86039 (1999 NC43) and 2008 DJ is discussed.