Olga P. Popova

Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization

Deep Impact? On 15 February 2013, the Russian district of Chelyabinsk, with a population of more than 1 million, suffered the impact and atmospheric explosion of a 20-meter-wide asteroid—the largest impact on Earth by an asteroid since 1908. Popova et al. (p. 1069, published online 7 November; see the Perspective by Chapman) provide a comprehensive description of this event and of the body that caused it, including detailed information on the asteroid orbit and atmospheric trajectory, damage assessment, and meteorite recovery and characterization. A detailed study of a recent asteroid impact provides an opportunity to calibrate the damage caused by these rare events. [Also see Perspective by Chapman] The asteroid impact near the Russian city of Chelyabinsk on 15 February 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and laboratory techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorology, meteoritics, and cosmochemistry and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.

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.

Radiation emitted during the flight of asteroids and comets through the atmosphere

Abstract The radiation emitted during the flight of a cosmic body through the atmosphere is an important factor in the bodys interaction with a planet. A variety of radiation aspects are considered in this paper. Altitudes of interest here range from the dense atmosphere to above 100 km where nonequilibrium and nonstationary ionic kinetic and molecular dynamics play an important role. Structure, intensity and the spectrum of radiative shock waves generated in the atmospheres of the Earth and Mars are investigated on the basis of the detailed numerical simulations. Ignition of fires, creation of a thin thermal layer at the ground, which essentially changes flow pattern after the impact, and contamination of the atmosphere by soot, aerosols and dust may pose a threat to humanity. These problems are also considered. Registered radiation of 1–10 m bodies can supply the necessary data on their properties and refine size-frequency curves. A relatively simple method to obtain the body characteristics in flight is developed and some observational data are analyzed. In the proposed scenario the body is heavily fragmented and intensely radiates due to an increase of its effective area.

Screening of Meteoroids by Ablation Vapor in High-Velocity Meteors

The ablation is calculated of non-fragmenting 10−2 to 10 cm sized meteoroids for typical Leonids with a velocity of 72 km/s at altitudes of about 90–110 km. At altitudes below the onset of intensive evaporation, a dense vapor cloud is formed around the body. This vapor cloud screens the meteoroid surface from direct impacts of air molecules. A particle beam model is developed that describes the air meteoroid interaction. Based on this model, the physical parameters of the vapor cloud are calculated and compared with observations collected during the Leonid Multi-Instrument Aircraft Campaign. General agreement with measured temperatures is found. In addition, boundaries of the models applicability to specific encounter conditions are given.

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.

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.

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. 67 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.

Methods and Means of Information-Analytical Assessment of Asteroid and Comet Hazard

This paper contains a description of methods and software tools for creation of the informationanalytical system for monitoring hazardous space objects. The paper presents the structure of the system and a description of its functional components that enable rapid assessment of the NEO hazard and forecast of the effects of dangerous celestial bodies colliding with the Earth. The results of the system’s operation regarding the modeling the motion of space objects are also included in this work.

Radiation from a Superbolide

Numerical simulation of the destruction, evaporation, deceleration, and emission of the Chelyabinsk superbolide has been carried out. The model assumes that the main energy is radiated in the stage when the asteroid is already completely destroyed and does not have solidity (quasi-liquid approximation). The radiation transfer during the motion is taken into account in the approximation of radiative heat conductivity and volumetric emission. The distributions of temperatures and densities are obtained at the moments when the bolide is at different altitudes. The intensity of radiation at the Earth’s surface is calculated at certain times by solving the radiative transfer equation along the rays passing through the luminous region using the air and LL-chondrite vapor absorption coefficients. The features of superbolide radiation, the contribution of air and vapor to radiation, the size of the luminous region, and the radiation spectrum have been considered. The calculated efficiency of radiation—17% of the kinetic energy of a cosmic body—agrees with the results of observations. It is shown that due to anisotropy of the superbolide radiation, the determination of luminous efficiency from measurements can depend on the observation point. For estimations, the pointsource approximation can be used, but in general, the source luminous efficiency is unknown, and its location is determined with some error; therefore, numerical simulation is required to reliably estimate the consequences of space body falls.