Until now, not a single confirmed case of human killing by a meteorite is known. And at the same time, even a small celestial body, which, unfortunately, has invaded the Earth's atmosphere, has a colossal destructive potential comparable to nuclear weapons. Sometimes, as recent events have shown, guests from heaven can catch us by surprise.
The fireball that flew over Chelyabinsk and made so much noise literally and figuratively amazed everyone with its incredible glow and shock wave, which crumbled glass, carried out the gate and tore off the facing panels from the walls. Much has been written about the consequences, much less has been said about the essence of this phenomenon. To understand in more detail the processes occurring with small celestial bodies that met the planet Earth on their way, "PM" turned to the Institute of Dynamics of Geospheres of the Russian Academy of Sciences, where they have long been studying and mathematical modeling of the movement of meteoroids, that is, celestial bodies entering the Earth's atmosphere. And here's what we managed to find out.
Knocked out of the belt
Bodies like Chelyabinsk come from the main asteroid belt, which lies between the orbits of Mars and Jupiter. It is not so close to the Earth, but sometimes the asteroid belt is shaken by cataclysms: as a result of collisions, larger objects disintegrate into smaller ones, and some of the debris pass into the category of near-Earth cosmic bodies - now their orbits cross the orbit of our planet. Sometimes celestial stones are kicked out of the belt by disturbances caused by large planets. As the data on the trajectory of the Chelyabinsk meteorite show, it represented the so-called Apollo group - a group of small celestial bodies moving around the Sun in elliptical orbits that intersect the Earth's orbit, and their perihelion (that is, the closest distance from the Sun) is less than the perihelion of the Earth's orbit.
Since we are most often talking about debris, these objects have an irregular shape. Most of them are composed of a rock called "chondrite". This name was given to her because of chondrules - spherical or elliptical inclusions with a diameter of about 1 mm (less often - more), surrounded by a debris or fine-crystalline matrix. Chondrites are of different types, but also iron specimens are found among meteoroids. It is interesting that there are fewer metal bodies, no more than 5% of the total, but iron certainly predominates among the found meteorites and their fragments. The reasons are simple: firstly, chondrites are visually difficult to distinguish from ordinary earth stones and are difficult to detect, and secondly, iron is stronger, and an iron meteorite has more chances to break through the dense layers of the atmosphere and not scatter into small fragments.
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Incredible speeds
The fate of a meteoroid depends not only on its size and the physicochemical properties of its substance, but also on the rate of entry into the atmosphere, which can vary over a fairly wide range. But in any case, we are talking about ultra-high speeds, significantly exceeding the speed of movement not even of supersonic aircraft, but also of orbital spacecraft. The average speed of entry into the atmosphere is 19 km / s, however, if a meteoroid comes into contact with the Earth on courses close to the oncoming one, the speed can reach 50 km / s, that is, 180,000 km / h. The smallest rate of entry into the atmosphere will be when the Earth and a small celestial body move, as it were, in neighboring orbits, next to each other, until our planet attracts a meteoroid.
The higher the rate of entry of a celestial body into the atmosphere, the stronger the load on it, the further from the Earth it begins to collapse and the higher the probability that it will collapse before reaching the surface of our planet. In Namibia, surrounded by a carefully constructed enclosure in the shape of a small amphitheater, lies a huge block of metal, 84% iron, as well as nickel and cobalt. The lump weighs 60 tons, while it is the largest solid piece of cosmic matter ever found on Earth. The meteorite fell to Earth about 80,000 years ago, without even leaving a crater after it fell. Probably, due to some coincidence of circumstances, the rate of its fall was minimal, since the metal Sikhote-Alin meteorite (1947,Primorsky Krai) fell apart into many pieces and, when falling, created a whole crater field, as well as a huge area of dispersion of small debris, which are still collected in the Ussuri taiga.
What's exploding there?
Even before the meteorite falls to the ground, it can, as the Chelyabinsk case clearly showed, be very, very dangerous. A celestial body bursting into the atmosphere at a tremendous speed generates a shock wave in which the air heats up to temperatures over 10,000 degrees. Radiation of shock-heated air causes the evaporation of the meteoroid. Thanks to these processes, it is enveloped in a halo of glowing ionized gas - plasma. A high pressure zone is formed behind the shock wave, which tests the strength of the frontal part of the meteorite. On the sides, the pressure is significantly lower. As a result of the resulting pressure gradient, the meteorite will most likely begin to collapse. How exactly this happens depends on the specific size, shape and structural features of the given meteoroid: cracks, recesses, cavities. Another thing is important - when the fireball is destroyed, its cross-sectional area increases, which instantly leads to an increase in energy release. The area of gas that the body captures increases, more and more kinetic energy is converted into heat. The rapid growth of energy release in a limited area of space in a short time is nothing but an explosion. It is at the moment of destruction that the glow of the car increases sharply (a bright flash occurs). And the surface area of the shock wave and, accordingly, the mass of the shock-heated air grows abruptly.like an explosion. It is at the moment of destruction that the glow of the car increases sharply (a bright flash occurs). And the surface area of the shock wave and, accordingly, the mass of the shock-heated air grows abruptly.like an explosion. It is at the moment of destruction that the glow of the car increases sharply (a bright flash occurs). And the surface area of the shock wave and, accordingly, the mass of the shock-heated air grows abruptly.
When a conventional or nuclear weapon explodes, the shock wave has a spherical shape, but in the case of a meteorite, of course, this is not the case. When a small celestial body enters the atmosphere, it forms a conventionally conical shock wave (the meteoroid is at the same time on the tip of the cone) - approximately the same as that created in front of the nose of a supersonic aircraft.
The shockwave generated by the destruction of a meteorite can bring much more trouble than the fall of a large debris. In the photo - a hole in the ice of Lake Chebarkul, presumably pierced by a piece of the Chelyabinsk meteorite.
But the difference is already observed here: after all, the aircraft have a streamlined shape, and a car crashing into dense layers does not have to be streamlined at all. Irregularities in its shape create additional turbulence. With a decrease in flight altitude and an increase in air density, aerodynamic loads increase. At altitudes of about 50 km, they are comparable to the strength of most stone meteoroids, and the meteoroids are likely to begin to collapse. Each separate stage of destruction carries with it an additional release of energy, the shock wave takes the form of a strongly distorted cone, crushes, due to which, during the passage of a meteorite, there can be several successive surges of excess pressure, which are felt on the ground as a series of powerful claps. In the Chelyabinsk case, there were at least three such claps.
The impact of a shock wave on the Earth's surface depends on the flight path, mass and speed of the body. The Chelyabinsk meteorite flew along a very flat trajectory, and its shock wave only touched the urban areas at the edge. Most of the meteorites (75%) enter the atmosphere along trajectories inclined to the Earth's surface at an angle of more than 30 degrees, and here everything depends on the altitude at which the main phase of its deceleration occurs, usually associated with destruction and a sharp increase in energy release. If this height is great, the shock wave will reach the Earth in a weakened form. If the destruction occurs at lower altitudes, the shock wave can "clean out" a huge area, much like it happens in an atmospheric nuclear explosion. Or as in the impact of the Tunguska meteorite.
How the stone evaporated
Back in the 1950s, to simulate the processes occurring during the flight of a meteoroid through the atmosphere, an original model was created, which consisted of a detonation cord (simulating the phase of flight before destruction) and a charge attached to its end (simulating expansion). Copper wires representing the forest were fixed vertically under the model of the brass surface. Experiments have shown that, as a result of the detonation of the main charge, the wires, bending, gave a very realistic picture of forest felling, similar to that observed in the Podkamennaya Tunguska area. Traces of the Tunguska meteorite have not yet been found, and the popular hypothesis that the body that collided with the Earth in 1908 was the ice core of a small comet is not at all considered the only reliable one. Modern calculations show that a body of greater mass, entering the atmosphere,it plunges deeper into it before the stage of deceleration, and its fragments are exposed to strong radiation for a longer time, which increases the likelihood of their evaporation.
The Tunguska meteorite could well have been stone, however, being shattered at a relatively low altitude, it could generate a cloud of very small debris, which evaporated from contact with hot gases. Only a shock wave reached the ground, which produced destruction on an area of more than 2000 km², comparable to the action of a thermonuclear charge with a power of 10-20 Mt. This refers to both dynamic impact and taiga fires generated by a light flash. The only factor that did not work in this case, unlike a nuclear explosion, is radiation. The action of the frontal part of the shock wave left in itself a memory in the form of a "telegraph forest" - the trunks resisted, but every branch was chopped off.
Despite the fact that meteorites fall on the Earth quite often, the statistics of instrumental observations of the entry of small celestial bodies into the atmosphere is still insufficient.
According to preliminary estimates, the energy release during the destruction of the Chelyabinsk meteorite is considered equivalent to 300 kt of TNT, which is about 20 times more than the power of the uranium "Baby" dropped on Hiroshima. If the trajectory of the car's flight were close to vertical, and the place of the fall would fall on urban development, colossal casualties and destruction would be inevitable. So how big is the risk of a recurrence, and should the meteorite threat be taken seriously?
A useful precaution
Yes, not a single meteorite, fortunately, has killed anyone yet, but the threat from the sky is not so insignificant as to be ignored. Celestial bodies of the Tunguska type fall to the Earth about once every 1000 years, which means that on average every year they completely "clean up" 2.5 km² of territory. The fall of a body of the Chelyabinsk type was noted for the last time in 1963 in the region of the islands of South Africa - then the energy release during destruction was also about 300 kt.
Currently, the astronomical community has been tasked with identifying and tracking all celestial bodies over 100 m across in orbits close to the Earth. But smaller meteoroids can also do trouble, the total monitoring of which is not yet possible: this requires special and numerous observation instruments. To date, the entry of only 20 meteoroid bodies into the atmosphere has been observed using astronomical instruments. There is only one known case when the fall of a relatively large meteorite (about 4 m in diameter) was predicted in about a day (it fell in Sudan in October 2008). And meanwhile, a warning about a cosmic cataclysm even in a day is not bad at all. If a celestial body threatens to fall on a settlement, the settlement can be evacuated within 24 hours. And of course, a day is enough for somethingto remind people once again: if you see a bright flash in the sky, you need to hide, and not stick your face to the window glass.
Oleg Makarov