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date: 05 March 2021

Chelyabinsk Meteoritefree

  • Olga PopovaOlga PopovaInstitute for Dynamics of Geospheres, Russian Academy of Sciences

Summary

The asteroid impact near the Russian city of Chelyabinsk on February 15, 2013, was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding 1 million. On clear morning at 9:20 a.m. local time, an asteroid about 19 m in size entered the Earth atmosphere near southern Ural Mountains (Russia) and, with its bright illumination, attracted the attention of hundreds of thousands of people. Dust trail in the atmosphere after the bolide was tens of kilometers long and was visible for several hours. Thousands of different size meteorites were found in the areas south-southwest of Chelyabinsk.

A powerful airburst, which was formed due to meteoroid energy deposition, shattered thousands of windows and doors in Chelyabinsk and wide surroundings, with flying glass injuring many residents.

The entrance and destruction of the 500-kt Chelyabinsk asteroid produced a number of observable effects, including light and thermal radiation; acoustic, infrasound, blast, and seismic waves; and release of interplanetary substance. This unexpected and unusual event is the most well-documented bolide airburst, and it attracted worldwide attention. The airburst was observed globally by multiple instruments. Analyses of the observational data allowed determination of the size of the body that caused the superbolide, its velocity, its trajectory, its behavior in the atmosphere, the strength of the blast wave, and other characteristics. The entry of the 19-m-diameter Chelyabinsk asteroid provides a unique opportunity to calibrate the different approaches used to model meteoroid entry and to calculate the damaging effects.

The recovered meteorite material was characterized as brecciated LL5 ordinary chondrite, in which three different lithologies can be distinguished (light-colored, dark-colored, and impact-melt). The structure and properties of meteorites demonstrate that before encountering Earth, the Chelyabinsk asteroid had experienced a very complex history involving at least a few impacts with other bodies and thermal metamorphism.

The Chelyabinsk airburst of February 15, 2013, was exceptional because of the large kinetic energy of the impacting body and the damaging airburst that was generated. Before the event, decameter-sized objects were considered to be safe. With the Chelyabinsk event, it is possible, for the first time, to link the damage from an impact event to a well-determined impact energy in order to assess the future hazards of asteroids to lives and property.

Portrait of the Event

On February 15, 2013, at 9:20 a.m. local time (nearly at sunrise; 3:20 universal time [UT]) the citizens of the Russian city Chelyabinsk (an industrial city with a population exceeding 1 million and located near the southern Ural Mountains, ~1500 km south-east from Moscow) and its wide surroundings were amazed by a bright bolide on a clear morning sky. The luminous path of the asteroid through the atmosphere started at about 100-km altitude and lasted about 17 s. The brightness grew strongly, and the maximum was observed approximately 11–12 s after the appearance of the meteor. The meteor brightness peaked at 30 km above the surface, approximately 40 km south of the Chelyabinsk city center (Borovička et al., 2013; Brown et al., 2013; Popova et al., 2013). At that point the bolide appeared brighter than the Sun, even for people 100 km away. Distinct fragments were seen below the point of maximal brightness, and about 60 s later the largest surviving piece reached the ice-covered Lake Chebarkul, 70 km west of Chelyabinsk, creating a 7-m-wide hole in the ice. Thousands of smaller meteorites were found in a strewn field tens of kilometers long (Badyukov, Dudorov, & Khaibrakhmanov, 2014; Popova et al., 2013). An impressive dust trail in the atmosphere formed immediately after the bolide. It gradually dispersed and remained visible for several hours, attracting the attention of local residents. The dust trail was also clearly observed from satellites in the Earth’s orbit (Miller et al., 2013; Proud, 2013).

An air blast wave reached the ground in the Chelyabinsk region within several minutes (video records indicate a time interval of 77 s to 3 min and above) after the bolide, depending on the location. The airburst was strong enough to create widespread glass damage or other minor structural damage. The shock wave hit a densely populated area, and more than 1,600 people asked for medical assistance at hospitals (Kartashova et al., 2018; Popova et al., 2013). The air blast wave was followed by a number of weaker secondary sonic booms, which lasted for several minutes.

The Chelyabinsk impact is the most well-documented event of this kind and provides a unique opportunity to calibrate the different approaches used to model meteoroid entry and to calculate the damaging effects of the resulting shock waves and radiations. A better understanding of what happened might help future impact hazard mitigation efforts.

Similar Events

Cosmic objects striking Earth may result in extreme consequences of different scale. Examples of such impacts include the Chicxulub impact that caused a crater 180 km in diameter and is connected with the Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary [K–T] dinosaur extinction event) (Alvarez, Alvarez, Asaro, & Michel, 1980), the impressive 1.1-km-wide Meteor Crater (Barringer, 1909), the tree-fall area of about 2,000 km2 caused by the famous Tunguska airburst event (Longo et al., 2005), and others. The largest impacts (from objects with diameters >1 km) are rare events and occur on geological timescales. However, even much more frequent, smaller bodies with sizes from a meter to tens of meters can carry a lot of energy. Typically the energy of cosmic impactors is expressed in kiloton (kt) and megaton (Mt) TNT (1 kt TNT = 4.184 × 1012 J) because their energy is comparable to the largest explosions and other high-energy events (Table 1). For example, a stone asteroid 6 m in diameter, with a density of 3,300 kg/m3, entering the Earth’s atmosphere with a speed of 20 km/s, has a kinetic energy equivalent to 17 kt of TNT, which is comparable to that of the atomic bomb dropped on Hiroshima (Table 1).

Table 1. Comparison of Known Bolide Energies With Natural and Artificial Explosions

Bolide

Energy, kt TNT

Atmospheric explosions

Energy, kt TNT

Tunguska (1908)

5,000–20,000

Largest nuclear explosion (USSR, 1961)

50,000

Chelyabinsk (2013)

500

Mount St. Helen volcanic eruption (United States, 1980)

24,000

South African (1963)

500–1,500

Pinatubo volcanic eruption (Indonesia, 1991)

25,000

Indonesia (2009)

33–50

Hiroshima bomb

13–18

Marshall Islands (1994)

40

Largest conventional explosion (United States, 1985)

4.3

Sikhote Alin (1947)

10

Almahata Sitta (2008)

1.3

The largest impact in modern recorded history was the 1908 Tunguska event in Siberia, for which kinetic energy estimates range broadly from 3−5 Mt (Boslough & Crawford, 1997) to 10−50 Mt (Svetsov & Shuvalov, 2008) because of a lack of good observations at the time (Table 1). Another event with energy closer to that of the Chelyabinsk one probably occurred on August 3, 1963, although only an infrasound signal was recorded for that event (Silber, ReVelle, Brown, & Edwards, 2009, Table 1). Numerous smaller bolide events have also been recorded; about a dozen bolides had energies between 10 and 40 kt TNT. The two largest events of these occurred near the Marshall Islands in 1994 (McCord et al., 1995) and near Sulawesi, Indonesia, in 2009 (Silber, Le Pichon, & Brown, 2011) (Figure 1). At the time of this paper preparation, information about a new 170-kt impact over the Bering Sea (about 400 km from Kamchatka peninsula) on December 18, 2018, was released by the U.S. government satellite observational system. In addition, at least two meteorological satellites observed the Bering Sea bolide entry, and an infrasound signal was recorded by the International Monitoring System. More information about this event will probably appear in the future.

Figure 1. The fireballs recorded optically by the U.S. government sensors. Locations of the Chelyabinsk (February 15, 2013) and Bering Sea (December 18, 2018) are marked.

Observational Data

The entrance and destruction of Chelyabinsk-like celestial bodies in the Earth’s atmosphere produces a number of observable effects, including light and thermal radiation; acoustic, infrasound, blast, and seismic waves; and release of interplanetary substance.

Because of the almost clear atmospheric conditions, the extremely bright Chelyabinsk = superbolide—that is, a bolide brighter than –17 magnitude (Ceplecha et al., 1999)—was widely observed from a dozen regions of Russia and two regions of Kazakhstan (Borovička et al., 2016; Emel’yanenko et al., 2013). The information about these observations became available because of numerous video records taken by citizens and posted on the Internet at different websites. Rich observational data were collected by virtue of many security and vehicle dashboard cameras operating in the area at the time. The most complete catalogue was published by Borovička et al. (2016) and is available online. The current version of the catalog contains 960 videos, 400 of which show at least part of the bolide in flight. The most distant bolide records came from the Samara region, more than 700 km from the bolide. The records of the trail are more concentrated in the vicinity of the bolide, and the trail was visible at distances of more than 500 km, for example, from Orenburg (Borovička et al., 2016). The sonic booms and the ground damage extended to about 100 km from the brightest part of the bolide, perpendicular to the trajectory (Popova et al., 2013).

The bolide was spectacular and impressive (Figure 2; see also numerous videos listed in the catalogue [Borovička et al., 2016] and posted on YouTube). It appeared as a small bright spot in the morning sky and became brighter than the Sun in the locations closer to the main flare. Individual fragments were seen in the videos during the final stages of the trajectory, below a height of 26 km, where the fragments were separated enough to be resolved. That provided an opportunity to study the fragmentation of the meteoroid.

Figure 2. A few video frames demonstrating the complicated fragmentation scenario of the Chelyabinsk meteorite.

Source: Extracted from the video film Chelyabinsk Meteor Dust Traveled Around the World (Credit: Sergey Zhabin).

The asteroid light curve, the meteoroid trajectory and orbit, the detailed behavior of the asteroid, its fragments, and its wake in the atmosphere were determined based on the collected videos (Borovička et al., 2013; Brown et al., 2013; Popova et al., 2013). The video records also provided data that allowed estimations of damage and injuries caused by the entry, and the effects of this event on the public (Brown et al., 2013; Kartashova et al., 2018; Popova et al., 2013).

Optical radiation from the superbolide was recorded by the satellite-based U.S. government sensors. These data timed the peak brightness to 03:20:32.2 UTC (coordinated universal time) on February 15, 2013, with an integrated radiated energy of 3.753 × 1014 J and a peak brightness of 2.73 × 1013 W/sr (Brown et al., 2013). Satellite data also enabled estimates of the approximate geographical location of the peak brightness. The light curve of the meteor is the total radiated intensity of the fireball at optical wavelengths as a function of time or altitude. The Chelyabinsk light curve was reconstructed independently by several authors (Borovička et al., 2013; Brown et al., 2013; Emel’yanenko et al., 2013; Popova et al., 2013) from selected video records. The extracted light curves have uncertainty caused by unknown properties of video cameras and automatic gain control. All obtained light curves have three peaks, although their relative brightness and durations vary. Calibration of the light curve was done by the power maximum measured by the U.S. government sensors (Brown et al., 2013) and by comparison with the near-full Moon in similar lighting conditions as the fireball (Popova et al., 2013). The maximum occurred at a height of 30 km, and the absolute (as seen from a 100-km distance) stellar magnitude reached about −28 magnitude, meaning that the bolide was about three times brighter than the Sun (magnitude −26.7).

The main source of perturbations in the atmosphere was a shock wave, resulting in subsequent seismic, acoustic, and infrasound waves. Acoustic waves (20 Hz–20 kHz) mostly propagate through relatively short distances. The infrasound low-frequency acoustic waves (from 20 Hz to 3 × 10–3 Hz) weakly decay in the atmosphere and can be detected over quite large distances. These perturbations can propagate in atmospheric waveguides formed at various altitudes by temperature gradients, wind velocity, and direction over distances up to several thousand kilometers. Infrasound from the Chelyabinsk airburst was detected at ~20 stations of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) stations (Brown et al., 2013; Le Pichon et al., 2013) as well as other stations all over the globe (Popova et al., 2013).

The meteorite fragments that survived the atmospheric entry hit the ground at subsonic terminal velocity (Popova et al., 2013) and did not cause any seismic shaking detectable at regional distances. However, because of direct coupling of the atmospheric shock wave with the ground, seismic waves were recorded hundreds to thousands of kilometers away (Heimann, González, Wang, Cesca, & Dahm, 2013; Tauzin, Debayle, Quantin, & Coltice, 2013). The magnitude of the seismic event was estimated as 2.7–4, according to different sources.

Several Earth-viewing environmental satellites in both geostationary and polar orbits viewed the Chelyabinsk region within minutes of the superbolide, capturing the debris trail left as it passed through the middle atmosphere (Miller et al., 2013).

The fall of Chelyabinsk meteorite was accompanied by a number of dynamic ionospheric phenomena (Berngardt et al., 2013a, 2013b; Givishvili et al., 2013; Gokhberg, Olshanskaya, Steblov, & Shalimov, 2013; Tertyshnikov, Alpatov, Gluhov, Perminova, & Davydenko, 2013; Yang et al., 2014). The dynamic ionospheric effects include formation of medium-scale traveling ionospheric disturbances (MSTIDs) at F-layer heights, detected directly by coherent radar and GPS network at distances less than 1,000 km from the epicenter (Berngardt et al., 2013b; Gokhberg et al., 2013; Yang et al., 2014), and an indirect evidence of the same radial waves at E-layer heights at distances less than 700 km (Kutelev & Berngardt, 2013).

In addition to the instrumental data on the event, official information about the damage and injuries was available from the Russian Emergency Ministry, its Chelyabinsk office, and from local authorities. Furthermore, a fact-finding mission was organized by the Russian Academy of Science’s Institute for Dynamics of Geospheres and Institute of Astronomy. The mission was supported by Peter Jenniskens (Search for Extraterrestrial Intelligence [SETI] Institute, United States) and researchers from the Chelyabinsk State University, the South Ural State University in Chelyabinsk, and the Ural Federal University in Yekaterinburg. This collaborative effort aimed to secure as much information as possible to help determine the initial kinetic energy of the impact, the manner in which this energy was deposited in the atmosphere, the nature and properties of the shock wave, and the extent of the damage and injuries it caused on the ground (Popova et al., 2013, 2014). In a series of road trips, some 50 villages and towns in different directions from the city were visited between 3 and 5 weeks after the event, during which time about 150 locals were interviewed, and sites of glass damage were visited to verify the damage (Figure 3). Telephone interviews with some 500 residents of the Chelyabinsk area (0.04% of Chelyabinsk residents) were conducted by the Public Opinion Foundation (FOM) on February 23 and 24, 2013. About 1,800 residents filled out a special, elaborated web-based query form, starting on February 21, 2013 (Kartashova et al., 2018; Popova et al., 2013).

Figure 3. Map of glass damage on the ground with modeled overpressure footprints, meteorite finds, trajectory, and fireball brightness. Solid orange circles show locations where field surveys reported glass damage; open black circles show where field surveys found no damage; and solid red circles show the most damaged villages in each district reported by the government. Each point, irrespective of population density, represents one of many villages or city districts scattered throughout the area. Grayscale contours show modeled blast footprints representing two overpressure levels (Δ‎p >1,000 and >500 Pa) from burst of two kinetic energies (300 and 520 kT). Moving from the inside out, the levels represent: 300 kT, Δ‎p > 1,000 Pa; 520 kT, Δ‎p > 1,000 Pa; 300 kT, Δ‎p > 500 Pa; and 520 kT, Δ‎p > 500 Pa. Also shown are the locations of meteorite finds (yellow points) and the ground-projected fireball trajectory (black line), moving from 97 km altitude right to 14 km altitude left. White shows the fireball brightness on a linear scale.

Source: Popova et al. (2013). Reprinted with permission from AAAS.

Numerous meteorite samples were recovered and collected out of snow by scientists and local residents, mainly during the first 10 days after the event, before a subsequent extensive snowfall. Between February 19 and 25, 2013, a series of dedicated searches for meteorites was organized by the Chelyabinsk State University, the Ural Federal University in Yekaterinburg, and the Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences (Badyukov et al., 2014; Popova et al., 2013). The searches were continued after the snow melt, resulting in many more finds with known locations.

This unexpected and unusual event attracted worldwide attention. Analyses of the observational data allowed determination of the size of the body that caused the superbolide, its velocity, trajectory, behavior in the atmosphere, the strength of the blast wave, the affected area, and other characteristics.

Meteorites and Dust

The Chelyabinsk fireball, with an almost instant recovery of a large number of fresh meteorites, is one of the most spectacular meteorite fall events in recent history. Recovery of material from the fall along a well-defined path yielded hundreds of kilograms of material (Figure 3). The majority of the thousands of collected fragments are composed of relatively small centimeter-scale pieces. The largest fragment, which reached Lake Chebarkul, was excavated on October 16, 2013, and found to weigh ≥570 kg (Figure 4). The impacting mass may have been larger, up to about 600–650 kg, with a dozen kg-weighted pieces that were collected from the lake bottom and some additional pieces that are still there (Popova et al., 2013; Ruzicka, Grossman, Bouvier, & Herd, 2015; Ruzicka, Grossman, Bouvier, & Agee, 2017). A 7 m sized hole was created in 70 cm thick ice on Lake Chebarkul (Figure 5) by the decelerated fragment, which fell vertically, in line with the trajectory. Small meteorite fragments were found over an area up to 50 m from the impact location (Figure 5). A lakeshore video security camera, pointed to the site, recorded the cloud of ice and snow caused by the impact. The total mass of meteorites is estimated as only 0.03−0.05% (4–6 tons) of the initial mass (Popova et al., 2013).

Figure 4. Main mass of the Chelyabinsk fall at the Chelyabinsk State Museum of Local History, shortly after recovery from Chebarkul Lake. Photo courtesy of Andrey Yarantsev.

Source: Popova et al. (2013). Reprinted with permission from AAAS.

Figure 5. Small meteorite fragments (left) recovered by a UrFU research team from Chebarkul Lake near the hole in the ice layer; shown right is an airborne photograph taken by Eduard Kalinin from a Diamond C2 aircraft shortly after the hole’s discovery on February 16 at 11:05:34 UT.

Source: Popova et al. (2013). Reprinted with permission from AAAS.

More material remained, however, in the atmosphere, forming a dust and vapor trail up to 2 km wide and extending along the fireball trajectory from altitudes of 18 to 70 km (Figure 6). The dust trail left in the atmosphere was mostly formed by micron-sized dust and represents unablated residuals of tiny fragments and may include recondensed particles. Total mass of the dust may be from a few percent to 25% of the initial mass (Popova et al., 2013). Within a few days after the event, the dust circled the globe, forming an optically thin dust ring around the Northern Hemisphere (Gorkavyi, Rault, Newman, da Silva, & Dudorov, 2013). The dust remained detectable in the atmosphere for three months (Rieger, Bourassa, & Degenstein, 2014).

Figure 6. Smoke train 46–73 s after formation, from a composite of several photographs by Evgeny Tvorogov. Approximate altitude markers in kilometers.

Source: Popova et al. (2013). Reprinted with permission from AAAS.

The recovered meteorite material was characterized as brecciated LL5 ordinary chondrite. Based on the magnetic susceptibility being in the intermediate range between LL and L chondrites, the Chelyabinsk meteorites are richer in metallic iron as compared to other LL chondrites. The measured bulk and grain densities (3,220 kg/m3 and 3,540 kg/m3, correspondingly) and the porosity (8.2%) closely resemble other LL chondrites (Kohout et al., 2014). LL chondrites, like Chelyabinsk meteorite, account for about 14% of all known ordinary chondrites, which corresponds to 7,683 LL meteorites listed in the Meteoritical Bulletin Database (2020). In addition, the Japanese Hayabusa spacecraft visited asteroid Itokawa and returned samples to Earth. This LL material recovered by Hayabusa is complementary to many LL chondrite samples in world collections.

Three types of meteorite material were found within the recovered meteorites, described as the light-colored, dark-colored, and impact-melt lithologies. All are of LL5 composition, with the impact-melt lithology being close to whole-rock melt and the dark-colored lithology being shock-darkened because of partial melting of iron metal and sulfides (Kohout et al., 2014). The impact melt is silicate-rich and is present within the light-colored and dark-colored lithology stones as dark inter-granular veins. In the dark-colored lithology, additional fine-grained metal and sulfide-rich melt forming a dense network of fine veins impregnating the inter- and intra-granular pore space is present (Figure 7A). This metal and sulfide-rich melt is related to a higher shock experienced by the dark-colored lithology and is the main darkening agent.

Figure 7. (a) Chelyabinsk meteorite (diameter ~4 cm) showing shock veins. (b) Iron (Fe) element map of a shock vein. Note the metal layer (shown in green) located ~20 microns inside the vein.

Source: Popova et al. (2013). Reprinted with permission from AAAS.

Some meteorites samples broken in laboratory tests fragmented along shock veins—a possible weakness in the material that could have contributed to the abundant dust formation. A peculiar feature is that some shock veins exhibit a metal layer located ~20 microns inside the vein, which follows the outer contours of the vein (Figure 7B), indicating that metal initially segregated from the most rapidly solidifying rims of the vein. This could contribute to weakness.

Some studies suggested that, before encountering Earth, the Chelyabinsk asteroid had experienced a very complex history involving multiple stages of impact with other bodies and thermal metamorphism followed by fragmentation (Righter et al., 2015). Other research assumes only few impacts (Trieloff et al., 2018). An energetic impact event at 1.7 ± 0.1 Ga ago resulted in the formation of the Chelyabinsk breccia (i.e., a rock formed from angular-shaped pieces cemented together in a matrix), and then a more recent impact at 30 Ma ago (the youngest one reported for any of the LL chondrites) changed all three lithologies to varying degrees, depending on their retentivity. Finally, the breccia was ejected 1–1.5 Ma ago from the asteroid (Trieloff et al., 2018). The cosmic ray exposure age was measured to be 1.2 Myr, one of the lowest among LL chondrites (Popova et al., 2013; Povinec et al., 2015).

Impactor Properties

Different scientific groups estimated the projectile energy by various methods. The best impact energy estimate is in the range of 500 ± 100 kt, which is about 25 times the yield of Trinity, the first atomic explosion. A combination of seismic data, infrasound waves, radiated energy measured by U.S. government satellites, the energy of radiated light derived from video records, the extent of the damage, and modeling efforts provided these estimates (Borovička et al., 2013; Brown et al., 2013; Popova et al., 2013, 2014).

The bolide trajectory was computed from calibrated videos by Borovička et al. (2013) and Popova et al. (2013), and results are in good agreement (Table 2). The bolide was visible from about 95-km down to 13-km height, with a trajectory more than 270 km long at a slope of 18° relatively to the horizontal, and an initial entry velocity of about 19 km/s. The bolide passed only 35 km to the south (horizontal ground distance) from the center of Chelyabinsk city. At that point the bolide height was 28 km, very close to the point of maximum brightness, which was reached at the height of 30 km (Borovička, 2016).

Table 2. Bolide Trajectory and Velocity According to Two Detailed Papers

Boroviˇcka et al. (2013)

Popova et al. (2013)

Beginning

Longitude

64.477

64.565

Latitude

54.545

54.445

Height (km)

95.0

97.1

Velocity (km/s)

19.03

19.16

Time (UT)

3:20:21.07

3:20:20.8

End

Longitude

60.5883

60.625

Latitude

54.9361

54.931

Height (km)

12.57

13.6

Velocity (km/s)

3.2

4.9

Time (UT)

3:20:37.80

3:20:36.8

Entryradiant

Azimuth

103.5

103.2

Elevation

18.55

18.3

From the impactor energy estimate of 500 kt and the velocity of 19 km/s, the mass of about 12,000 Mt and diameter of 19 m was determined.

From the known trajectory and entry velocity, the pre-encounter heliocentric orbit of the asteroid was computed. Orbital elements derived by two teams are in good agreement (Borovička, 2016). The orbit is a typical near-Earth asteroid orbit, with perihelion in the vicinity of Venus’s orbit, aphelion approximately in the middle between the orbits of Mars and Jupiter, and a low inclination of 5 degrees. The most likely origin of the Chelyabinsk asteroid is in the inner asteroid belt. The Flora asteroid family or the related Baptistina family are considered as the most likely sources of the Chelyabinsk meteoroid (Reddy et al., 2014). These families seem to be common sources of LL chondrites (Vernazza et al., 2008; Vokrouhlicky, Bottke, & Nesvorny, 2017).

Damage and Injuries

The powerful airburst, which was formed due to meteoroid energy deposition, caused significant damage in 11 municipal districts of Chelyabinsk Oblast. The damage mainly consisted of shattered and broken glass from windows and doors, but some locations closer to the trajectory also showed window frames being pushed in by the airburst, as well as other minor structural damage such as cracks in walls (Popova et al., 2013). According to the official data, 7,320 buildings were affected. In Chelyabinsk itself, 3,613 apartment buildings (about 44% of the total number) had shattered and broken glass. These damages were not evenly distributed in the city. Structural damage included the collapse of a zinc factory roof. The most damaged areas included the most populated Chelyabinsk regions and districts close to the trajectory, such as Korkino and Yemanzhelinsk. The Chelyabinsk airburst caused damage out to 120 km from its trajectory. The blast wave resulted in over $33M (1 billion rubles) of damage on the ground.

The resulting damage map (Figure 3) demonstrates that the shock wave had a cylindrical component, extending furthest perpendicular to the trajectory. There was little coherence of the shock wave in the forward direction, where the disturbance was of long duration, shaking buildings and making people run outside, but causing no damage.

Combining the known trajectory with the arrival times of the blast wave at various sites, it was proven that the blast wave causing damage originated at various heights between 25 and 45 km, rather than at a single point (Brown et al., 2013; Popova et al., 2013). It was therefore produced by the supersonic flight of the fragmenting asteroid.

It was reported that 1,613 people asked for medical assistance at hospitals (Akimov et al., 2015), and 69 people were hospitalized, 2 of whom were in serious condition (Kartashova et al., 2018; Popova et al., 2013). The most severe injuries included cut wounds from broken glass shattered by the shock wave, broken bones, and brain injuries. Injuries due to the airburst were more severe than those due to thermal and UV radiation. A significant number of people were either in panic or stressed out following the airburst arrival, so that post-event injuries included effects from stress (Kartashova et al., 2018). Never before in recorded history were people injured from an asteroid impact in the same way.

Modeling

Modeling of the bolide light curve provides an understanding of how the meteoroid’s energy was deposited in the atmosphere. The modeling also allows inference of information about meteoroid internal structure. The known data are the trajectory and velocity, the observed deceleration toward the end, the trajectories and decelerations of individually observed fragments, the bolide light curve, and arrival times of sonic booms.

Since the time of the event, many studies have been dedicated to the modeling of the meteoroid light curve and deceleration, and the common feature of all reliable models is application of a fragmentation scenario. Different fragmentation modeling approaches were used. A liquid-like or “pancake” model, which assumes that the meteoroid is disrupted into a swarm of small bodies, which continue their flight as a single mass with an increasing, pancake-like cross-section, was applied to the Chelyabinsk event in several papers (see, e.g., Avramenko, Glazyrin, Ionov, & Karpeev, 2014; Collins, Lynch, McAdam, & Davison, 2017; McMullan & Collins, 2019). Progressive fragmentation (PF) models, which suggest that formed fragments continue their flight independently and may be disrupted further, was used by Brown et al. (2013). A semi-empirical fragmentation model, which is PF type and considers different kinds of fragments (individual, dust, and eroding) was applied by Borovička et al. (2013). Hybrid models take into account splitting into separate fragments; treated as a set of liquid-like or solid particles, they combined a progressive fragmentation approach (formation of individual fragments) with releases of dust or dispersing debris clouds (Mathias, Wheeler, & Dotson, 2017; Popova et al., 2013; Register, Mathias, & Wheeler, 2017; Wheeler, Mathias, Stokan, & Brown, 2018; Wheeler, Register, & Mathias, 2017; and others). In general all models provide a possibility for describing large flares observed in the Chelyabinsk meteor event and its energy deposition curve. Hybrid and PF models allow a description of a meteorites-strewn field (Borovička et al., 2013; Popova et al., 2013).

The dynamic pressure acting at fragmentation points provides information about the strength of the asteroid. Whereas the (tensile) strength of stony meteorites is typically about 50 MPa, the strengths of meter-sized meteoroids were found to be much lower by their atmospheric fragmentation analysis (Popova et al., 2011). The Chelyabinsk object followed this trend. The first significant fragmentation of Chelyabinsk occurred at a height of about 50–45 km, where the dynamic pressure was ∼ 0.5 MPa. Large-scale disruption occurred at heights 39–30 km under pressures 1–5 MPa. All models agree that more than 99% of the initial meteoroid mass was converted into dust and small fragments, probably of sub-kilogram mass (< 10 cm). Fragments that survived the main disruption were finally broken at heights of 26–21 km, under pressures of 10–18 MPa. Only one large fragment (∼ 60 cm) survived and reached Chebarkul Lake.

Fragments that survived much of the entry (Figure 2) hold only a small amount of initial energy. Shuvalov, Svetsov, Popova, and Glazachev (2017b) even modeled Chelyabinsk as a strengthless, liquid-like body and were able to reproduce the light curve reasonably well (of course, no meteorites on ground were produced). Increasing the complexity of the material model to better represent asteroid rock masses changes the detailed mechanics of the breakup, but only has a limited effect on the burst altitude and energy deposition (Robertson & Mathias, 2017).

The light curve is considered to be the most accurate estimate of the energy release profile in the atmosphere. The motion, deceleration, and fragmentation of the meteoroid in the atmosphere resulted in the energy deposition and is accompanied by the formation of a shock wave, which caused damage at large distances. The characteristics of the emerging shock wave and the details of its impact on the surface depend on the energy release profile (Aftosmis et al., 2016; Collins et al., 2017; Mathias et al., 2017; Shuvalov, Popova, Svettsov, Trubetskaya, & Glazachev, 2016; Shuvalov et al., 2017a, 2017b). The Chelyabinsk meteoroid energy deposition curve is extensively used now as a test case and a basis of different asteroid impact risk models, which are being developed to assess the risk that potential asteroid strikes pose to Earth’s population (Mathias et al., 2017; Rumpf, Lewis, & Atkinson, 2017; Shuvalov et al., 2017a, 2017b; Wheeler et al., 2017, 2018).

Hydrodynamic and computational fluid dynamics simulations of the blast wave propagation through the atmosphere (Figure 8) have been performed using different assumptions about the blast source energy to approximate the energy deposition along the meteoroid trajectory—spherical charge, line-source, pancake model, proportionality of energy deposition to emitted light (Aftosmis, Nemic, Mathias, & Berger, 2016; Avramenko et al., 2014; Brown et al., 2013; Collins et al., 2017; Popova et al., 2013) or in the frame of quasi-liquid-like (QL) model (Shuvalov et al., 2017b). Both the models based on the energy deposition estimates from the light curve and the QL model are able to reproduce the main characteristics of overpressure footprints (sizes, maximum overpressure) in satisfactory agreement with the observations. Modeling allows reproduction of not only the shock wave but also the radiation fluxes on the Earth surfaces (Shuvalov et al., 2017b) and some features of the formation and initial evolution of the trail (Artemieva, Shuvalov, & Khazins, 2019).

Figure 8. (a) Modeling result for formation and propagation of the shock wave. Shock wave is shown as a thin white line and was modeled assuming that energy deposition is proportional to the light curve (Popova et al., 2013; Shuvalov et al., 2017b). The formed structure along trajectory resembles the observed wake. (b) Chelyabinsk meteoroid wake observed from Troitsk (about 85 km south from the meteoroid trajectory).

Source: Photo by Lyubov-none.

Lessons From the Chelyabinsk Event

With Chelyabinsk, it is possible for the first time to link the damage from an impact event to a well-determined impact energy in order to assess the future hazards of asteroids to lives and property. Although about 90% of nearby objects greater than 1 km in diameter have been cataloged, most of the objects in the decameter size range remain undiscovered and could pose significant risk, as demonstrated by Chelyabinsk. Before the event, objects in the 20 m size range were considered to be safe. As a small object that approached Earth from the Sun, Chelyabinsk impacted the atmosphere with no prior warning. This unexpected and unusual event attracted worldwide attention and caused significant damage in a populated area.

Thanks to the many security and vehicle dashboard cameras operating in the area at the time, the Chelyabinsk airburst is the best-observed impact to date and provides a valuable test case. The statistics of large bolides (Brown et al., 2013) and asteroid discoveries (Harris & D’Abramo, 2015) now agree better than in the past and suggest that impacts of Chelyabinsk-like sizes occur globally once per 40 ± 20 years on average.

Such a large airburst also provides ground-truth data for validation of entry models of ground damage caused by impacts, an essential component of future planetary defense efforts. It was a reminder of the potential hazard of meteoroids and small asteroids, the vast majority of which are undetected, and motivates the need for accurate hazard and risk assessment.

Further Reading

  • Adushkin, V. V., & Nemchinov, I. V. (Eds.). (2008). Catastrophic events caused by cosmic objects. Berlin, Germany: Springer.
  • Artemieva, N., & Shuvalov, V. (2016). From Tunguska to Chelyabinsk via Jupiter. Annual Review of Earth Planetary Sciences, 44(1), 37–56.
  • Borovička, J., Spurný, P., Brown, P., Wiegert, P., Kalenda, P., Clark, D., & Shrbený, L. (2013). The trajectory, structure and origin of the Chelyabinsk asteroidal impactor. Nature, 503, 235–237.
  • Borovička, J. (2016). The Chelyabinsk event. Astronomy in focus. Paper presented at the IAU 29th General Assembly, 2015. Proceedings of the IAU (Vol. 29A, pp. 247–252).
  • Borovička, J., Macke, R. J., Campbell-Brown, M. D., Levasseur-Regourd, A.-C., Rietmeijer, F. J. M., & Kohourt, T. (2019). Physical and chemical properties of meteoroids. In G. O. Ryabova, D. J. Asher, & M. D. Campbell-Brown (Eds.), Meteoroids: Sources of meteors on the Earth and beyond (pp. 37–62). Cambridge, UK: Cambridge University Press.
  • Borovička, J., Spurný, P., & Brown, P. (2015). Small near-earth asteroids as a source of meteorites. In P. Michel, F. E. DeMeo, & W. F. Bottke (Eds.), Asteroids IV (pp. 257–280). Tucson: University of Arizona Press.
  • Brown, P. G., Assink, J. D., Astiz, L., Blaauw, R., Boslough, M. B., Borovicka, J., . . . Krzeminski, Z. (2013). A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors. Nature, 503(7475), 238–241.
  • Kring, D. A., & Boslough, M. (2014). Chelyabinsk: Portrait of an asteroid airburst. Physics Today, 67(9), 32–37.
  • Morrison, D., & Robertson, D. K. (2019). Introduction to Icarus special papers on Tunguska. Icarus, 327, 1–3. (And papers in this issue.)
  • Svetsov, V., Shuvalov, V., Collins, G., & Popova, O. (2019). Impact hazard of large meteoroids and small asteroids. In G. O. Ryabova, D. J. Asher, & M. D. Campbell-Brown (Eds.), Meteoroids: Sources of meteors on the Earth and beyond (pp. 275–298). Cambridge, UK: Cambridge University Press.

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