by Viktor BARELKO, Dr. Sc. (Chemistry), RAS Institute of Problems of Chemical Physics (Chernogolovka, Moscow Region), Mikhail DROZDOV, Cand. Sc. (Phys. & Math.), Branch of the RAS Institute of Energy Problems of Chemical Physics (Chernogolovka, Moscow Region), Maxim KUZNETSOV, Dr. Sc. (Chemistry), All-Russia Research Institute of Civil Defense and Emergency Situations RF MES (Moscow)

The fall of the Chelyabinsk meteorite on February 15, 2013, was a subject for publications by many periodicals including Science in Russia (No. 4, 2013). Director of the Vernadsky Institute of Geochemistry and Analytical Chemistry and Chairman of the Committee for Meteorites of the RAS Presidium Acad. Eric Galimov in his paper summed up preliminary results of the studies of chemical composition of the meteorite, and Director of the RAS Institute for Space Research Acad. Lev Zeleny and his colleagues exceeded the limits of the specific event in the sky over Chelyabinsk and discussed some aspects of the asteroid-comet danger. But a problem concerning the physical mechanism of explosion-like catastrophic phenomena, accompanying entry of meteorites into dense atmospheric layers and resulting in large-scale areas of destruction on the Earth's surface, was not discussed.

Specialists of various profiles made the first attempts to answer this question more than 100 years ago after the fall of the Tunguska meteorite to the Earth in 1908*. For the past decades different hypotheses were put forward. But they were reduced mostly to dispersion

* See: E. Galimov, M. Nazarov, "Centennial of the Tunguska Event", Science in Russia, No. 3, 2008.--Ed.

models of the fireball body at the expense of creation in this object, which moves at a hypersonic speed in dense layers of the earthly atmosphere, of destructive mechanical and thermal stresses. Unfortunately, many naturally emerging and seemingly evident questions remain without adequate answers under such approach. Here we shall try to specify them.

Is it enough to use only ideas of a shock wave generated by a body flying in the atmosphere at a hypersonic speed for explanation of a scale of catastrophic consequences of the meteorite impact? What is the nature and dynamic regularities of the intense loss of its mass in the form of a steam-gas plume, accompanying the fireball movement and what can we say about the physical mechanism of this factor and its influence on energy characteristics of the said blast wave? What (if not explosion!) determined in a jiffy termination of existence of the Chelyabinsk fireball at a height of 10-20 km? How can we explain such an insignificant volume of its fragments on the ground surface, though it possessed an enormous initial mass? It should be noted in this context that no fixed presence of any substance of the Tunguska meteorite had been revealed at all. At the same time the analysis of the Chelyabinsk catastrophe consequences helped establish: it was managed to collect fragments of a total weight of some hundreds of kilograms against an assumed overall mass of the cosmic body from 6 to 10 thous. t in the damaged area of hundreds of square kilometers with involvement of modern search and survey methods!

It may seem strange but among the numerous analysts presenting their versions of the Chelyabinsk event it is difficult to find professionals in the field of combustion, explosion and detonation processes. Perhaps it is connected with the fact that the event we are interested in cannot be referred to conventional objects of the given section of physics?

We think that the cause of the meteorite explosion should be sought among gas-detonation mechanisms which form a supersonic blast wave front. As the presence of chemical sources of explosive gas release in the meteorite in the given case is excluded, it appears well-grounded to address the process of volumetric boiling up of the body overheated to several thousands of degrees. In other words, we suggest considering the role of "steam explosion" as a factor accompanying transformation of meteorites (fireballs) in dense atmospheric layers.

The concept "steam explosion" has been known in science and engineering practice over a period of more than 150 years from the time of creation of steam boilers and invention of steam engines. In case of emergency depressurizing overheated water in a high-pressure boiler boiled up in no time, which caused formation of a blast wave damaging the boiler and leading to tragic consequences.

We can imagine a similar scheme with some approximations also for description of meteorite explosion dynamics within the limits of the steam-gas detonation concept. A solid cosmic body enters dense atmospheric layers at a rattling good speed (10-20 km/s), whereby a hot boundary layer adiabatically compressed to high pressure parameters is formed on its surface. The object is overheated well above the boiling point of the substance used for its manufacture, as a result of which, as the fireball slows down and pressure compressing it decreases, the fireball body mass boils up for an ultra-short period of time. The substance, transformed to a steam-gas state and still compressed to high pressure parameters, decomposes like an explosion, i.e. "volumetric steam explosion" takes place, which forms a blast wave with catastrophic consequences.

An insignificant mass of the found fragments testifies in favor of the suggested hypothesis. The blast wave disperses a steam-gas cloud of the exploded meteorite substance in a large atmospheric area, therefore it is not possible to collect more or less substantia] volume of its residual products on the Earth surface.

Regrettably the theoretical bases explaining the effect of a steam explosion are not created yet as well as its mathematical models. Due to this fact it is necessary to look for objects for experimental studies of the problem at hand. We assume that the effect of the so-called "exploding wires", at one time an object of numerous investigations*, is applicable as a laboratory model describing a meteorite explosion. In the course of experiments a very short pulse of high density electric current (10⁴-10⁶ A/mm²) was passed through a thin metal wire (0.1-1 mm in diameter) placed in the reactor. Almost instantly (10^-5-10^-7 s) it was overheated above the boiling point of the material, then exploding sublimated entirely scattering metal nanoparticles at supersonic speeds over the whole space and walls of the reactor. Such electric explosion is accompanied by an emerging blast wave with a pressure in its front reaching several thousands of atmospheres due to the mode of superfast heating of a wire element at a speed above 1.10⁷ K.s^-1 to the temperature parameters exceeding 10⁴ K. The electroexplosive method is used now as a technological tool for production of

* See: V. Shpak, "Safety Fuse: History Continues", Science in Russia, No. 5. 2012.-Ed.

metallic and non-metallic nanosize powders of materials possessing substantial stored energy.

The explosion picture of a massive fireball is naturally more complicated than that of a thin wire. But under certain parameters of its movement in the atmosphere, overheating, even local, can be such that there forms in a limited layer a steam-gas phase of superhigh pressure, which "detonates". It can take place successively by recurrent explosive actions--3 such explosions preceded the instantaneous end of the Chelyabinsk fireball. The last recorded flash prior to disappearance of the object was at a height of 10-20 km.

Another version cannot be excluded either, which explains intensification of heat-exchanging processes in a cosmic body volume. As a result of mechanical and thermal stresses this body disperses to small-size fragments when moving in the atmosphere. And then the movement of a "mass" of fragments provides conditions for realization of a uniform overheating mode of the fragmented mass of the object.

The suggested idea of the steam explosion theory is not limited, in our opinion, only by its application to meteorite objects. We believe that this mechanism "activates" also in volcanism. In particular, in case of "nonvolcanic eruptions" they occur under contact of magma and its flows with water-containing fluid media in the Earth's crust (or with glacial coatings on volcanic domes) starting from powerful outbursts of water vapor, and they cannot be explained by anything but the action of the steam explosion mechanism. Moreover, in our opinion, the process proper of opening of the crater and a subsequent blast wave outburst to a height of several kilometers of a steam-gas "stream" with carried away fragments of magma and rock chippings is a result of steam explosion of an overheated magmatic mass under a volcanic dome. At a level of everyday life this process illustrates a "shot" of a champagne bottle at its opening due to volumetric ebullition of wine oversaturated with carbonic acid gas.

The suggested concept, in our opinion, is extremely important as the ideas about a steam explosion can be also used for explanation of the nature of such technogenic catastrophes as explosions in nuclear power steam-generating reactors (in particular, as applied to the mechanism of the Chernobyl catastrophe). Moreover,

one of the authors of this paper used the steam explosion concept in his expert opinion on the causes of the accident at the Sayano-Shushenskaya hydroelectric power-station in 2009.

Certain scientific undertaking within the framework of studies of this problem was initiated in our works on "autowave crisis of boiling" and "detonation mechanisms in physics of boiling ". Basing on the formulated ideas we suggested an engineering solution related to a design of nuclear fuel elements and reduction of accidents in nuclear-power engineering similar to that in Chernobyl.

As regards the choice of a laboratory object necessary for shaping of scientific foundations for the "steam explosion" phenomenon, we shall refer again to an "exploding wire". This model will be helpful in determining many important parameters such as critical values of discharge pulse duration, rate of object heating and a value of "injected" heating energy to this object. Besides, it will help explain a critical value of overheating of the object above boiling temperature necessary for explosion realization, measure dynamic characteristics of explosion, study mechanisms of initiation, speeds of wave propagation of overheated mass boiling and explosion action time, determine parameters of acoustic emission pattern, and measure a pressure field in a blast wave. Finally, it can confirm molecular dispersion of the object in the process of explosion, i.e. its sublimation mechanism. The obtained data will make up a basis for development of a thermal and gas dynamic theory of the "steam explosion" phenomenon.

It should be stressed that steam explosion should be referred to those nonlinear phenomena, physical bases of which are based on involvement of phenomenology of combustion, explosion and detonation, in particular, to description of metastable decay modes of the substance similar to explosion in solid state physics and metal science. We also considered examples of development of interdisciplinary concepts of such kind as applied to cosmochemistry (mechanisms of superfast solid-phase chemical transformations near 0 K) and geotectonics (earthquakes as a result of explosive polymorphic transformations in the Earth's crust). This problem was touched in the paper by Viktor Barelko and his coauthors published in Natural Science, No. 12, Vol. 2, 2010. The analogy becomes rather obvious if we compare the role of the mechanism of "explosive crystallization" of the supercooled melt in magmatic processes and the role of the mechanism of steam explosion of overheated medium in meteorites.

The authors express gratitude to Mikhail Zelensky, Cand. Sc. (Geol. & Miner.), a senior research assistant of the Institute of Experimental Mineralogy for his valuable advice.