Which bodies are called meteorites and which are asteroids. asteroids

On warm summer nights, it is pleasant to walk under the starry sky, look at the wonderful constellations on it, make wishes at the sight of a falling star. Or was it a comet? Or maybe a meteorite? Probably, there are more experts in astronomy among romantics and lovers than among visitors to planetariums.

mysterious space

Questions that constantly arise during contemplation require answers, and heavenly riddles require clues and scientific explanations. Here, for example, what is the difference between an asteroid and a meteorite? Not every student (and even an adult) can immediately answer this question. But let's start in order.

asteroids

To understand how an asteroid differs from a meteorite, you need to define the concept of "asteroid". This word from the ancient Greek language is translated as “like a star”, since these celestial bodies, when observed through a telescope, resemble stars rather than planets. Asteroids until 2006 were often called minor planets. Indeed, the movement of asteroids as a whole does not differ from the planetary movement, because it also occurs around the Sun. Asteroids differ from ordinary planets in their small size. For example, the largest asteroid Ceres is only 770 km across.

Where are these star-like space dwellers located? Most asteroids move in long-studied orbits in the space between Jupiter and Mars. But some small planets still cross the orbit of Mars (like the asteroid Icarus) and other planets, and sometimes even come closer to the Sun than Mercury.

meteorites

Unlike asteroids, meteorites are not inhabitants of space, but its messengers. Each of the earthlings can see the meteorite with their own eyes and touch it with their own hands. A large number of them are kept in museums and private collections, but it must be said that meteorites look rather unattractive. Most of them are gray or brownish-black pieces of stone and iron.

So, we managed to figure out how an asteroid differs from a meteorite. But what can unite them? It is believed that meteorites are fragments of small asteroids. Stones rushing in space collide with each other, and their fragments sometimes reach the surface of the Earth.

The most famous meteorite in Russia is the Tunguska meteorite, which fell in the deep taiga on June 30, 1908. In the recent past, namely in February 2013, the Chelyabinsk meteorite attracted everyone's attention, whose numerous fragments were found near Chebarkul Lake in the Chelyabinsk region.

Thanks to meteorites, peculiar guests from outer space, scientists, and with them all the inhabitants of the Earth, have an excellent opportunity to learn about the composition of celestial bodies and get an idea of ​​the origin of the universe.

Meteora

The words "meteor" and "meteorite" come from the same Greek root, meaning "heavenly" in translation. We know, and how it differs from a meteor is not difficult to understand.

A meteor is not a specific celestial object, but an atmospheric phenomenon that looks like It occurs when fragments of comets and asteroids burn up in the Earth's atmosphere.

A meteor is a shooting star. It may appear to observers to fly back into outer space or burn up in the Earth's atmosphere.

Understanding how meteors differ from asteroids and meteorites is also easy. The last two celestial objects are concretely tangible (even if theoretically in the case of an asteroid), and the meteor is a glow resulting from the combustion of cosmic fragments.

Comets

No less wonderful celestial body that an earthly observer can admire is a comet. How are comets different from asteroids and meteorites?

The word "comet" is also of ancient Greek origin and literally translates as "hairy", "shaggy". Comets come from the outer part of the solar system, and, accordingly, have a different composition than asteroids that formed near the Sun.

In addition to the difference in composition, there is a more obvious difference in the structure of these celestial bodies. When approaching the Sun, a comet, unlike an asteroid, exhibits a nebulous coma shell and a tail consisting of gas and dust. Volatile substances of the comet, as it heats up, actively stand out and evaporate, turning it into the most beautiful luminous celestial object.

In addition, asteroids move in orbits, and their movement in outer space resembles the smooth and measured movement of ordinary planets. Unlike asteroids, comets are more extreme in their movements. Its orbit is highly elongated. The comet either approaches the Sun closely, or moves away from it at a considerable distance.

A comet differs from a meteorite in that it is in motion. A meteorite is the result of a collision of a celestial body with the earth's surface.

The heavenly world and the earthly world

It must be said that watching the night sky is doubly pleasant when its unearthly inhabitants are well known and understandable to you. And what a pleasure to tell your interlocutor about the world of stars and unusual events in outer space!

And it’s not even about the question of how an asteroid differs from a meteorite, but about the awareness of the close connection and deep interaction between the earthly and cosmic worlds, which must be established as actively as the relationship between one person and another.

The content of the article

METEOR. The word "meteor" in Greek was used to describe various atmospheric phenomena, but now it refers to phenomena that occur when solid particles from space enter the upper atmosphere. In a narrow sense, a "meteor" is a luminous band along the path of a decaying particle. However, in everyday life, this word often denotes the particle itself, although scientifically it is called a meteoroid. If part of the meteoroid reaches the surface, then it is called a meteorite. Meteors are popularly called "shooting stars". Very bright meteors are called fireballs; sometimes this term refers only to meteor events accompanied by sound phenomena.

Appearance frequency.

The number of meteors that an observer can see in a given period of time is not constant. In good conditions, away from city lights and in the absence of bright moonlight, an observer can see 5–10 meteors per hour. For most meteors, the glow lasts about a second and looks fainter than the brightest stars. After midnight, meteors appear more often, since the observer at this time is located on the forward side of the Earth in the course of orbital motion, which receives more particles. Each observer can see meteors within a radius of about 500 km around him. In just a day, hundreds of millions of meteors appear in the Earth's atmosphere. The total mass of particles entering the atmosphere is estimated at thousands of tons per day - an insignificant amount compared to the mass of the Earth itself. Measurements from spacecraft show that about 100 tons of dust particles also fall on Earth per day, too small to cause the appearance of visible meteors.

Meteor observation.

Visual observations provide a lot of statistical data about meteors, but special instruments are needed to accurately determine their brightness, height, and flight speed. For nearly a century, astronomers have been using cameras to photograph meteor trails. A rotating shutter (shutter) in front of the camera lens makes the meteor trail look like a dotted line, which helps to accurately determine time intervals. Typically, this shutter makes 5 to 60 exposures per second. If two observers, separated by a distance of tens of kilometers, simultaneously photograph the same meteor, then it is possible to accurately determine the height of the particle's flight, the length of its track, and, in time intervals, the flight speed.

Since the 1940s, astronomers have been observing meteors using radar. Cosmic particles themselves are too small to be detected, but as they travel through the atmosphere they leave a plasma trail that reflects radio waves. Unlike photography, the radar is effective not only at night, but also during the day and in cloudy weather. The radar detects small meteoroids that the camera cannot see. From photographs, the flight path is determined more accurately, and the radar allows you to accurately measure distance and speed. Cm. RADAR; RADAR ASTRONOMY.

Television equipment is also used to observe meteors. Image intensifier tubes make it possible to register weak meteors. Cameras with CCD matrices are also used. In 1992, while recording a sporting event on a video camera, a flight of a bright fireball was recorded, ending in a meteorite fall.

speed and height.

The speed with which meteoroids enter the atmosphere lies in the range from 11 to 72 km/s. The first value is the speed acquired by the body only due to the attraction of the Earth. (A spacecraft must get the same speed in order to break out of the Earth's gravitational field.) A meteoroid that arrived from distant regions of the solar system, due to attraction to the Sun, acquires a speed of 42 km / s near the earth's orbit. The Earth's orbital speed is about 30 km/s. If the meeting takes place head-on, then their relative speed is 72 km/s. Any particle coming from interstellar space must have an even greater speed. The absence of such fast particles proves that all meteoroids are members of the solar system.

The height at which the meteor begins to glow or is noted by the radar depends on the speed of entry of the particle. For fast meteoroids, this height can exceed 110 km, and the particle is completely destroyed at an altitude of about 80 km. For slow meteoroids, this happens lower, where the density of the air is greater. Meteors, comparable in brightness to the brightest stars, are formed by particles with a mass of tenths of a gram. Larger meteoroids usually take longer to break up and reach low altitudes. They are significantly slowed down due to friction in the atmosphere. Rare particles fall below 40 km. If a meteoroid reaches heights of 10–30 km, then its speed becomes less than 5 km/s, and it can fall to the surface in the form of a meteorite.

Orbits.

Knowing the meteoroid's speed and the direction from which it approached Earth, an astronomer can calculate its orbit before impact. The earth and the meteoroid collide if their orbits intersect and they simultaneously find themselves at this intersection point. The orbits of meteoroids are both almost circular and extremely elliptical, going beyond planetary orbits.

If a meteoroid is approaching the Earth slowly, then it is moving around the Sun in the same direction as the Earth: counterclockwise, as viewed from the north pole of the orbit. Most meteoroid orbits go beyond the Earth's orbit, and their planes are not very inclined to the ecliptic. The fall of almost all meteorites is associated with meteoroids that had velocities of less than 25 km/s; their orbits lie entirely within Jupiter's orbit. Most of the time these objects spend between the orbits of Jupiter and Mars, in the belt of minor planets - asteroids. Therefore, it is believed that asteroids serve as a source of meteorites. Unfortunately, we can only observe those meteoroids that cross the Earth's orbit; obviously, this group does not fully represent all the small bodies of the solar system.

In fast meteoroids, the orbits are more elongated and more inclined to the ecliptic. If a meteoroid flies up at a speed of more than 42 km / s, then it moves around the Sun in the opposite direction to the direction of the planets. The fact that many comets move in such orbits indicates that these meteoroids are fragments of comets.

meteor showers.

On some days of the year, meteors appear much more often than usual. This phenomenon is called a meteor shower, when tens of thousands of meteors are observed per hour, creating an amazing phenomenon of "starry rain" throughout the sky. If you trace the paths of meteors in the sky, it will seem that they all fly out of the same point, called the radiant of the shower. This perspective phenomenon, similar to rails converging at the horizon, indicates that all particles are moving along parallel paths.

Astronomers have identified several dozen meteor showers, many of which show annual activity lasting from a few hours to several weeks. Most streams are named after the constellation in which their radiant lies, for example, the Perseids, which have a radiant in the constellation Perseus, the Geminids, with a radiant in Gemini.

After the amazing star shower caused by the Leonid shower in 1833, W. Clark and D. Olmstead suggested that it was associated with a certain comet. At the beginning of 1867, K. Peters, D. Schiaparelli and T. Oppolzer independently proved this connection by establishing the similarity of the orbits of Comet 1866 I (Comet Temple-Tutl) and the Leonid meteor shower 1866.

Meteor showers are observed when the Earth crosses the trajectory of a swarm of particles formed during the destruction of a comet. Approaching the Sun, the comet is heated by its rays and loses matter. For several centuries, under the influence of gravitational perturbations from the planets, these particles form an elongated swarm along the comet's orbit. If the Earth crosses this stream, we can observe a shower of stars every year, even if the comet itself is far from the Earth at that moment. Because the particles are unevenly distributed along the orbit, the intensity of rain can vary from year to year. The old streams are so expanded that the Earth crosses them for several days. In cross section, some streams are more like a ribbon than a cord.

The ability to observe the flow depends on the direction of arrival of particles to the Earth. If the radiant is located high in the northern sky, then the stream is not visible from the southern hemisphere of the Earth (and vice versa). Meteor showers can only be seen if the radiant is above the horizon. If the radiant hits the daytime sky, then the meteors are not visible, but they can be detected by radar. Narrow streams under the influence of planets, especially Jupiter, can change their orbits. If at the same time they no longer cross the earth's orbit, they become unobservable.

The December Geminid shower is associated with the remnants of a minor planet or the inactive nucleus of an old comet. There are indications that the Earth is colliding with other groups of meteoroids generated by asteroids, but these flows are very weak.

Fireballs.

Meteors that are brighter than the brightest planets are often referred to as fireballs. Fireballs are sometimes observed brighter than the full moon and extremely rarely those that flare brighter than the sun. Bolides arise from the largest meteoroids. Among them are many fragments of asteroids, which are denser and stronger than fragments of cometary nuclei. But still, most asteroid meteoroids are destroyed in the dense layers of the atmosphere. Some of them fall to the surface in the form of meteorites. Due to the high brightness of the flash fireballs seem much closer than in reality. Therefore, it is necessary to compare observations of fireballs from different places before organizing a search for meteorites. Astronomers have estimated that about 12 fireballs around the Earth every day end up in the fall of more than a kilogram of meteorites.

physical processes.

The destruction of a meteoroid in the atmosphere occurs by ablation, i.e. high-temperature splitting off of atoms from its surface under the action of incoming air particles. The hot gas trail remaining behind the meteoroid emits light, but not as a result of chemical reactions, but as a result of the recombination of atoms excited by impacts. The spectra of meteors show many bright emission lines, among which the lines of iron, sodium, calcium, magnesium and silicon predominate. Lines of atmospheric nitrogen and oxygen are also visible. The chemical composition of meteoroids determined from the spectrum is consistent with data on comets and asteroids, as well as on interplanetary dust collected in the upper atmosphere.

Many meteors, especially fast ones, leave a luminous trail behind them that is observed for a second or two, and sometimes for much longer. When large meteorites fell, the trail was observed for several minutes. The glow of oxygen atoms at altitudes of approx. 100 km can be explained by traces lasting no more than a second. The longer trails are due to the complex interaction of the meteoroid with the atoms and molecules of the atmosphere. Dust particles along the bolide's path can form a bright trail if the upper atmosphere where they are scattered is illuminated by the Sun when the observer below has deep twilight.

Meteoroid speeds are hypersonic. When a meteoroid reaches relatively dense layers of the atmosphere, a powerful shock wave arises, and strong sounds can be carried for tens or more kilometers. These sounds are reminiscent of thunder or distant cannonade. Because of the distance, the sound arrives a minute or two after the car appears. For several decades, astronomers have been arguing about the reality of the anomalous sound that some observers heard directly at the time of the appearance of the fireball and described as crackling or whistling. Studies have shown that sound is caused by disturbances in the electric field near the fireball, under the influence of which objects close to the observer emit sound - hair, fur, trees.

meteorite hazard.

Large meteoroids can destroy spacecraft, and small dust particles constantly wear away their surface. The impact of even a small meteoroid can give the satellite an electrical charge that will disable electronic systems. The risk is generally low, but still, spacecraft launches are sometimes delayed if a strong meteor shower is expected.

Orbits of meteors and meteorites

To date, Soviet and foreign observers have published several catalogs of meteor radiants and orbits, each numbering several thousand meteors. So there is more than enough material for their statistical analysis.

One of the most important results of this analysis is that almost all meteoroids belong to the solar system, and are not aliens from interstellar spaces. Here's how to show it.

Even if a meteor body came to us from the very borders of the solar system, its speed relative to the Sun at a distance of the earth's orbit will be equal to the parabolic speed at this distance, which is several times greater than the circular one. The earth moves with an almost circular speed of 30 km/s, therefore, the parabolic speed in the region of the earth's orbit is 30=42 km/s. Even if a meteoroid flies towards the Earth, its speed relative to the Earth will be equal to 30+42=72 km/s. This is the upper limit of the geocentric velocity of meteors.

How is its lower limit determined? Let the meteor body move near the Earth along its orbit with the same speed as the Earth. The geocentric velocity of such a body will initially be close to zero. But gradually, under the influence of the Earth's gravity, the particle will begin to fall to the Earth and accelerate to the well-known second cosmic velocity of 11.2 km/s. With this speed, it will enter the Earth's atmosphere. This is the lower limit of the extra-atmospheric speed of meteors.

It is more difficult to determine the orbits of meteorites. We have already said that meteorite falls are extremely rare and, moreover, unpredictable phenomena. No one can say in advance when and where the meteorite will fall. Analysis of the testimonies of random eyewitnesses of the fall gives extremely low accuracy in determining the radiant, and it is completely impossible to determine the speed in this way.

But on April 7, 1959, several stations of the meteor service of Czechoslovakia photographed a bright fireball, which ended with the fall of several fragments of the Pribram meteorite. The atmospheric trajectory and orbit in the solar system of this meteorite have been accurately calculated. This event inspired astronomers. On the prairies of the United States, a network of stations was organized, equipped with the same type of camera sets, especially for shooting bright fireballs. They called it the Prairie Web. Another network of stations - European - was deployed on the territory of Czechoslovakia, the GDR and the FRG.

The prairie network for 10 years of work recorded the flight of 2500 bright fireballs. American scientists hoped that by continuing their downward trajectories, they would be able to find at least dozens of fallen meteorites.

Their expectations were not met. Only one (!) of 2500 fireballs ended on January 4, 1970 with the fall of the Lost City meteorite. Seven years later, when the Prairie Network was no longer working, the flight of the Inisfree meteorite was photographed from Canada. This happened on February 5, 1977. Of the European fireballs, not one (after Pribram) ended in a meteorite fallout. Meanwhile, among the fireballs photographed, many were very bright, many times brighter than the full moon. But the meteorites did not fall out after their passage. This mystery was resolved in the mid-70s, which we will discuss below.

Thus, along with many thousands of meteor orbits, we have only three (!) exact meteorite orbits. To these we can add several dozen approximate orbits calculated by I. S. Astapovich, A. N. Simonenko, V. I. Tsvetkov and other astronomers based on an analysis of eyewitness testimony.

In the statistical analysis of the elements of meteor orbits, several selective factors must be taken into account, leading to the fact that some meteors are observed more often than others. So, geometric factorP 1 determines the relative visibility of meteors with different radiant zenith distances. For meteors recorded by radar (the so-called radio meteors), what matters is the geometry of the reflection of radio waves from the ion-electron trace and the radiation pattern of the antenna. Physical factor P 2 determines the dependence of meteor visibility on speed. Namely, as we will see later, the greater the speed of the meteoroid, the brighter the meteor will be observed. The brightness of a meteor, observed visually or recorded photographically, is proportional to the 4th or 5th power of the speed. This means, for example, that a meteor with a speed of 60 km/s will be 400-1000 times brighter than a meteor with a speed of 15 km/s (if the masses of the meteoroids generating them are equal). For radio meteors, there is a similar dependence of the intensity of the reflected signal (radio brightness of the meteor) on the speed, although it is more complex. Finally, there is more astronomical factor P 3 , the meaning of which is that the meeting of the Earth with meteor particles moving in the solar system in different orbits has a different probability.

After taking into account all three factors, it is possible to construct the distribution of meteors over the elements of their orbits, corrected for selective effects.

All meteors are divided into in-line, i.e. those belonging to known meteor showers, and sporadic, components of the meteor background. The line between them is to some extent conditional. About twenty major meteor showers are known. They are called by the Latin names of the constellations where the radiant is located: Perseids, Lyrids, Orionids, Aquarids, Geminids. If two or more meteor showers operate in a given constellation at different times, they are designated by the nearest star: (-Aquarids, -Aquarids, -Perseids, etc.

The total number of meteor showers is much greater. Thus, the catalog of A. K. Terent'eva, compiled from photographic and best visual observations up to 1967, contains 360 meteor showers. From the analysis of 16,800 radio meteor orbits, V. N. Lebedinets, V. N. Korpusov and A. K. Sosnova identified 715 meteor showers and associations (a meteor association is a group of meteor orbits, the genetic proximity of which has been established with less confidence than in the case of a meteor shower ).

For a number of meteor showers, their genetic relationship with comets has been reliably established. Thus, the orbit of the Leonid meteor shower, observed annually in mid-November, practically coincides with the orbit of the comet 1866 I. Once every 33 years spectacular meteor showers are observed with a radiant in the constellation Leo. The most intense rains were observed in 1799, 1832 and 1866. Then during two periods (1899-1900 and 1932-1933) there were no meteor showers. Apparently, the position of the Earth during the period of its encounter with the flow was unfavorable for observations - it did not pass through the densest part of the swarm. But on November 17, 1966, the Leonid meteor shower was repeated. It was observed by US astronomers and winterers from 14 Soviet polar stations in the Arctic, where it was at that time the polar night (on the main territory of the USSR at that time it was day). The number of meteors reached 100,000 per hour, but the meteor shower lasted only 20 minutes, while in 1832 and 1866. it went on for several hours. This can be explained in two ways: either the swarm consists of separate clusters-clouds of various sizes and the Earth in different years passes through one or the other clouds, or in 1966 the Earth crossed the swarm not in diameter, but along a small chord. Comet 1866 I also has an orbital period of 33 years, further confirming its role as the swarm's progenitor comet.

Similarly comet 1862 III is the ancestor of the August Perseid meteor shower. Unlike the Leonids, the Perseids do not produce meteor showers. This means that the swarm matter is more or less evenly distributed along its orbit. It can therefore be assumed that the Perseids are an "older" meteor flood than the Leonids.

Relatively recently, the Draconids meteor shower formed, giving spectacular meteor showers on October 9-10, 1933 and 1946. The ancestor of this stream is the comet Giacobini-Zinner (1926 VI). Its period is 6.5 years, so meteor showers were observed at intervals of 13 years (the two periods of the comet correspond almost exactly to 13 revolutions of the Earth). But neither in 1959 nor in 1972 were Draconid meteor showers observed. During these years, the Earth passed far from the swarm's orbit. For 1985, the forecast was more favorable. Indeed, on the evening of October 8, a spectacular meteor shower was observed in the Far East, although it was inferior in number and duration to the rain of 1946. It was daytime in most of the territory of our country, but the astronomers of Dushanbe and Kazan observed the meteor shower using radar installations.

Comet Biela, which broke up in 1846 before the eyes of astronomers into two parts, was no longer observed in 1872, but astronomers witnessed two powerful meteor showers - in 1872 and 1885. This stream was called Andromeda (after the constellation) or Bielida (after the comet). Unfortunately, for a whole century it has not been repeated, although the period of revolution of this comet is also 6.5 years. Biela's comet is one of the lost - it has not been observed for 130 years. Most likely, it really fell apart, giving rise to the Andromedid meteor shower.

Halley's famous comet is associated with two meteor showers: the Aquarids observed in May (radiant in Aquarius) and the Orionids observed in October (radiant in Orion). This means that the orbit of the Earth intersects with the orbit of the comet not at one point, like most comets, but at two. In connection with the approach of Halley's comet to the Sun and to the Earth in early 1986, the attention of astronomers and amateur astronomers was drawn to these two streams. Observations of the Aquarid shower in May 1986 in the USSR confirmed its increased activity with a predominance of bright meteors.

Thus, from the established connections between meteor showers and comets, an important cosmogonic conclusion follows: the meteor bodies of the streams are nothing but products of the destruction of comets. As for sporadic meteors, they are most likely the remnants of disintegrated streams. Indeed, the trajectory of meteor particles is strongly affected by the attraction of the planets, especially the giant planets of the Jupiter group. Disturbances from the planets lead to dissipation, and then to the complete decay of the flow. True, this process takes thousands, tens and hundreds of thousands of years, but it works constantly and inexorably. The entire meteor complex is gradually being updated.

Let us turn to the distribution of meteor orbits according to the values ​​of their elements. First of all, we note the important fact that these distributions different for meteors recorded by photomethod (photometeors) and radar (radiometeors). The reason for this is that the radar method makes it possible to register much fainter meteors than photography, which means that the data of this method (after taking into account the physical factor) refer on average to much smaller bodies than the data of the photographic method. Bright meteors that can be photographed correspond to bodies with a mass of more than 0.1 g, while radio meteors collected in the catalog of B. L. Kashcheev, V. N. Lebedints and M. F. Lagutin correspond to bodies with a mass of 10 -3 ~ 10 - 4 y.

Analysis of the meteor orbits of this catalog showed that the entire meteor complex can be divided into two components: flat and spherical. The spherical component includes orbits with arbitrary inclinations to the ecliptic, with a predominance of orbits with large eccentricities and semiaxes. The flat component includes orbits with small inclinations ( i < 35°), небольшими размерами (a< 5 a. e.) and rather large eccentricities. In 1966, V. N. Lebedinets hypothesized that meteor bodies with a spherical component are formed due to the decay of long-period comets, but their orbits are greatly changed under the influence of the Poynting-Robertson effect.

This effect is as follows. Small particles are very effectively affected not only by the attraction of the Sun, but also by light pressure. Why light pressure acts precisely on small particles is clear from the following. The pressure of the sun's rays is proportional surface area particle, or the square of its radius, while the attraction of the Sun is its mass, or ultimately its volume, i.e. the cube of the radius. The ratio of the light pressure (more precisely, the acceleration imparted by it) to the acceleration of the gravitational force will thus be inversely proportional to the radius of the particle and will be greater in the case of small particles.

If a small particle revolves around the Sun, then due to the addition of the speeds of light and the particle, according to the parallelogram rule, the light will fall slightly in front (For readers familiar with the theory of relativity, this interpretation may raise objections: after all, the speed of light does not add up to the speed of the source or receiver of light But a rigorous consideration of this phenomenon, as well as the phenomenon of the annual aberration of starlight (the apparent displacement of stars forward along the Earth's motion) close to it in nature within the framework of the theory of relativity, leads to the same result. a change in the direction of the beam incident on the particle due to its transition from one frame of reference to another.) and will slightly slow down its movement around the Sun. Because of this, the particle in a very gentle spiral will gradually approach the Sun, its orbit will be deformed. This effect was qualitatively described in 1903 by J. Poynting and mathematically substantiated in 1937 by G. Robertson. We will meet with manifestations of this effect more than once.

Based on the analysis of the elements of the orbits of meteor bodies with a spherical component, VN Lebedinets developed a model for the evolution of interplanetary dust. He calculated that in order to maintain the equilibrium state of this component, long-period comets should eject an average of 10 15 g of dust annually. This is the mass of a relatively small comet.

As for the meteor bodies of the flat component, they are apparently formed as a result of the decay of short-period comets. However, not everything is clear yet. The typical orbits of these comets differ from the orbits of meteors of the flat component (comets have large perihelion distances and smaller eccentricities), and their transformation cannot be explained by the Poynting-Robertson effect. We are not aware of comets with such orbits as active meteor showers of the Geminids, Arietids, -Aquarids and others. Meanwhile, to replenish the flat component, it is necessary that one new comet with an orbit of this type be formed once every several hundred years. These comets, however, are extremely short-lived (mainly due to small perihelion distances and short orbital periods), and perhaps that is why not a single such comet has yet come into our field of vision.

An analysis of the orbits of photometeors by the American astronomers F. Whipple, R. McCroskey, and A. Posen showed significantly different results. Most large meteoroids (with masses greater than 1 g) move in orbits similar to those of short-period comets ( a < 5 а. е., i< 35° e> 0.7). Approximately 20% of these bodies have orbits close to those of long-period comets. Apparently, each component of meteor bodies of such sizes is a product of the decay of the corresponding comets. When moving to smaller bodies (up to 0.1 g), the number of orbits of small sizes increases noticeably (a< 2 a. e.). This is consistent with the fact discovered by Soviet scientists that such orbits predominate in radio meteors of the flat component.

Let us now turn to the orbits of meteorites. As already mentioned, exact orbits have been determined for only three meteorites. Their elements are given in table. one ( v is the speed at which the meteorite enters the atmosphere, q, q" - distances from the Sun at perihelion and aphelion).

The close similarity between the orbits of the Lost City and Inisfree meteorite and some difference from them in the orbit of the Pribram meteorite is striking. But the most important thing is that all three meteorites in aphelion cross the so-called asteroid belt (minor planets), the boundaries of which conditionally correspond to distances of 2.0-4.2 AU. e. The orbital inclinations of all three meteorites are small, unlike most small meteoroids.

But maybe it's just a coincidence? After all, three orbits is too little material for statistics and any conclusions. A. N. Simonenko in 1975-1979 studied more than 50 orbits of meteorites, determined by an approximate method: the radiant was determined from the testimony of eyewitnesses, and the entry velocity was estimated from the location of the radiant relative to apex(The point on the celestial sphere, to which the movement of the Earth is currently directed in its orbit). Obviously, for oncoming (fast) meteorites, the radiant should be located not far from the apex, and for overtaking (slow) meteorites - near the point of the celestial sphere opposite to the apex - antiapex.

Table 1. Elements of the exact orbits of three meteorites

It turned out that the radiants of all 50 meteorites are grouped around the antiapex and cannot be separated from it further than 30-40 o. This means that all meteorites are catching up, that they move around the Sun in the forward direction (like the Earth and all the planets) and their orbits cannot have an inclination to the ecliptic exceeding 30-40 °.

Let's face it, this conclusion is not strictly justified. In her calculations of the elements of the orbits of 50 meteorites, A. N. Simonenko proceeded from the assumption previously formulated by her and B. Yu. Levin that the speed of entry of meteorite-forming bodies into the Earth's atmosphere cannot exceed 22 km/s. This assumption was based first on the theoretical analysis of B. Yu. Levin, who back in 1946; showed that at high speeds a meteoroid entering the atmosphere must be completely destroyed (due to evaporation, crushing, melting) and does not fall out in the form of a meteorite. This conclusion was confirmed by the results of observations of the Prairie and European fireball networks, when none of the large meteoroids that flew in at speeds greater than 22 km/s fell out in the form of a meteorite. The speed of the Pribram meteorite, as can be seen from Table. 1 is close to this upper limit, but still does not reach it.

Having taken the value of 22 km/s as the upper limit for the entry velocity of meteorites, we thereby already predetermine that only overtaking meteoroids can break through the “atmospheric barrier” and fall to Earth as meteorites. This conclusion means that those meteorites that we collect and study in our laboratories moved in the solar system along orbits of a strictly defined class (their classification will be discussed later). But it does not mean at all that they exhaust the entire complex of bodies of the same size and mass (and, possibly, the same structure and composition, although this is not at all necessary) moving in the solar system. It is possible that many bodies (and even most of them) move in completely different orbits and simply cannot break through the "atmospheric barrier" of the Earth. The negligible percentage of meteorites that fell compared to the number of bright fireballs photographed by both fireball networks (about 0.1%) seems to support such a conclusion. But we come to different conclusions if we adopt other methods of analysis of observations. One of them, based on the determination of the density of meteoroids from the height of their destruction, will be discussed further. Another method is based on a comparison of the orbits of meteorites and asteroids. Since the meteorite fell to the Earth, it is obvious that its orbit intersected with the Earth's orbit. Of the entire mass of known asteroids (about 2500), only 50 have orbits that intersect the orbit of the Earth. All three meteorites with precise orbits at aphelion crossed the asteroid belt (Fig. 5). Their orbits are close to the orbits of asteroids of the Amur and Apollo groups, passing near the Earth's orbit or crossing it. About 80 such asteroids are known. The orbits of these asteroids are usually divided into five groups: I - 0.42<q<0,67 а. е.; II -0,76<q<0,81 а. е.; III - 1,04< q<1,20 а. е.; IV-small orbits; V is a large inclination of the orbits. Between groups I- II and II- III noticeable intervals, called the hatches of Venus and the Earth. Most asteroids (20) belong to the group III, but this is due to the convenience of observing them near perihelion, when they come close to the Earth and are in opposition to the Sun.

If we distribute the 51 orbits of meteorites known to us into the same groups, then 5 of them can be attributed to the group I; 10 - to the group II, 31 - to the group III and 5 - to the group IV. None of the meteorites belongs to the group V. It can be seen that here, too, the vast majority of orbits belong to the group III, although the factor of convenience of observation does not apply here. But it is not difficult to realize that fragments of asteroids of this group must enter the Earth's atmosphere at very low velocities, and therefore they must experience relatively weak destruction in the atmosphere. The meteorites Lost City and Inisfree belong to this group, while Pribram belongs to the group II.

All these circumstances, along with some others (for example, with a comparison of the optical properties of the surfaces of asteroids and meteorites), allow us to draw a very important conclusion: meteorites are fragments of asteroids, and not just any, but belonging to the Amur and Apollo groups. This immediately gives us the opportunity to judge the composition and structure of asteroids based on the analysis of the substance of meteorites, which is an important step forward in understanding the nature and origin of both.

But we must immediately draw another important conclusion: meteorites have other origin, than the bodies that create the phenomenon of meteors: the first are fragments of asteroids, the second are the decay products of comets.

Rice. 5. Orbits of the meteorites Pribram, Lost City and Inisfree. Points of their meeting with the Earth are marked

Thus, meteors cannot be considered “small meteorites” - in addition to the terminological difference between these concepts, which was mentioned at the beginning of the book (the author of this book, back in 1940, proposed (together with G. O. Zateishchikov) to call the cosmic body itself meteor, and the phenomenon of a "shooting star" - meteor flight. However, this proposal, which greatly simplified meteor terminology, was not accepted.), there is also a genetic difference between the bodies that create the phenomenon of meteors and meteorites: they are formed in different ways, due to the decay of various bodies of the solar system.

Rice. 6. Diagram of distribution of orbits of small bodies in coordinates a-e

Points - fireballs of the Prairie network; circles - meteor showers (according to V. I. Tsvetkov)

The question of the origin of meteoroids can be approached in another way. Let's build a diagram (Fig. 6), plotting along the vertical axis the values ​​of the semi-major axis of the orbit a(or 1/ a), a on the horizontal - the eccentricity of the orbit e. By values a, e Let us plot points on this diagram corresponding to the orbits of known comets, asteroids, meteorites, bright fireballs, meteor showers, and meteors of various classes. Let us also draw two very important lines corresponding to the conditions q=1 and q" = 1. It is obvious that all points for meteoric bodies will be located between these lines, since only inside the region bounded by them, the condition of intersection of the meteoroid orbit with the Earth's orbit is realized.

Many astronomers, starting with F. Whipple, tried to find and plot on a- e-diagram in the form of lines, criteria delimiting the orbits of asteroidal and cometary types. A comparison of these criteria was made by the Czechoslovak meteor researcher L. Kresak. Since they give similar results, we have carried out in Fig. 6 one averaged "contact line" q"= 4.6. Above and to the right of it are comet-type orbits, below and to the left - asteroidal. On this chart, we plotted points corresponding to 334 race cars from the catalog of R. McCrosky, K. Shao and A. Posen. It can be seen that most of the points lie below the demarcation line. Only 47 out of 334 points are located above this line (15%), and with a slight upward shift, their number will decrease to 26 (8%). These points probably correspond to bodies of cometary origin. It is interesting that many points seem to "snuggle" to the line q = 1, and two points even go beyond the bounded area. This means that the orbits of these two bodies did not cross the Earth's orbit, but only passed close, but the Earth's gravity forced these bodies to fall on it, giving rise to the spectacular phenomenon of bright fireballs.

It is possible to make another comparison of the orbital characteristics of the small bodies of the Solar System. When building a- e- diagrams, we did not take into account the third important element of the orbit - its inclination to the ecliptic i. It is proved that some combination of elements of the orbits of the bodies of the Solar System, called the Jacobi constant and expressed by the formula

where a- the semi-major axis of the orbit in astronomical units, retains its value, despite the change in individual elements under the influence of perturbations from the major planets. Value U e has the meaning of some speed, expressed in units of the circular velocity of the Earth. It is easy to prove that it is equal to the geocentric velocity of a body crossing the Earth's orbit.

Fig.7. Distribution of asteroid orbits (1), fireballs of the Prairie Network ( 2 ), meteorites (3), comets (4) and meteor showers (3) by the Jacobi constant U e and major axle a

Let's build a new diagram (Fig. 7), plotting the Jacobi constant along the vertical axis U e (dimensionless) and the corresponding geocentric velocity v 0 , and along the horizontal axis - 1/ a. Let us plot points on it corresponding to the orbits of asteroids of the Amur and Apollo groups, meteorites, short-period comets (long-period comets go beyond the diagram), and fireballs of the McCrosky, Shao, and Posen catalogs (bolides are marked with crosses, which correspond to the most friable bodies, see below),

We can immediately note the following properties of these orbits. The orbits of fireballs are close to the orbits of asteroids of the Amur and Apollo groups. The orbits of meteorites are also close to the orbits of asteroids of these groups, but for them U e <0,6 (геоцентрическая скорость меньше 22 км/с, о чем мы уже говорили выше). Орбиты комет расположены значительно левее орбит прочих тел, т. е. у них больше значения a. Only Encke's comet fell into the thick of fireball orbits (There is a hypothesis put forward by I. T. Zotkin and developed by L. Kresak that the Tunguska meteorite is a fragment of Encke's comet. For more details, see the end of Chapter 4).

The similarity of the orbits of the asteroids of the Apollo group with the orbits of some short-period comets and their sharp difference from the orbits of other asteroids led the Irish astronomer E. Epik (an Estonian by nationality) in 1963 to the unexpected conclusion that these asteroids are not small planets, but "dried" nuclei of comets . Indeed, the orbits of the asteroids Adonis, Sisyphus, and 1974 MA are very close to those of Comet Encke, the only "living" comet that could be assigned to the Apollo group by its orbital characteristics. At the same time, comets are known that retained their typical cometary appearance only at the first appearance. Comet Arend-Rigo already in 1958 (second appearance) had a completely star-shaped appearance, and, had it been discovered in 1958 or 1963, it could well have been classified as an asteroid. The same can be said about the comets Kulin and Neuimin-1.

According to Epic, the time of the loss of all volatile components by the nucleus of Encke's comet is measured in thousands of years, while the dynamic time of its existence is measured in millions of years. Therefore, a comet must spend most of its life in a "dried" state, in the form of an asteroid of the Apollo group. Apparently, Encke's comet has been moving in its orbit for no more than 5,000 years.

The Geminid meteor shower falls on the diagram in the asteroidal region, and the asteroid Icarus has the closest orbit to it. For the Geminids, the progenitor comet is unknown. According to Epic, the Geminid shower is the result of the breakup of a once-existing comet of the same group as Comet Encke.

Despite its originality, Epik's hypothesis deserves serious consideration and careful testing. The direct way of such verification is the study of Encke's comet and asteroids of the Apollo group from automatic interplanetary stations.

The most weighty objection to the above hypothesis is that not only stony meteorites (Pribram, Lost City, Inisfree), but also iron ones (Sikhote-Alin) have orbits close to those of asteroids of the Apollo group. But an analysis of the structure and composition of these meteorites (see below) shows that they were formed in the depths of parent bodies tens of kilometers in diameter. It is unlikely that these bodies could be the nuclei of comets. In addition, we know that meteorites are never associated with either comets or meteor showers. Therefore, we come to the conclusion that among the asteroids of the Apollo group there should be at least two subgroups: meteorite-forming and "dried" nuclei of comets. Asteroids can be assigned to the first subgroup I- IV classes mentioned above, with the exception of such asteroids I class like Adonis and Daedalus having too much value U e. The second subgroup includes asteroids of the Icarus type and 1974 MA (the second of them belongs to V class, Icarus falls out of this classification).

Thus, the question of the origin of large meteoroids cannot yet be considered fully clarified. However, we will return to their nature later.

The influx of meteoric matter to Earth

A huge number of meteoroids are constantly falling to the Earth. And the fact that most of them evaporate or break up into tiny grains in the atmosphere does not change things: due to the fallout of meteoroids, the mass of the Earth is constantly increasing. But what is this increase in the mass of the Earth? Can it have cosmogonic significance?

In order to estimate the influx of meteoric matter to the Earth, it is necessary to determine what the mass distribution of meteoroids looks like, in other words, how the number of meteoroids changes with mass.

It has long been established that the distribution of meteoroids by mass is expressed by the following power law:

N m= N 0 M - S,

where N 0 - number of meteoric bodies of unit mass, N m - number of bodies of mass M and more S is the so-called integral mass index. This value has been repeatedly determined for various meteor showers, sporadic meteors, meteorites, and asteroids. Its values ​​for a number of definitions are presented in Fig. 8, borrowed from the famous Canadian meteor researcher P. Millman. When S=1 the mass flux brought by meteoric bodies is the same in any equal intervals of the mass logarithm; if S>1, then most of the mass flow is supplied by small bodies, if S<1, то большие тела. Из рис. 8 видно, что величина S takes on different values ​​in different mass ranges, but averageS=1. For visual and photographic meteors over many data S\u003d 1.35, for fireballs, according to R. McCrosky, S=0.6. In the region of small particles (M<10 -9 г) S also decreases to 0.6.

Rice. 8. Change parameter Swith the mass of small bodies of the solar system (according to P. Millman)

1 - lunar craters; 2- meteor particles (satellite data); 3 - meteors; 4 - meteorites; 5 - asteroids

One way to study the mass distribution of small meteor particles is to study microcraters on surfaces specially exposed for this purpose in interplanetary space or on the Moon, since it has been proven that all small and the vast majority of large lunar craters are of impact, meteorite origin. Going from crater diameters D to the values ​​of the mass of the bodies that formed them is produced by the formula

D= km 1/ b,

where in the cgs system k=3.3, for small bodies (10 -4 cm or less) b=3, for large bodies (up to meter) b=2,8.

However, one must keep in mind that microcraters on the surface of the Moon can be destroyed due to various forms of erosion: meteorite, from the solar wind, thermal destruction. Therefore, their observed number may be less than the number of formed craters.

Combining all methods of studying meteoric matter: counting microcraters on spacecraft, readings of meteor particle counters on satellites, radar, visual and photographic observations of meteors, counting meteorite falls, statistics of asteroids, it is possible to draw up a summary graph of the distribution of meteoroids by mass and calculate the total influx of meteoric matter to the ground. We present here a graph (Fig. 9) constructed by V. N. Lebedints on the basis of numerous series of observations by different methods in different countries, as well as summary and theoretical curves. The distribution model adopted by V. N. Lebedints is shown as a solid line. Attention is drawn to the break of this curve near M=10 -6 g and a noticeable deflection in the mass range 10 -11 -10 -15 g.

This deflection is explained by the already known Poynting-Robertson effect. As we know, light pressure slows down the orbital motion of very small particles (their dimensions are on the order of 10 -4 -10 -5 cm) and causes them to gradually fall out onto the Sun. Therefore, in this range of masses, the curve has a deflection. Even smaller particles have diameters comparable or smaller than the wavelength of light, and light pressure does not act on them: due to the phenomenon of diffraction, light waves go around them without exerting pressure.

Let's move on to estimating the total mass inflow. Let we want to determine this influx in the mass interval from M 1 to M 2 , and M 2 > M 1 Then from the mass distribution law written above it follows that the influx of mass Ф m is equal to:

at S 1

at S=1

Rice. 9. Distribution of meteoroids by mass (according to VN Lebedints) The "dip" in the mass range 10 -11 -10 -15 g is associated with the Poynting-Robertson effect; N-number of particles per square meter per second from the celestial hemisphere

These formulas have a number of remarkable properties. Namely, at S=1 mass flux Ф m depends only on the mass ratio M 2 M 1(given No) ; at S<1 and M 2 >> M 1 f m depends practically only on the value greater mass M 2 and does not depend on M 1 ; at S>1 and M 2 > M 1 flux F m depends practically only on the value smaller massM 1 and does not depend on M 2 These properties of mass influx formulas and variability S, shown in fig. 8, clearly show how dangerous it is to average the value S and straighten the distribution curve in Fig. 9, which some researchers have already tried to do. Calculations of the mass inflow have to be done at intervals, then summing up the results.

Table 2. Estimates of the influx of meteor matter to Earth based on astronomical data

Various scientists, using different methods of analysis, received different estimates, not much, however, diverging from each other. In table. Table 2 shows the most reasonable estimates for the last 20 years.

As you can see, the extreme values ​​of these estimates differ by almost 10 times, and the last two estimates - by 3 times. However, V. N. Lebedinets considers the number he obtained to be only the most probable and indicates the extreme possible limits of the mass inflow (0.5-6) ​​10 4 tons / year. Refinement of the estimate of the influx of meteor matter to the Earth is a task for the near future.

In addition to astronomical methods for determining this important quantity, there are also cosmochemical methods based on calculations of the content of cosmogenic elements in certain sediments, namely in deep-sea sediments: silts and red clays, glaciers and snow deposits in Antarctica, Greenland and other places. Most often, the content of iron, nickel, iridium, osmium, isotopes of carbon 14 C, helium 3 He, aluminum 26 A1, chlorine 38 C is determined l, some isotopes of argon. To calculate the mass influx by this method, the total content of the element under study in the taken sample (core) is determined, then the average content of the same element or isotope in terrestrial rocks (the so-called earth background) is subtracted from it. The resulting number is multiplied by the density of the core, by the rate of sedimentation (i.e., the accumulation of those deposits from which the core was taken) and by the surface area of ​​the Earth and divided by the relative content of this element in the most common class of meteorites - in chondrites. The result of such a calculation is the influx of meteoric matter to the Earth, but determined by cosmochemical means. Let's call it FK.

Although the cosmochemical method has been used for more than 30 years, its results are in poor agreement with each other and with the results obtained by the astronomical method. True, J. Barker and E. Anders, by measuring the content of iridium and osmium in deep-sea clays at the bottom of the Pacific Ocean, obtained in 1964 and 1968. estimates of mass inflow (5 - 10) 10 4 t/year, which is close to the highest estimates obtained by the astronomical method. In 1964, O. Schaeffer and co-workers determined the value of the mass inflow of 4 10 4 t/year from the content of helium-3 in the same clays. But for chlorine-38, they also received a value 10 times greater. E. V. Sobotovich and his collaborators on the content of osmium in red clays (from the bottom of the Pacific Ocean) obtained FK = 10 7 t/year, and on the content of the same osmium in the Caucasian glaciers - 10 6 t/year. Indian researchers D. Lal and V. Venkatavaradan calculated Fc = 4 10 6 t/year from the content of aluminum-26 in deep-sea sediments, and J. Brokas and J. Picciotto calculated from the content of nickel in snow deposits of Antarctica - (4-10) 10 6 t/year.

What is the reason for such a low accuracy of the cosmochemical method, which gives discrepancies within three orders of magnitude? The following explanations for this fact are possible:

1) the concentration of the measured elements in most meteoric matter (which, as we have seen, is mainly of cometary origin) is different from that accepted for chondrites;

2) there are processes that we do not take into account that increase the concentration of measured elements in bottom sediments (for example, underwater volcanism, gas release, etc.);

3) the rate of sedimentation is determined incorrectly.

Obviously, cosmochemical methods still need to be improved. We will therefore proceed from the data of astronomical methods. Let's accept the estimate of the influx of meteoric matter obtained by the author and see how much of this matter fell out during the entire time of the Earth's existence as a planet. Multiplying the annual influx (5 10 4 t) by the age of the Earth (4.6 10 9 years), we get approximately 2 10 14 t. throughout this entire period. Recall that the mass of the Earth is 6 10 21 tons. Our estimate of the increase is an insignificant fraction (one thirty-millionth) of the mass of the Earth. If we accept the estimate of the influx of meteor matter obtained by V. N. Lebedints, this fraction will drop to one hundred millionth. Of course, this increase did not play any role in the development of the Earth. But this conclusion refers to the modern period. Previously, especially in the early stages of the evolution of the solar system and the Earth as a planet, the fallout on it of the remnants of a pre-planetary dust cloud and larger fragments undoubtedly played a significant role not only in increasing the mass of the Earth, but also in its heating. However, we will not consider this issue here.

The structure and composition of meteorites

Meteorites are usually divided into two groups according to the method of their detection: falls and finds. Falls are meteorites observed during the fall and picked up immediately after it. Finds are meteorites found by chance, sometimes during excavation and field work or during hiking trips, excursions, etc. (The found meteorite is of great value for science. Therefore, it should be immediately sent to the Committee on Meteorites of the USSR Academy of Sciences: Moscow , 117312, M. Ulyanova St., 3. Those who find a meteorite are paid a cash prize.If the meteorite is very large, it is necessary to break it off and send a small piece.Prior to receipt of a notice from the Committee on Meteorites or until the arrival of a representative of the Committee, a stone suspected of cosmic origin in no case should it be split into pieces, handed out, damaged.It is necessary to take all measures to preserve this stone or stones, if several are collected, and also to remember or mark the places of finds.)

According to their composition, meteorites are divided into three main classes: stony, stony-iron and iron. To conduct their statistics, only falls are used, since the number of finds depends not only on the number of meteorites that once fell, but also on the attention they attract from random eyewitnesses. Here, iron meteorites have an undeniable advantage: a person is more likely to pay attention to a piece of iron, moreover of an unusual appearance (melted, with pits), than to a stone that differs little from ordinary stones.

Among the falls, 92% are stony meteorites, 2% are stony iron and 6% are iron.

Often, meteorites break apart in flight into several (sometimes very many) fragments, and then meteor Rain. It is customary to consider a meteor shower the simultaneous fall of six or more individual copies meteorites (as it is customary to call fragments falling to Earth each separately, in contrast to fragments, formed during the crushing of meteorites from hitting the ground).

Meteor showers are most often stone, but occasionally iron meteor showers also fall (for example, the Sikhote-Alin, which fell on February 12, 1947 in the Far East).

Let us proceed to the description of the structure and composition of meteorites by types.

stone meteorites. The most common class of stony meteorites are the so-called chondrites(see incl.). More than 90% of stony meteorites belong to them. These meteorites got their name from rounded grains - chondrus, of which they are composed. Chondrules have different sizes: from microscopic to centimeter, they account for up to 50% of the volume of the meteorite. The rest of the substance (interchondral) does not differ in composition from the substance of chondrules.

The origin of chondrules has not yet been elucidated. They are never found in terrestrial minerals. It is possible that chondrules are frozen droplets formed during the crystallization of meteorite matter. In terrestrial rocks, such grains must be crushed by the monstrous pressure of the layers lying above, while meteorites were formed in the depths of parent bodies tens of kilometers in size (the average size of asteroids), where pressure even in the center is relatively small.

Basically, chondrites are composed of iron-magnesian silicates. Among them, the first place is occupied by olivine ( Fe, Mg) 2 Si0 4 - it accounts for 25 to 60% of the substance of meteorites of this class. In second place are hypersthene and bronzite ( Fe, Mg) 2 Si 2 O 6 (20-35%). Nickel iron (kamacite and taenite) is from 8 to 21%, iron sulfite FeS - troilite - 5%.

Chondrites are divided into several subclasses. Among them, ordinary, enstatite and carbonaceous chondrites are distinguished. Ordinary chondrites, in turn, are divided into three groups: H - with a high content of nickel iron (16-21%), L-with low (about 8%) and LL-c is very low (less than 8%). In enstatite chondrites, the main components are enstatite and clinoenstatite. Mg2 Si 2 Q 6 , which account for 40-60% of the total composition. Enstatite chondrites are also distinguished by a high content of kamacite (17-28%) and troilite (7-15%). They also contain plagioclase. PNaAlSi 3 O 8 - m CaAlSi 2 O 8 - up to 5-10%.

Carbonaceous chondrites stand apart. They are distinguished by their dark color, for which they got their name. But this color is given to them not by an increased carbon content, but by finely divided grains of magnetite. Fe3 O 4 . Carbonaceous chondrites contain many hydrated silicates such as montmorillonite ( Al, Mg) 3 (0 h) 4 Si 4 0 8 , serpentine Mg 6 ( OH) 8 Si 4 O 10 , and, as a result, a lot of bound water (up to 20%). With the transition of carbonaceous chondrites from type C I to type C III, the proportion of hydrated silicates decreases, and they give way to olivine, clinohypersthene, and clinoenstatite. Carbonaceous matter in type C chondrites I is 8%, C II - 5%, for C III - 2%.

Cosmogonists consider the substance of carbonaceous chondrites to be the closest in composition to the primary substance of the pre-planetary cloud that once surrounded the Sun. Therefore, these very rare meteorites are subjected to careful analysis, including isotopic analysis.

From the spectra of bright meteors, it is sometimes possible to determine the chemical composition of the bodies that give rise to them. A comparison of the ratios of the content of iron, magnesium and sodium in meteor bodies from the Draconid stream and in chondrites of various types, carried out in 1974 by the Soviet meteoritologist A. A. Yavnel, showed that the bodies included in the Draconid stream are close in composition to carbonaceous chondrites of the class With I. In 1981, the author of this book, continuing his research according to the method of A. A. Yavnel, proved that sporadic meteoroids are similar in composition to chondrites C I, and those that form the Perseid stream, to class C III. Unfortunately, data on the spectra of meteors, which make it possible to determine the chemical composition of the bodies that give rise to them, are still insufficient.

Another class of stony meteorites - achondrites- characterized by the absence of chondrules, a low content of iron and elements close to it (nickel, cobalt, chromium). There are several groups of achondrites, differing in the main minerals (orthoenstatite, olivine, orthopyroxene, pigeonite). All achondrites account for about 10% of stony meteorites.

It is curious that if you take the substance of chondrites and melt it, then two fractions that do not mix with each other are formed: one of them is nickel iron, similar in composition to iron meteorites, the other is silicate, which is close in composition to achondrites. Since the number of both is almost the same (among all meteorites, 9% are achondrites and 8% are iron and iron-stone), one can think that these classes of meteorites are formed during the remelting of chondrite substance in the bowels of the parent bodies.

iron meteorites(see photo) are 98% nickel iron. The latter has two stable modifications: poor in nickel kamacite(6-7% nickel) and rich in nickel taenite(30-50% nickel). Kamacite is arranged in the form of four systems of parallel plates separated by interlayers of taenite. Kamacite plates are located along the faces of an octahedron (octahedron), therefore such meteorites are called octahedrites. Less common are iron meteorites. hexahedrites, having a cubic crystal structure. Even more rare ataxites- meteorites, devoid of any ordered structure.

The thickness of kamacite plates in octahedrites varies from a few millimeters to hundredths of a millimeter. According to this thickness, coarse- and fine-structured octahedrites are distinguished.

If a part of the octahedrite surface is ground down and the section is etched with acid, then a characteristic pattern will appear in the form of a system of intersecting bands, called Widmanstätten figures(see incl.) named after the scientist A. Widmanstetten, who first discovered them in 1808. These figures appear only in octahedrites and are not observed in iron meteorites of other classes and in terrestrial iron. Their origin is associated with the kamacite-taenite structure of octahedrites. According to the Widmashnettten figures, one can easily establish the cosmic nature of the "suspicious" piece of iron found.

Another characteristic feature of meteorites (both iron and stone) is the presence on the surface of many pits with smooth edges approximately 1/10 the size of the meteorite itself. These pits, clearly visible in the photograph (see incl.), are called regmaglypts. They are formed already in the atmosphere as a result of the formation of turbulent vortices near the surface of the body that has entered it, which, as it were, scrape out pits-regmaglipts (This explanation was proposed and substantiated by the author of this book in 1963).

The third external sign of meteorites is the presence on their surface of a dark melting crust thickness from hundredths to one millimeter.

Iron stone meteorites half metal, half silicate. They are divided into two subclasses: pallasites, in which the metal fraction forms a kind of sponge, in the pores of which silicates are located, and mesosiderites, where, on the contrary, the pores of the silicate sponge are filled with nickel iron. In pallasites, silicates consist mainly of olivine, in mesosiderites - of orthopyroxene. Pallasites got their name from the first Pallas Iron meteorite found in our country. This meteorite was discovered more than 200 years ago and taken from Siberia to St. Petersburg by Academician PS Pallas.

The study of meteorites makes it possible to reconstruct their history. We have already noted that the structure of meteorites indicates their occurrence in the interior of parent bodies. The phase ratio of, for example, nickel iron (kamacite-taenite), the distribution of nickel across the taenite layers, and other characteristic features even make it possible to judge the size of the primary parent bodies. In most cases, these were bodies with a diameter of 150-400 km, i.e., like the largest asteroids. Studies of the structure and composition of meteorites force us to reject the hypothesis that is very popular among non-specialists about the existence and decay between the orbits of Mars and Jupiter of the hypothetical planet Phaeton, several thousand kilometers in size. Meteorites falling to Earth formed in the depths many parent bodies different sizes. The analysis of the orbits of asteroids carried out by Academician of the Academy of Sciences of the Azerbaijan SSR G. F. Sultanov leads to the same conclusion (about the multiplicity of parent bodies).

By the ratio of radioactive isotopes and their decay products in meteorites, one can also determine their age. Isotopes with the longest half-lives, such as rubidium-87, uranium-235 and uranium-238, give us the age substances meteorites. It turns out to be 4.5 billion years, which corresponds to the age of the oldest terrestrial and lunar rocks and is considered the age of our entire solar system (more precisely, the period elapsed from the completion of the formation of the planets).

The above isotopes, decaying, form respectively strontium-87, lead-207 and lead-206. These substances, like the original isotopes, are in the solid state. But there is a large group of isotopes whose final decay products are gases. So, potassium-40, decaying, forms argon-40, and uranium and thorium - helium-3. But with a sharp heating of the parent body, helium and argon escape, and therefore the potassium-argon and uranium-helium ages give only the time of subsequent slow cooling. An analysis of these ages shows that they are sometimes measured in billions of years (but often much less than 4.5 billion years), and sometimes in hundreds of millions of years. For many meteorites, the uranium-helium age is 1-2 billion years less than the potassium-argon age, which indicates repeated collisions of this parent body with other bodies. Such collisions are the most likely sources of sudden heating of small bodies to temperatures of hundreds of degrees. And since helium volatilizes at lower temperatures than argon, the helium ages may indicate the time of a later, not very strong collision, when the increase in temperature was not enough to volatilize argon.

All these processes were experienced by the substance of the meteorite even during its stay in the parent body, so to speak, before its birth as an independent celestial body. But here the meteorite in one way or another separated from the parent body, "was born into the world." When did it happen? The period elapsed from this event is called space age meteorite.

To determine cosmic ages, a method based on the phenomenon of the interaction of a meteorite with galactic cosmic rays is used. This is the name given to energetic charged particles (most often protons) coming from the boundless expanses of our Galaxy. Penetrating the body of a meteorite, they leave their traces (tracks). From the density of the tracks, one can determine the time of their accumulation, i.e., the space age of the meteorite.

The cosmic age of iron meteorites is hundreds of millions of years, and that of stone meteorites is millions and tens of millions of years. This difference is most likely due to the lower strength of stony meteorites, which break apart from collisions with each other into small pieces and "do not live" to the age of one hundred million years. An indirect confirmation of this view is the relative abundance of stone meteorite showers compared to iron ones.

Concluding this review of our knowledge of meteorites, let us now turn to what the study of meteor phenomena gives us.

The objects of the solar system, in accordance with the rules of the International Astronomical Union, are divided into the following categories:

planets - bodies that revolve around the Sun are in hydrostatic equilibrium (that is, they have a shape close to spherical), and have also cleared the vicinity of their orbit from other smaller objects. There are eight planets in the solar system - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

dwarf planets also revolve around the Sun and have a spherical shape, but their gravity is not enough to clear their trajectory from other bodies. So far, the International Astronomical Union has recognized five dwarf planets - Ceres (a former asteroid), Pluto (a former planet), as well as Haumea, Makemake and Eris.

planetary satellites- bodies that do not revolve around the Sun, but around the planets.

Comets- bodies that revolve around the sun and consist mainly of frozen gas and ice. When approaching the Sun, they have a tail, the length of which can reach millions of kilometers, and a coma - a spherical gaseous shell around a solid core.

asteroids- all other inert stone bodies. The orbits of most asteroids are concentrated between the orbits of Mars and Jupiter - in the main asteroid belt. Beyond the orbit of Pluto, there is an outer belt of asteroids - the Kuiper belt.

Meteora- fragments of space objects, particles a few centimeters in size, which enter the atmosphere at a speed of tens of kilometers per second and burn out, giving rise to a bright flare - a shooting star. Astronomers are aware of many meteor showers that are associated with the orbits of comets.

Meteorite- a space object or its fragment, which managed to "survive" the flight through the atmosphere and fell to the ground.

fireball- a very bright meteor, brighter than Venus. It's a fireball with a smoky tail trailing behind it. The flight of the fireball can be accompanied by thunderous sounds, it can end with an explosion, and sometimes with the fallout of meteorites. Numerous video clips filmed by residents of Chelyabinsk show exactly the flight of the bolide.

Damocloids- celestial bodies of the solar system that have orbits similar to those of comets in terms of parameters (large eccentricity and inclination to the ecliptic plane), but do not show cometary activity in the form of a coma or cometary tail. The name Damocloids was named after the first representative of the class - the asteroid (5335) Damocles. As of January 2010, 41 damocloids were known.

Damocloids are relatively small - the largest of them, 2002 XU 93, has a diameter of 72 km, and the average diameter is about 8 km. Measurements of the albedo of four of them (0.02-0.04) showed that damocloids are among the darkest bodies in the solar system, having, nevertheless, a reddish tint. Due to their large eccentricities, their orbits are very elongated, and at aphelion they are farther than Uranus (up to 571.7 AU in 1996 PW), and at perihelion they are closer than Jupiter, and sometimes even Mars.

It is believed that damocloids are the nuclei of Halley-type comets, which originated in the Oort cloud and lost their volatile substances. This hypothesis is considered correct because quite a few objects that were considered damocloids were subsequently found to have a coma and were classified as comets. Another strong evidence is that the orbits of most damocloids are strongly inclined to the plane of the ecliptic, sometimes more than 90 degrees - that is, some of them revolve around the Sun in the opposite direction to the movement of the major planets, which sharply distinguishes them from asteroids. The first of these bodies, discovered in 1999, was named (20461) Diorets - "asteroid" in reverse.

RIA Novosti http://ria.ru/science/20130219/923705193.html#ixzz3byxzmfDT

Asteroids, comets, meteors, meteorites - astronomical objects that seem the same to the uninitiated in the basics of the science of celestial bodies. In fact, they differ in several ways. The properties that characterize asteroids, comets, are easy to remember. They also have a certain similarity: such objects are classified as small bodies, often classified as space debris. About what a meteor is, how it differs from an asteroid or a comet, what are their properties and origin, and will be discussed below.

tailed wanderers

Comets are space objects consisting of frozen gases and stone. They originate in remote areas of the solar system. Modern scientists suggest that the main sources of comets are the interconnected Kuiper belt and scattered disk, as well as the hypothetically existing

Comets have highly elongated orbits. As they approach the Sun, they form a coma and a tail. These elements consist of evaporating gaseous substances ammonia, methane), dust and stones. The head of a comet, or coma, is a shell of tiny particles, distinguished by brightness and visibility. It has a spherical shape and reaches its maximum size when approaching the Sun at a distance of 1.5-2 astronomical units.

In front of the coma is the nucleus of the comet. It, as a rule, has a relatively small size and an elongated shape. At a considerable distance from the Sun, the nucleus is all that remains of the comet. It consists of frozen gases and rocks.

Types of comets

The classification of these is based on the periodicity of their circulation around the star. Comets that fly around the Sun in less than 200 years are called short-period comets. Most often, they fall into the inner regions of our planetary system from the Kuiper belt or scattered disk. Long-period comets revolve with a period of more than 200 years. Their "homeland" is the Oort cloud.

"Minor planets"

Asteroids are made up of solid rocks. In size, they are much inferior to the planets, although some representatives of these space objects have satellites. Most of the minor planets, as they were called before, are concentrated in the main one located between the orbits of Mars and Jupiter.

The total number of such cosmic bodies known in 2015 exceeded 670,000. Despite such an impressive number, the contribution of asteroids to the mass of all objects in the solar system is insignificant - only 3-3.6 * 10 21 kg. This is only 4% of the similar parameter of the Moon.

Not all small bodies are classified as asteroids. The selection criterion is the diameter. If it exceeds 30 m, then the object is classified as an asteroid. Bodies with smaller dimensions are called meteoroids.

Classification of asteroids

The grouping of these cosmic bodies is based on several parameters. Asteroids are grouped according to the features of their orbits and the spectrum of visible light that was reflected from their surface.

According to the second criterion, three main classes are distinguished:

• carbon (C);
• silicate (S);
• metal (M).

Approximately 75% of all asteroids known today fall into the first category. With the improvement of equipment and a more detailed study of such objects, the classification is expanding.

meteoroids

A meteoroid is another type of cosmic body. They are not asteroids, comets, meteors or meteorites. The peculiarity of these objects is their small size. Meteoroids in their dimensions are located between asteroids and cosmic dust. Thus, they include bodies with a diameter of less than 30 m. Some scientists define a meteoroid as a solid body with a diameter of 100 microns to 10 m. By their origin, they are primary or secondary, that is, formed after the destruction of larger objects.

When entering the Earth's atmosphere, the meteoroid begins to glow. And here we are already approaching the answer to the question of what a meteor is.

Shooting star

Sometimes, among the flickering stars in the night sky, one suddenly flares up, describes a small arc and disappears. Anyone who has seen this at least once knows what a meteor is. These are "shooting stars" that have nothing to do with real stars. A meteor is actually an atmospheric phenomenon that occurs when small objects (the same meteoroids) enter the air shell of our planet. The observed brightness of the flash directly depends on the initial dimensions of the cosmic body. If the brightness of a meteor exceeds the fifth, it is called a fireball.

Observation

Such phenomena can only be admired from planets with an atmosphere. Meteors on the Moon or on Mercury cannot be observed, since they do not have an air shell.

Under the right conditions, "shooting stars" can be seen every night. It is best to admire meteors in good weather and at a considerable distance from a more or less powerful source of artificial light. Also, there should be no moon in the sky. In this case, it will be possible to notice up to 5 meteors per hour with the naked eye. The objects that give rise to such single "shooting stars" revolve around the Sun in a variety of orbits. Therefore, the place and time of their appearance in the sky cannot be accurately predicted.

streams

Meteors, photos of which are also presented in the article, as a rule, have a slightly different origin. They are part of one of several swarms of small cosmic bodies revolving around the star along a certain trajectory. In their case, the ideal period for observing (the time when, by looking at the sky, anyone can quickly understand what a meteor is) is pretty well defined.

A swarm of similar space objects is also called a meteor shower. Most often they are formed during the destruction of the nucleus of a comet. Individual swarm particles move parallel to each other. However, from the surface of the Earth, they seem to be flying out of a certain small area of ​​the sky. This section is called the radiant of the stream. The name of the meteor swarm, as a rule, is given by the constellation in which its visual center (radiant) is located, or by the name of the comet, the disintegration of which led to its appearance.

Meteors, photos of which are easy to obtain with special equipment, belong to such large streams as the Perseids, Quadrantids, Eta Aquarids, Lyrids, Geminids. In total, the existence of 64 streams has been recognized to date, and about 300 more are awaiting confirmation.

heavenly stones

Meteorites, asteroids, meteors and comets are related concepts according to certain criteria. The first are space objects that have fallen to Earth. Most often, their source is asteroids, less often - comets. Meteorites carry invaluable data about various corners of the solar system outside the Earth.

Most of these bodies that have fallen on our planet are very small. The most impressive meteorites in terms of their dimensions leave traces after the impact, which are quite noticeable even after millions of years. Well known is the crater near Winslow, Arizona. The fall of a meteorite in 1908 allegedly caused the Tunguska phenomenon.

Such large objects "visit" the Earth every few million years. Most of the found meteorites are rather modest in size, but do not become less valuable for science.

According to scientists, such objects can tell a lot about the time of the formation of the solar system. Presumably, they carry particles of the substance that young planets were made of. Some meteorites come to us from Mars or the Moon. Such space wanderers allow you to learn something new about nearby objects without huge expenses for distant expeditions.

To memorize the differences between the objects described in the article, it is possible to summarize the transformation of such bodies in space. An asteroid, consisting of solid rock, or a comet, which is an ice block, when destroyed, gives rise to meteoroids, which, when entering the planet's atmosphere, flare up as meteors, burn out in it or fall, turning into meteorites. The latter enrich our knowledge of all the previous ones.

Meteorites, comets, meteors, as well as asteroids and meteoroids are participants in continuous cosmic motion. The study of these objects contributes greatly to our understanding of the universe. As the equipment improves, astrophysicists receive more and more data on such objects. The relatively recently completed mission of the Rosetta probe unequivocally demonstrated how much information can be obtained from a detailed study of such cosmic bodies.