What are meteorites?

Until now, there is not a single confirmed case of the murder of a person with a meteorite. At the same time, even a small celestial body, invading, unfortunately, into the Earth’s atmosphere, has a tremendous destructive potential comparable to that of nuclear munitions. Sometimes, as recent events have shown, guests from the sky are able to take us by surprise.

Flying over Chelyabinsk and literally making a lot of noise, the car struck everyone with its incredible glow and shock wave, which crumbled the glass, carried out the gates and tore the facing panels from the walls. The consequences were written a lot, much less was said about the essence of this phenomenon. In order to understand in more detail the processes that occur with small celestial bodies that met the planet Earth in their path, the PM turned to the Institute of Dynamics of the Geospheres of the Russian Academy of Sciences, where they have long been studying and mathematical modeling of the motion of meteoroids, that is, celestial bodies entering the Earth’s atmosphere. And that’s what we managed to find out.

Embossed from the belt

Bodies like Chelyabinsk originate from the main belt of asteroids, which lies between the orbits of Mars and Jupiter. This is not so close to the Earth, but sometimes the asteroid belt is shaken by cataclysms: larger objects collide as a result of collisions into smaller ones, and some of the debris go into the category of near-Earth cosmic bodies – now their orbits cross the orbit of our planet. Sometimes celestial stones are knocked out of the belt by disturbances caused by large planets. As the data on the trajectory of the Chelyabinsk meteorite show, he 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 (ie the nearest distance from the Sun) is less than the perihelion of the earth’s orbit.
 

 
Since this is mostly about debris, these objects have an irregular shape. Most of them are made of rock, called chondrit. This name is given to her because of the chondrules – spherical or elliptical inclusions with a diameter of about 1 mm (more rarely – more), surrounded by clastic or small-crystalline matrix. Hondrites are of different types, but also meteoroid specimens are found from iron. It is interesting that metal bodies are smaller, not more than 5% of the total number, however, among the meteorites and their fragments, iron certainly prevails. The reasons are simple: firstly, the chondrites are visually difficult to distinguish from ordinary earth stones and find them hard, and secondly, the iron is stronger, and the chances to break through the dense layers of the atmosphere and not to scatter into smaller fragments from the iron meteorite more.

Unthinkable speed

The fate of a meteoroid depends not only on its size and the physico-chemical properties of its substance, but also on the rate of entry into the atmosphere, which can vary in a fairly large range. But in any case, we are talking about ultra-high speeds, far exceeding the speed of even not supersonic aircraft, but also orbital spacecraft. The average speed of entering the atmosphere is 19 km / s, however, if the meteoroid comes into contact with the Earth in courses close to the oncoming one, the speed can reach 50 km / s, that is, 180,000 km / h. The smallest speed of entry into the atmosphere will be when the Earth and the small celestial body move as if in neighboring orbits, next to each other, until our planet attracts a meteoroid.

The higher the speed of entry of the celestial body into the atmosphere, the stronger the load on it, the farther from the Earth it begins to collapse and the higher the probability that it will collapse, never reaching the surface of our planet. In Namibia, surrounded by a carefully made fence, shaped like a small amphitheater, lies a huge metal block, consisting of 84% of iron, and also of nickel and cobalt. Weigh a block of 60 tons, while it is the largest single piece of cosmic matter ever found on Earth. The meteorite fell to Earth about 80,000 years ago, not leaving even a crater after the fall. Probably due to some combination of circumstances, the rate of its fall was minimal, since the metal Sikhote-Alin meteorite (1947, Primorskii krai), comparable in mass and metal, collapsed into many pieces, and when it was created, created a whole crater field, as well as a huge scattering area small debris, which are collected in the Ussuri taiga until now.
 
 
 
What is it that explodes?

Even before the meteorite falls to the ground, it can, as the Chelyabinsk case has visually shown, be very, very dangerous. Entering the atmosphere at a giant speed, the celestial body generates a shock wave in which air is heated to temperatures of more than 10,000 degrees. Radiation of shock-heated air causes evaporation of the meteoroid. Due to these processes, it envelops the halo of the glowing ionized gas – plasma. Behind the shock wave, a high pressure zone is formed, which tests the frontal part of the meteorite for strength. On the sides, however, the pressure is much lower. As a result of the gradient of pressure, a meteorite with a high degree of probability will begin to collapse. How exactly this will happen depends on the specific size, shape and features of the structure of the given meteoroid: cracks, depressions, cavities. Another important thing is that when the car is destroyed, its cross-sectional area increases, which immediately leads to an increase in energy release. The area of ​​gas that the body grasps increases, more and more kinetic energy is converted into thermal energy. The rapid growth of energy release in a limited area of ​​space in a short time is nothing short of an explosion. It is at the moment of destruction that the glow of the car sharply increases (there is a bright flash). And the area of ​​the surface of the shock wave and, accordingly, the mass of shock-heated air grows abruptly.

When a conventional or nuclear munition is blasted, the shock wave has a spherical shape, but in the case of a meteorite this is certainly not the case. When a small celestial body enters the atmosphere, it forms a conditionally conical shock wave (the meteoroid is at the tip of the cone) – about the same as it is created in front of the nose of a supersonic aircraft.
 

The shock wave that occurs when a meteorite is destroyed can bring far more trouble than the fall of a large fragment. In the photo there is a hole in the ice of Lake Chebarkul, allegedly pierced by a piece of Chelyabinsk meteorite.
 
But the difference is already observed here: after all, aircraft have a streamlined shape, and the car that cuts into dense layers does not have to be streamlined at all. The irregularities of its shape create additional twists. With decreasing flight altitude and increasing air density, aerodynamic loads increase. At altitudes of about 50 km they are compared with the strength of most stone meteoroids, and meteoroids are more likely to begin to collapse. Each separate phase of destruction carries with it an additional release of energy, the shock wave takes the form of a highly distorted cone, is crushed, which can cause several successive overpressures that are felt on the ground as a series of powerful claps during the flight of the meteorite. In the Chelyabinsk case of such cotton was at least three.

Rustle, which should not be

To the puzzles associated with large meteorites (fireballs) is the phenomenon of so-called electrophonic bolides. In this case, a person observing the passage of a small space body across the sky hears a rustling sound from the car. Obviously, a sound wave can not reach the observer’s ear so quickly. Apparently, the rustle arises from the interaction of the electromagnetic radiation coming from the fireball from the earth’s surface surrounding the observer. But what kind of interaction is unknown. According to some evidences, the electrophone effect was also observed during the flight of the Chelyabinsk bolide.

The impact of a shock wave on the Earth’s surface depends on the flight path, mass and speed of the body. Chelyabinsk meteorite flew along a very shallow trajectory, and its shock wave touched areas of urban development with only a margin. The majority of meteorites (75%) enter the atmosphere along trajectories inclined to the Earth’s surface at an angle of more than 30 degrees, and then everything depends on the height at which the main phase of its deceleration occurs, usually associated with destruction and a sharp increase in energy release. If this height is high, the shock wave will reach the Earth in a weakened form. If the destruction occurs at lower altitudes, the shock wave can “clean up” a huge area, approximately as it happens in an atmospheric nuclear explosion. Or as if striking a Tunguska meteorite.

As the stone evaporated

Back in the 1950s, an original model consisting of a detonation cord (simulating the flight phase before destruction) and an attached charge (imitating an expansion) was created to simulate the processes occurring during the passage of the meteoroid through the atmosphere. The brass surface model was fixed vertically with copper wires representing the forest. Experiments have shown that as a result of detonation of the main charge, the wires, bending, gave a very realistic picture of the forest fall, similar to that observed in the Podkamennaya Tunguska area. Traces of the Tunguska meteorite have not been found so far, and the popular hypothesis that the body that collided with the Earth in 1908 was the icy nucleus of a small comet is not at all considered to be the only reliable one. Modern calculations show that the body of a larger mass enters the atmosphere and dives deeper into it until the braking phase, and its fragments are exposed to strong radiation for a longer time, which increases the probability of their evaporation.

The Tunguska meteorite could well have been a stone meteorite, however, having been shattered at a relatively low altitude, it could have produced a cloud of very small debris which, after contact with the glowing gases, evaporated. Only the shock wave reached the ground, which produced on the area more than 2000 km² of destruction, comparable to the action of a thermonuclear charge of 10-20 Mt. It refers to both the 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 a memory in the form of a “telegraph forest” – the trunks stood still, but the branches were chopped off all to one.
 

Despite the fact that meteorites fall to the Earth quite often, statistics of instrumental observations of the entry into the atmosphere of small celestial bodies is still insufficient.
 
The energy release during the destruction of the Chelyabinsk meteorite is estimated, according to preliminary estimates, to be equivalent to 300 kt of TNT, which is about 20 times greater than the power of the uranium “Baby” dropped on Hiroshima. If the trajectory of the car’s flight was close to vertical, and the place of fall would have to be in urban development, colossal sacrifices and destruction would be unavoidable. So how big is the risk of recurrence, and should we take seriously the meteorite threat?

Random megabomb

The Earth’s population is unevenly distributed over the planet, most of the Earth is occupied by oceans and low-habitable territories, so that the entry of a large meteorite into a populated locality is not very likely. However, in an unfavorable confluence of circumstances, a meteorite of the Chelyabinsk type could cause damage comparable to the results of nuclear bombardment. When a Tungus type meteorite hits a large metropolis, the latter would be completely destroyed.

The processes that occur when a meteoroid is destroyed in dense layers of the atmosphere are somewhat different from the explosion of a charge consisting of TNT or another explosive. In the explosion of explosives, first a detonation wave propagates through the explosive, and then dense detonation products scatter to the sides and generate a shock wave in the air. There is no explosive in the meteorite. Due to its enormous speed, it is rather an analog of dense explosion products. The shock wave that it forms is not spherical, but conditionally approximated to a conical shape.

Excessive caution

Yes, not a single meteorite has fortunately killed anyone, but the threat from the sky is not so insignificant that it does not reckon with it. Celestial bodies such as the Tungus fall to the Earth about once every 1000 years, which means that on average each year they completely “clean up” 2.5 km² of the territory. The fall of the Chelyabinsk type body was noted last time in 1963 in the region of the islands of South Africa – then the energy release during destruction was also about 300 kt.

At present, the astronomical community is tasked to identify and track all celestial bodies with a size of more than 100 m in diameter close to the earth’s orbits. But troubles can also be done by smaller meteoroids, the total monitoring of which is not yet possible: special and numerous monitoring instruments are needed for this. To date, the entry of only 20 meteoroids into the atmosphere has been observed with the help of astronomical instruments. Only one case is known when the fall of a relatively large meteorite (diameter about 4 m) was predicted about a day (it fell in Sudan in October 2008). And meanwhile the warning about the space cataclysm even for a day is quite good. If the celestial body threatens to fall on a settlement, in 24 hours the settlement can be evacuated. And, of course, there is enough time to remind people: if you see a bright flash in the sky, you have to hide, and not stick to the window pane.