The largest asteroid in the solar system. What is interesting about the largest asteroids and their movement? Report on the largest asteroids

Any cosmic body with a diameter of more than 3 kilometers threatens the Earth in a collision with the disappearance of civilization. Therefore, it is so important to know about the largest asteroids and their movement in orbits, because among the 670 thousand objects of the solar system there are very unusual specimens. The bulk of large celestial bodies is located in the so-called asteroid belt, far from the Earth, so there is no direct threat to us. As they were discovered, they were called female names from Roman and Greek mythology, and then, with an increase in the number of discoveries, this rule was no longer observed.

Ceres

This rather large celestial body (diameter 975 * 909 km) has been anything since its discovery: both a full-fledged planet of the solar system, and an asteroid, and since 2006 it has acquired a new status - a dwarf planet. The last name is the most correct, since Ceres is not the main one in its orbit, but only the largest in the asteroid belt. It was discovered quite by accident by the Italian astronomer Piazzi in 1801.

Ceres is spherical (unusual for asteroids) with a rocky core and a crust of water ice and minerals. The distance between the nearest point of the orbit of this satellite of the Sun and the Earth is 263 million kilometers. Its path lies between Mars and Jupiter, but there is some tendency to chaotic movement (which increases the chances of colliding with other asteroids and changing the orbit). It is not visible to the naked eye from the surface of our planet - this is a star of only 7 magnitudes.

Pallas

The size is 582 * 556 kilometers, and it is also part of the asteroid belt. The angle of Pallas' axis of rotation is very high - 34 degrees (for other celestial bodies it does not exceed 10). Pallas moves in an orbit with a large degree of deviation, which is why its distance from the Sun changes all the time. It is a silicon-rich carbon asteroid and is of further interest from a mining standpoint.

Vesta

This is the heaviest asteroid at the moment, although it is inferior in size to the previous ones. Due to the composition of the rock, Vesta reflects 4 times more light than the same Ceres, although its diameter is half that. It turns out that this is the only asteroid whose movement can be observed with the naked eye from the surface of the Earth, when it approaches every 3-4 years at a minimum distance of 177 million kilometers. Its movement is carried out along the inside of the asteroid belt and never crosses our orbit.

It is interesting that with a length of 576 kilometers on its surface there is a crater with a diameter of 460 kilometers. In general, the entire asteroid belt around Jupiter is a giant quarry, where celestial bodies collide with each other, scatter into pieces and change their orbits - but how Vesta survived a collision with such a large object and retained its integrity remains a mystery. Its core is made of heavy metal, and the crust is made of light rocks.

Hygiea

This asteroid does not intersect with our orbit and revolves around the Sun. A very dim celestial body, although it has a diameter of 407 kilometers, was discovered later than the others. This is the most common type of asteroid, with a carbonaceous content. Normally, a telescope is required to observe Hygia, but at its closest approach on Earth, it can be viewed with binoculars.

Today, an asteroid, falling to Earth, will bring with it victims, destruction and cataclysms. But, despite the fact that astronomers call this type of celestial bodies "space debris", it is to them that we owe the appearance of life on our planet. In 2010, independently of each other, two groups of researchers discovered on the asteroid Themis (included in the 20 largest) water ice, complex hydrocarbons and molecules, the isotopic composition of which coincides with that of the earth.

Asteroids, or minor planets, are much inferior in size to such bodies of the solar system as the Earth, Venus and even Mercury. However, they cannot be considered full-fledged "inhabitants" of our piece of the Galaxy.

main belt

The asteroids of the solar system are concentrated in several zones. Their most impressive part is located between the orbits of Mars and Jupiter. This cluster of small bodies was called the Main Mass of all the objects located here by space standards is negligible: it is only 4% of the lunar mass. Moreover, the largest asteroids make a decisive contribution to this parameter. Both their movement and the movement of their smaller counterparts, as well as parameters such as composition, shape and origin, attracted the attention of astronomers at the beginning of the 19th century: Ceres, previously considered the largest asteroid, and now classified as a dwarf planet, was discovered the first of January 1801.

Beyond Neptune

The Kuiper belt, the Orta cloud and the scattered disk began to be considered and studied as places of accumulation of a large number of small ones somewhat later. The first of these is located beyond the orbit of Neptune. It was opened only in 1992. According to researchers, the Kuiper belt is much longer and more massive than a similar formation between Mars and Jupiter. Small bodies located here differ from the objects of the Main Belt in composition: methane, ammonia and water prevail here over solid rocks and metals, characteristic of the "inhabitants" of the Asteroid Belt.

The existence of the Horta cloud has not been proven today, but it is consistent with many theories describing the solar system. Presumably, the Horta cloud, which is a spherical region, is located beyond the orbits of the planets, at a distance approximately from the Sun. Space objects consisting of ammonia, methane and water ice are located here.

The scattered disk region overlaps somewhat with the Kuiper belt. Scientists do not yet know its origin. Objects consisting of different types of ice are also placed here.

Comparison of a comet with an asteroid

For an accurate understanding of the essence of the issue, it is necessary to separate two astronomical concepts: “comet” and “asteroid”. Until 2006, there was no certainty about the differences between these objects. At the IAU General Assembly in that year, specific signs were assigned to the comet and asteroid, allowing each to be more or less confidently assigned to a certain category.

A comet is an object moving in a very elongated orbit. When approaching the Sun, as a result of sublimation of ice located near the surface, the comet forms a coma - a cloud of dust and gas, which grows as the distance between the object and the luminary decreases and is often accompanied by the formation of a "tail".

Asteroids do not form coma and, as a rule, have less elongated orbits. Those of them that move along trajectories similar to comets are considered to be the nuclei of the so-called extinct comets (objects that have lost all volatile substances and therefore do not form a coma are called extinct or degenerate comets).

The largest asteroids and their movement

There are very few objects really large by space standards in the Main Asteroid Belt. Most of the mass of all bodies located between Jupiter and Mars falls on four objects - these are Ceres, Vesta, Pallas and Hygiea. The first until 2006 was considered the largest asteroid, then it was given the status of Ceres - an almost round body with a diameter of about 1000 km. Its mass is approximately 32% of the total mass of all known belt objects.

The most massive object after Ceres is Vesta. In size, of the asteroids, only Pallas is ahead of it (after the recognition of Ceres as a dwarf planet). Pallas is also distinguished from the rest by an unusually strong axis tilt.

Hygiea is the fourth largest object in the Main Belt in terms of size and mass. Despite its size, it was discovered much later than several smaller asteroids. This is due to the fact that Hygiea is a very dim object.

All these bodies revolve around the Sun in the same direction as the planets and do not cross the Earth.

Orbit features

The largest asteroids and their movement obey the same laws as the movements of other similar bodies of the belt. Their orbits are constantly affected by the planets, especially the giant Jupiter.

All asteroids rotate in slightly eccentric orbits. The movement of asteroids affected by Jupiter follows several shifting orbits. These displacements can be described as fluctuations around some average position. For each such oscillation, the asteroid spends up to several hundred years; therefore, observational data are currently not enough to refine and verify theoretical constructions. However, in general, the hypothesis of changing orbits is generally accepted.

The result of shifting orbits is an increased possibility of collisions. In 2011, data was obtained suggesting that Ceres and Vesta may collide in the future.

The largest asteroids and their movement are constantly under the scrutiny of scientists. Features of the change in their orbits and other characteristics shed light on some cosmic regularities, which in the process of data analysis are often extrapolated to objects larger than asteroids. The movement of asteroids is also studied with the help of spacecraft, which temporarily become satellites of certain objects. One of them entered the orbit of Ceres on March 6, 2015.

At the very beginning of the XIX century. Italian astronomer Piazzi (1746-1826) accidentally discovered the first minor planet (asteroid). She was named Ceres. Subsequently, many other minor planets were discovered, forming an asteroid belt between the orbits of Mars and Jupiter.

asteroid movement

In photographs of the starry sky taken at long exposures, they appear as light dashes. More than 5500 minor planets have been registered. The total number of asteroids must be ten times greater. Asteroids whose orbits are established receive designations (sequential numbers) and names. Some new asteroids are named after great people (1379 Lomonosov), states (1541 Estonia, 1554 Yugoslavia), observatories (1373 Cincinnati - an American observatory, which is the International Asteroid Observation Center), etc.

Asteroids move around the Sun in the same direction as the major planets. Their revolutions have larger eccentricities (0.15 on average) than the orbits of the major planets. Therefore, some small planets go far beyond the asteroid belt. Some of them at aphelion move beyond the orbit of Saturn, others at perihelion approach Mars and Earth. For example, Hermes in October 1937 passed from the Earth at a distance of 580,000 km (only one and a half times further than the Moon), and the asteroid Icarus, discovered in 1949, when moving, even gets inside the orbit of Mercury and every 19 years approaches the Earth . The last time this happened was in June 1987. Then Icarus approached the Earth at a distance of several million kilometers, it was observed at many observatories. Of course, this is not the only case. It is possible, for example, that the collision of an asteroid with the Earth led to the death of dinosaurs 65 million years ago. And in March 1989, an asteroid about 300 m in size passed from the Earth at a distance of less than 650 thousand km. Therefore, it is no coincidence that scientists have begun to develop effective methods for the timely detection, and, if necessary, the destruction of dangerous asteroids.

Physical characteristics of asteroids

Asteroids are not visible to the naked eye. The largest asteroid is Ceres (diameter 1000 km). In general, asteroids have diameters from several kilometers to several tens of kilometers, and most asteroids are shapeless blocks. The masses of asteroids, although different, are too small for these celestial bodies to hold an atmosphere. The total mass of all the asteroids put together is about 20 times less than the mass of the moon. Of all the asteroids, one planet with a diameter of less than 1500 km would have turned out.

In recent years, satellites (!) have been discovered near some asteroids. The asteroid was photographed for the first time from a distance of only 16,000 km on October 29, 1991, from the board of the American Galileo spacecraft launched on October 18, 1982 to study Jupiter. Crossing the asteroid belt, Galileo photographed the minor planet 951, the asteroid Gaspra. This is a typical asteroid. The semi-major axis of its orbit is 2.21 AU. It turned out to be irregular in shape and may have been formed as a result of the collision of larger bodies in the asteroid belt. The photographs show craters (their diameter is 1-2 km, the consecrated part of the asteroid is 16x12 km). In the images, it is possible to distinguish the details of the surface of the Gaspra asteroid 60-100 m in size.

The shape and surface of the asteroid Ida.
North is up.
Animated by Typhoon Oner.
(Copyrighted © 1997 by A. Tayfun Oner).

1. General representations

Asteroids are solid rocky bodies that, like planets, move in elliptical orbits around the sun. But the sizes of these bodies are much smaller than those of ordinary planets, which is why they are also called minor planets. The diameters of asteroids range from several tens of meters (relatively) to 1000 km (the size of the largest asteroid Ceres). The term "asteroid" (or "stellar") was introduced by the famous 18th century astronomer William Herschel to characterize the appearance of these objects when observed through a telescope. Even with the largest ground-based telescopes, it is impossible to distinguish the visible disks of the largest asteroids. They are observed as point sources of light, although, like other planets, they themselves do not emit anything in the visible range, but only reflect the incident sunlight. The diameters of some asteroids have been measured using the "star occultation" method, at those fortunate moments when they were on the same line of sight with sufficiently bright stars. In most cases, their sizes are estimated using special astrophysical measurements and calculations. Most of the currently known asteroids move between the orbits of Mars and Jupiter at distances from the Sun of 2.2-3.2 astronomical units (hereinafter referred to as AU). In total, about 20,000 asteroids have been discovered to date, of which about 10,000 have been registered, that is, they have been assigned numbers or even proper names, and the orbits have been calculated with great accuracy. Proper names for asteroids are usually assigned by their discoverers, but in accordance with established international rules. In the beginning, when the minor planets were known a little more, their names were taken, as for other planets, from ancient Greek mythology. The annular region of space occupied by these bodies is called the main asteroid belt. With an average linear orbital velocity of about 20 km / s, the asteroids of the main belt spend from 3 to 9 Earth years per revolution around the Sun, depending on the distance from it. The inclinations of the planes of their orbits with respect to the plane of the ecliptic sometimes reach 70°, but are mostly in the range of 5-10°. On this basis, all known asteroids of the main belt are divided approximately equally into flat (with orbital inclinations up to 8°) and spherical subsystems.

During telescopic observations of asteroids, it was found that the brightness of the absolute majority of them changes in a short time (from several hours to several days). Astronomers have long assumed that these changes in the brightness of asteroids are associated with their rotation and are determined primarily by their irregular shape. The very first photographs of asteroids obtained with the help of spacecraft confirmed this and also showed that the surfaces of these bodies are pitted with craters or funnels of various sizes. Figures 1-3 show the first satellite images of asteroids taken by various spacecraft. Obviously, such forms and surfaces of small planets were formed during their numerous collisions with other solid celestial bodies. In the general case, when the shape of an asteroid observed from the Earth is unknown (since it is visible as a point object), then they try to approximate it using a triaxial ellipsoid.

Table 1 provides basic information about the largest or simply interesting asteroids.

Explanations for the table.

1 Ceres is the largest asteroid ever discovered. It was discovered by the Italian astronomer Giuseppe Piazzi on January 1, 1801 and named after the Roman goddess of fertility.

2 Pallas is the second largest asteroid, also the second to be discovered. This was done by the German astronomer Heinrich Olbers on March 28, 1802.

3 Juno - discovered by C. Harding in 1804

4 Vesta is the third largest asteroid, also discovered by G. Olbers in 1807. This body has observational signs of the presence of a basaltic crust covering the olivine mantle, which may be the result of melting and differentiation of its substance. The image of the visible disk of this asteroid was first obtained in 1995 using the American Space Telescope. Hubble in Earth orbit.

8 Flora is the largest asteroid of a large family of asteroids called by the same name, numbering several hundred members, which was first characterized by the Japanese astronomer K. Hirayama. Asteroids of this family have very close orbits, which probably confirms their joint origin from a common parent body, destroyed in a collision with some other body.

243 Ida is a main belt asteroid imaged by the Galileo spacecraft on August 28, 1993. These images made it possible to detect a small satellite of Ida, later named Dactyl. (See figures 2 and 3).

253 Matilda is an asteroid imaged by the NIAR spacecraft in June 1997 (See Fig. 4).

433 Eros is a near-Earth asteroid imaged by the NIAR spacecraft in February 1999.

951 Gaspra is a main belt asteroid that was first imaged by the Galileo spacecraft on October 29, 1991 (See Fig. 1).

1566 Icarus - an asteroid approaching the Earth and crossing its orbit, having a very large orbital eccentricity (0.8268).

1620 Geographer is a near-Earth asteroid that is either a double object or has a very irregular shape. This follows from the dependence of its brightness on the phase of rotation around its own axis, as well as from its radar images.

1862 Apollo - the largest asteroid of the same family of bodies approaching the Earth and crossing its orbit. The eccentricity of Apollo's orbit is quite large - 0.56.

2060 Chiron is an asteroid-comet that periodically exhibits cometary activity (regular increases in brightness near the perihelion of the orbit, that is, at a minimum distance from the Sun, which can be explained by the evaporation of volatile compounds that make up the asteroid), moving along an eccentric trajectory (eccentricity 0.3801) between orbits of Saturn and Uranus.

4179 Toutatis is a binary asteroid whose components appear to be in contact and measure approximately 2.5 km and 1.5 km. Images of this asteroid were obtained using radars located in Arecibo and Goldstone. Of all the currently known near-Earth asteroids in the 21st century, Toutatis should be at the closest distance (about 1.5 million km, September 29, 2004).

4769 Castalia is a double asteroid with approximately identical (0.75 km in diameter) components in contact. Its radio image was obtained using radar in Arecibo.

Image of asteroid 951 Gaspra

Rice. 1. Image of asteroid 951 Gaspra, obtained with the help of the Galileo spacecraft, in pseudo-colors, that is, as a combination of images through purple, green and red filters. The resulting colors are specially boosted to highlight subtle differences in surface detail. Areas of rock outcrops have a bluish tint, while areas covered with regolith (crushed material) have a reddish tint. The spatial resolution at each point of the image is 163 m. Gaspra has an irregular shape and approximate dimensions along 3 axes of 19 x 12 x 11 km. The sun illuminates the asteroid from the right.
Image of NASA GAL-09.

Image of asteroid 243 Ides

Rice. 2 Pseudocolor image of asteroid 243 Ida and its small moon Dactyl, taken by the Galileo spacecraft. The original images used to obtain the image shown in the figure were obtained from a distance of approximately 10,500 km. Color differences may indicate variations in the composition of the surface matter. The bright blue areas are probably covered with a substance consisting of iron-bearing minerals. The length of Ida is 58 km, and its axis of rotation is oriented vertically with a slight inclination to the right.
NASA GAL-11 image.

Rice. 3. Image of Dactyl, a small satellite of 243 Ida. It is not yet known whether it is a piece of Ida, broken off from it during some kind of collision, or an alien object captured by its gravitational field and moving in a circular orbit. This image was taken on August 28, 1993 through a neutral density filter from a distance of about 4000 km, 4 minutes before the closest approach to the asteroid. Dactyl measures approximately 1.2 x 1.4 x 1.6 km. Image of NASA GAL-04

Asteroid 253 Matilda

Rice. 4. Asteroid 253 Matilda. NASA image, NEAR spacecraft

2. How could the main asteroid belt have arisen?

The orbits of the bodies concentrated in the main belt are stable and have a shape close to circular or slightly eccentric. Here they move in a "safe" zone, where the gravitational influence of the big planets on them, and first of all, Jupiter, is minimal. The scientific facts available today show that it was Jupiter that played the main role in the fact that another planet could not arise on the site of the main asteroid belt during the birth of the solar system. But even at the beginning of our century, many scientists were still convinced that there used to be another large planet between Jupiter and Mars, which for some reason collapsed. Olbers was the first to express such a hypothesis, immediately after his discovery of Pallas. He also came up with the name of this hypothetical planet - Phaeton. Let's make a small digression and describe one episode from the history of the solar system - the history that is based on modern scientific facts. This is necessary, in particular, to understand the origin of the main belt asteroids. A great contribution to the formation of the modern theory of the origin of the solar system was made by Soviet scientists O.Yu. Schmidt and V.S. Safronov.

One of the largest bodies, formed in the orbit of Jupiter (at a distance of 5 AU from the Sun) about 4.5 billion years ago, began to increase in size faster than others. Being at the boundary of condensation of volatile compounds (H 2 , H 2 O, NH 3 , CO 2 , CH 4 , etc.), which flowed from the protoplanetary disk closer to the Sun and more heated, this body became the center of accumulation of matter, consisting of mainly from frozen gas condensates. Upon reaching a sufficiently large mass, it began to capture with its gravitational field the previously condensed matter located closer to the Sun, in the zone of the parent bodies of asteroids, and thus inhibit the growth of the latter. On the other hand, smaller bodies, not captured by the proto-Jupiter for any reason, but located in the sphere of its gravitational influence, were effectively scattered in different directions. Similarly, the ejection of bodies from the formation zone of Saturn probably took place, although not so intensively. These bodies also penetrated the belt of parent bodies of asteroids or planetesimals that had arisen earlier between the orbits of Mars and Jupiter, "sweeping" them out of this zone or subjecting them to crushing. And before that, the gradual growth of the parent bodies of asteroids was possible due to their low relative velocities (up to about 0.5 km/s), when the collisions of any objects ended in their unification, and not crushing. The increase in the flow of bodies thrown into the asteroid belt by Jupiter (and Saturn) during its growth led to the fact that the relative velocities of the parent bodies of the asteroids increased significantly (up to 3-5 km/s) and became more chaotic. Ultimately, the process of accumulation of parent bodies of asteroids was replaced by the process of their fragmentation during mutual collisions, and the potential for the formation of a sufficiently large planet at a given distance from the Sun disappeared forever.

3. Orbits of asteroids

Returning to the current state of the asteroid belt, it should be emphasized that Jupiter still continues to play a primary role in the evolution of asteroid orbits. The long-term gravitational influence (more than 4 billion years) of this giant planet on the asteroids of the main belt has led to the fact that there are a number of "forbidden" orbits or even zones in which there are practically no small planets, and if they get there, they cannot stay there for a long time. They are called gaps or Kirkwood hatches - after Daniel Kirkwood, the scientist who first discovered them. Such orbits are resonant, since the asteroids moving along them experience a strong gravitational effect from Jupiter. The periods of revolution corresponding to these orbits are in simple relations with the period of revolution of Jupiter (for example, 1:2; 3:7; 2:5; 1:3, etc.). If any asteroid or its fragment, as a result of a collision with another body, falls into a resonant or close to it orbit, then the semi-major axis and eccentricity of its orbit change quite quickly under the influence of the Jupiterian gravitational field. It all ends with the asteroid either leaving its resonant orbit and may even leave the main asteroid belt, or being doomed to new collisions with neighboring bodies. In this way, the corresponding Kirkwood space is "cleared" of any objects. However, it should be emphasized that there are no gaps or empty gaps in the main asteroid belt, if we imagine the instantaneous distribution of all the bodies included in it. All asteroids, at any moment of time, fill the asteroid belt fairly evenly, since, moving along elliptical orbits, they spend most of their time in the "foreign" zone. Another, "opposite" example of the gravitational influence of Jupiter: at the outer boundary of the main asteroid belt there are two narrow additional "rings", on the contrary, made up of asteroid orbits, the periods of revolution of which are in proportions of 2:3 and 1:1 with respect to the period of revolution Jupiter. Obviously, asteroids with a period of revolution corresponding to a ratio of 1:1 are directly in the orbit of Jupiter. But they move at a distance from it equal to the radius of Jupiter's orbit, either ahead or behind. Those asteroids that are ahead of Jupiter in their movement are called "Greeks", and those that follow him are called "Trojans" (as they are named after the heroes of the Trojan War). The movement of these small planets is quite stable, since they are located at the so-called "Lagrange points", where the gravitational forces acting on them are equalized. The common name for this group of asteroids is "Trojans". Unlike Trojans, which could gradually accumulate in the vicinity of Lagrange points during the long collisional evolution of different asteroids, there are families of asteroids with very close orbits of their constituent bodies, which were most likely formed as a result of relatively recent decays of their parent bodies. This, for example, is the family of the asteroid Flora, which already has about 60 members, and a number of others. Recently, scientists have been trying to determine the total number of such families of asteroids in order to estimate the initial number of their parent bodies.

4 Near Earth Asteroids

Near the inner edge of the main asteroid belt, there are other groups of bodies whose orbits go far beyond the main belt and may even intersect with the orbits of Mars, Earth, Venus, and even Mercury. First of all, these are the groups of Amur, Apollo and Aten asteroids (according to the names of the largest representatives included in these groups). The orbits of such asteroids are no longer as stable as those of the bodies of the main belt, but rather rapidly evolve under the influence of the gravitational fields not only of Jupiter, but also of the terrestrial planets. For this reason, such asteroids can move from one group to another, and the division of asteroids into the above groups is conditional, based on data on modern asteroid orbits. In particular, Amurians move in elliptical orbits, the perihelion distance (the minimum distance to the Sun) of which does not exceed 1.3 AU. The Apollos move in orbits with a perihelion distance of less than 1 AU. (recall that this is the average distance of the Earth from the Sun) and penetrate into the Earth's orbit. If for the Amurians and Apollonians the major semiaxis of the orbit exceeds 1 AU, then for the Atonians it is less than or of the order of this value, and these asteroids, therefore, move mainly inside the earth's orbit. It is obvious that the Apollos and Atons, crossing the Earth's orbit, can create a threat of collision with it. There is even a general definition of this group of small planets as "near-Earth asteroids" - these are bodies whose orbital sizes do not exceed 1.3 AU. To date, about 800 such objects have been discovered. But their total number can be much larger - up to 1500-2000 with dimensions of more than 1 km and up to 135,000 with dimensions of more than 100 m. The existing threat to the Earth from asteroids and other space bodies that are located or may end up in the Earth's environs, is widely discussed in scientific and public circles. For more on this, as well as the measures proposed to protect our planet, see a recently published book edited by A.A. Boyarchuk.

5. About other asteroid belts

There are also asteroid-like bodies beyond the orbit of Jupiter. Moreover, according to the latest data, it turned out that there are a lot of such bodies on the periphery of the solar system. This was first suggested by the American astronomer Gerard Kuiper back in 1951. He formulated the hypothesis that beyond the orbit of Neptune, at distances of about 30-50 AU. there may be a whole belt of bodies that serves as a source of short-period comets. Indeed, since the beginning of the 90s (with the introduction of the largest telescopes with a diameter of up to 10 m in the Hawaiian Islands), more than a hundred asteroid-like objects with diameters from about 100 to 800 km have been discovered beyond the orbit of Neptune. The totality of these bodies has been called the "Kuiper belt", although they are still not enough for a "full-fledged" belt. Nevertheless, according to some estimates, the number of bodies in it may be no less (if not more) than in the main asteroid belt. According to the parameters of the orbits, the newly discovered bodies were divided into two classes. About a third of all trans-Neptunian objects were assigned to the first, so-called "Plutino class". They move in a 3:2 resonance with Neptune along fairly elliptical orbits (major axes about 39 AU; eccentricities 0.11-0.35; orbital inclinations to the ecliptic 0-20 degrees), similar to the orbit of Pluto, from where the the name of this class. Currently, there are even discussions between scientists about whether to consider Pluto a full-fledged planet or only one of the objects of the above-named class. However, most likely, the status of Pluto will not change, since its average diameter (2390 km) is much larger than the diameters of known trans-Neptunian objects, and in addition, like most other planets in the solar system, it has a large satellite (Charon) and an atmosphere . The second class includes the so-called "typical Kuiper belt objects", since most of them (the remaining 2/3) are known and they move in orbits close to circular with semi-major axes in the range of 40-48 AU. and various slopes (0-40°). So far, the great remoteness and relatively small size have prevented the discovery of new similar bodies at a higher rate, although the largest telescopes and the most modern technology are used for this. Based on a comparison of these bodies with known asteroids in terms of optical characteristics, it is now believed that the former are the most primitive in our planetary system. This means that since the moment of its condensation from the protoplanetary nebula, their substance has undergone very small changes in comparison, for example, with the substance of the terrestrial planets. In fact, the absolute majority of these bodies in their composition can be comet nuclei, which will also be discussed in the "Comets" section.

A number of asteroid bodies have been discovered (over time, this number will probably increase) between the Kuiper belt and the main asteroid belt - this is the "class of Centaurs" - by analogy with the ancient Greek mythological centaurs (half-human, half-horse). One of their representatives is the asteroid Chiron, which would be more correctly called a comet asteroid, since it periodically exhibits cometary activity in the form of an emerging gaseous atmosphere (coma) and tail. They are formed from volatile compounds that make up the substance of this body, when it passes through the perihelion sections of the orbit. Chiron is one of the clearest examples of the absence of a sharp boundary between asteroids and comets in terms of composition of matter and, possibly, in terms of origin. It has a size of about 200 km, and its orbit overlaps with the orbits of Saturn and Uranus. Another name for objects of this class is the Kazimirchak-Polonskaya belt, after E.I. Polonskaya, who proved the existence of asteroid bodies between the giant planets.

6. A little about the methods of researching asteroids

Our understanding of the nature of asteroids is now based on three main sources of information: ground-based telescopic observations (optical and radar), images obtained from spacecraft approaching asteroids, and laboratory analysis of known terrestrial rocks and minerals, as well as meteorites that have fallen to Earth, which ( which will be discussed in the "Meteorites" section) are mainly considered fragments of asteroids, cometary nuclei and surfaces of terrestrial planets. But we still obtain the greatest amount of information about minor planets with the help of ground-based telescopic measurements. Therefore, asteroids are divided into so-called "spectral types" or classes, in accordance, first of all, with their observed optical characteristics. First of all, this is the albedo (the proportion of light reflected by the body from the amount of sunlight falling on it per unit time, if we consider the directions of the incident and reflected rays to be the same) and the general shape of the reflection spectrum of the body in the visible and near infrared ranges (which is obtained by simply dividing on each wavelength of the spectral brightness of the surface of the observed body by the spectral brightness at the same wavelength of the Sun itself). These optical characteristics are used to assess the chemical and mineralogical composition of the matter that makes up asteroids. Sometimes additional data (if any) is taken into account, for example, on the radar reflectivity of the asteroid, on the speed of its rotation around its own axis, etc.

The desire to divide asteroids into classes is explained by the desire of scientists to simplify or schematize the description of a huge number of small planets, although, as more thorough studies show, this is not always possible. Recently, it has already become necessary to introduce subclasses and smaller divisions of the spectral types of asteroids in order to characterize some common features of their individual groups. Before giving a general description of asteroids of different spectral types, let us explain how the composition of asteroid matter can be estimated using remote measurements. As already noted, it is believed that asteroids of one type have approximately the same albedo values ​​and reflection spectra similar in shape, which can be replaced by average (for a given type) values ​​or characteristics. These average values ​​for a certain type of asteroid are compared with similar values ​​for terrestrial rocks and minerals, as well as those meteorites, samples of which are available in terrestrial collections. The chemical and mineral composition of the samples, which are called "analogue samples", together with their spectral and other physical properties, as a rule, are already well studied in terrestrial laboratories. On the basis of such a comparison and selection of analogue samples, some average chemical and mineral composition of matter for asteroids of this type is determined in the first approximation. It turned out that, unlike terrestrial rocks, the substance of asteroids as a whole is much simpler or even primitive. This suggests that the physical and chemical processes in which asteroid matter was involved throughout the entire history of the existence of the solar system were not as diverse and complex as on the terrestrial planets. If about 4000 mineral species are now considered reliably established on Earth, then on asteroids there may be only a few hundred of them. This can be judged by the number of mineral species (about 300) found in meteorites that fell to the earth's surface, which may be fragments of asteroids. A wide variety of minerals on Earth arose not only because the formation of our planet (as well as other terrestrial planets) took place in a protoplanetary cloud much closer to the Sun, and therefore at higher temperatures. In addition to the fact that the silicate substance, metals and their compounds, being in a liquid or plastic state at such temperatures, were separated or differentiated by specific gravity in the Earth's gravitational field, the prevailing temperature conditions turned out to be favorable for the emergence of a constant gaseous or liquid oxidizing medium, the main components of which were oxygen and water. Their long and constant interaction with primary minerals and rocks of the earth's crust has led to the richness of minerals that we observe. Returning to asteroids, it should be noted that, according to remote data, they mainly consist of simpler silicate compounds. First of all, these are anhydrous silicates, such as pyroxenes (their generalized formula is ABZ 2 O 6, where positions "A" and "B" are occupied by cations of different metals, and "Z" - by Al or Si), olivines (A 2+ 2 SiO 4, where A 2+ \u003d Fe, Mg, Mn, Ni) and sometimes plagioclase (with the general formula (Na,Ca)Al(Al,Si)Si 2 O 8). They are called rock-forming minerals because they form the basis of most rocks. Silicate compounds of another type, widely present on asteroids, are hydrosilicates or layered silicates. These include serpentines (with the general formula A 3 Si 2 O 5? (OH), where A \u003d Mg, Fe 2+, Ni), chlorites (A 4-6 Z 4 O 10 (OH, O) 8, where A and Z are mainly cations of different metals) and a number of other minerals that contain hydroxyl (OH) in their composition. It can be assumed that not only simple oxides, compounds (for example, sulphurous) and alloys of iron and other metals (in particular FeNi), carbon (organic) compounds, but even metals and carbon in a free state are found on asteroids. This is evidenced by the results of a study of meteorite matter that constantly falls to the Earth (see the section "Meteorites").

7. Spectral types of asteroids

To date, the following main spectral classes or types of minor planets have been identified, denoted by Latin letters: A, B, C, F, G, D, P, E, M, Q, R, S, V, and T. Let us give a brief description of them.

Type A asteroids have a fairly high albedo and the reddest color, which is determined by a significant increase in their reflectivity towards long wavelengths. They may consist of high-temperature olivines (having a melting point in the range of 1100-1900 ° C) or a mixture of olivine with metals that correspond to the spectral characteristics of these asteroids. On the contrary, small planets of types B, C, F, and G have a low albedo (B-type bodies are somewhat lighter) and almost flat (or colorless) in the visible range, but the reflection spectrum sharply decreasing at short wavelengths. Therefore, it is believed that these asteroids are mainly composed of low-temperature hydrated silicates (which can decompose or melt at temperatures of 500-1500 ° C) with an admixture of carbon or organic compounds that have similar spectral characteristics. Asteroids with low albedo and reddish color were assigned to D- and P-types (D-bodies are redder). Such properties have silicates rich in carbon or organic substances. They consist, for example, of particles of interplanetary dust, which probably filled the near-solar protoplanetary disk even before the formation of planets. Based on this similarity, it can be assumed that D- and P-asteroids are the most ancient, little-altered bodies of the asteroid belt. Small E-type planets have the highest albedo values ​​(their surface matter can reflect up to 50% of the light falling on them) and a slightly reddish color. The mineral enstatite (this is a high-temperature variety of pyroxene) or other silicates containing iron in the free (non-oxidized) state, which, therefore, can be part of E-type asteroids, has the same spectral characteristics. Asteroids that are similar in their reflection spectra to P- and E-type bodies, but located between them in terms of albedo, are classified as M-type. It turned out that the optical properties of these objects are very similar to the properties of metals in the free state or metal compounds mixed with enstatite or other pyroxenes. There are now about 30 such asteroids. With the help of ground-based observations, such an interesting fact has recently been established as the presence of hydrated silicates on a significant part of these bodies. Although the cause of this unusual combination of high-temperature and low-temperature materials has not yet been finally established, it can be assumed that hydrosilicates could be introduced to M-type asteroids during their collisions with more primitive bodies. Of the remaining spectral classes, Q-, R-, S-, and V-type asteroids are quite similar in terms of albedo and the general shape of the reflection spectra in the visible range: they have a relatively high albedo (slightly lower for S-type bodies) and a reddish color. The differences between them boil down to the fact that the broad absorption band of about 1 micron present in their reflection spectra in the near infrared range has a different depth. This absorption band is characteristic of a mixture of pyroxenes and olivines, and the position of its center and depth depend on the proportion and total content of these minerals in the surface matter of asteroids. On the other hand, the depth of any absorption band in the reflection spectrum of a silicate substance decreases if it contains any opaque particles (for example, carbon, metals or their compounds) that screen diffusely reflected (that is, transmitted through the substance and carrying information about its composition) light. For these asteroids, the absorption band depth at 1 µm increases from S-to Q-, R-, and V-types. In accordance with the foregoing, the bodies of the listed types (except V) may consist of a mixture of olivines, pyroxenes, and metals. The substance of V-type asteroids may include, along with pyroxenes, feldspars, and be similar in composition to terrestrial basalts. And, finally, the last, T-type, includes asteroids that have a low albedo and a reddish reflection spectrum, which is similar to the spectra of P- and D-type bodies, but occupies an intermediate position between their spectra in slope. Therefore, the mineralogical composition of T-, P-, and D-type asteroids is considered to be approximately the same and corresponding to silicates rich in carbon or organic compounds.

When studying the distribution of asteroids of different types in space, a clear relationship was found between their supposed chemical and mineral composition and the distance to the Sun. It turned out that the simpler the mineral composition of a substance (the more volatile compounds it contains) these bodies have, the farther, as a rule, they are. In general, more than 75% of all asteroids are C-type and are located mainly in the peripheral part of the asteroid belt. Approximately 17% are S-type and dominate the interior of the asteroid belt. Most of the remaining asteroids are M-type and also move mainly in the middle part of the asteroid ring. The distribution maxima of these three types of asteroids are within the main belt. The maximum of the total distribution of E- and R-type asteroids somewhat extends beyond the inner boundary of the belt towards the Sun. It is interesting that the total distribution of P- and D-type asteroids tends to its maximum towards the periphery of the main belt and goes not only beyond the asteroid ring, but also beyond the orbit of Jupiter. It is possible that the distribution of P- and D-asteroids of the main belt overlaps with the Kazimirchak-Polonskaya asteroid belts located between the orbits of the giant planets.

In conclusion of the review of minor planets, we briefly outline the meaning of the general hypothesis about the origin of asteroids of various classes, which is increasingly being confirmed.

8. On the origin of minor planets

At the dawn of the formation of the Solar System, about 4.5 billion years ago, clumps of matter arose from the gas-dust disk surrounding the Sun due to turbulent and other non-stationary phenomena, which, during mutual inelastic collisions and gravitational interactions, united into planetesimals. With increasing distance from the Sun, the average temperature of the gas-dust substance decreased and, accordingly, its general chemical composition changed. The annular zone of the protoplanetary disk, from which the main asteroid belt subsequently formed, turned out to be near the condensation boundary of volatile compounds, in particular, water vapor. Firstly, this circumstance led to the accelerated growth of the Jupiter embryo, which was located near the indicated boundary and became the center of accumulation of hydrogen, nitrogen, carbon and their compounds, leaving the more heated central part of the solar system. Secondly, the gas-dust substance from which the asteroids were formed turned out to be very heterogeneous in composition depending on the distance from the Sun: the relative content of the simplest silicate compounds in it sharply decreased, while the content of volatile compounds increased with distance from the Sun in the region from 2, 0 to 3.5 a.u. As already mentioned, powerful perturbations from the rapidly growing embryo of Jupiter to the asteroid belt prevented the formation of a sufficiently large proto-planetary body in it. The process of accumulation of matter there was stopped when only a few dozen planetosimals of pre-planetary size (about 500-1000 km) had time to form, which then began to break up during collisions due to a rapid increase in their relative velocities (from 0.1 to 5 km / s). However, during this period, some parent bodies of asteroids, or at least those that contained a high proportion of silicate compounds and were closer to the Sun, were already heated or even experienced gravitational differentiation. Two possible mechanisms are now being considered for heating the interiors of such proto-asteroids: as a result of the decay of radioactive isotopes, or as a result of the action of induction currents induced in the substance of these bodies by powerful streams of charged particles from the young and active Sun. The parent bodies of asteroids that have survived for some reason to this day, according to scientists, are the largest asteroids 1 Ceres and 4 Vesta, the main information about which is given in Table. 1. In the process of gravitational differentiation of proto-asteroids, which experienced sufficient heating to melt their silicate substance, metal cores and other lighter silicate shells were separated, and in some cases even a basaltic crust (for example, at 4 Vesta), as in the terrestrial planets . But still, since the material in the asteroid zone contained a significant amount of volatile compounds, its average melting point was relatively low. As shown by mathematical modeling and numerical calculations, the melting temperature of such a silicate substance could be in the range of 500-1000 ° C. So, after differentiation and cooling, the parent bodies of asteroids experienced numerous collisions not only between themselves and their fragments, but also with bodies , invading the asteroid belt from the zones of Jupiter, Saturn and the more distant periphery of the solar system. As a result of a long impact evolution, proto-asteroids were fragmented into a huge number of smaller bodies that are now observed as asteroids. At relative velocities of about several kilometers per second, collisions of bodies consisting of several silicate shells with different mechanical strengths (the more metals are contained in a solid, the more durable it is), led to "stripping" from them and crushing to small fragments in the first place. the least durable outer silicate shells. Moreover, it is believed that asteroids of those spectral types that correspond to high-temperature silicates originate from different silicate shells of their parent bodies that have undergone melting and differentiation. In particular, M- and S-type asteroids can be entirely the cores of parent bodies (for example, S-asteroid 15 Eunomia and M-asteroid 16 Psyche with diameters of about 270 km) or their fragments due to the highest content of metals in them. . A- and R-type asteroids can be fragments of intermediate silicate shells, while E- and V-type asteroids can be fragments of outer shells of such parent bodies. Based on the analysis of the spatial distributions of E-, V-, R-, A-, M-, and S-type asteroids, one can also conclude that they have undergone the most intense thermal and impact reworking. This can probably be confirmed by the coincidence with the inner boundary of the main belt or the proximity to it of the distribution maxima of these types of asteroids. As for asteroids of other spectral types, they are considered either partially changed (metamorphic) due to collisions or local heating, which did not lead to their general melting (T, B, G and F), or primitive and little changed (D, P, C and Q). As already noted, the number of asteroids of these types increases towards the periphery of the main belt. There is no doubt that they all also experienced collisions and crushing, but this process was probably not so intense as to noticeably affect their observed characteristics and, accordingly, the chemical-mineral composition. (This issue will also be discussed in the "Meteorites" section). However, as shown by numerical simulation of collisions of asteroid-sized silicate bodies, many of the currently existing asteroids after mutual collisions could reaccumulate (that is, combine from the remaining fragments) and therefore are not monolithic bodies, but moving “heaps of cobblestones”. There are numerous observational confirmations (from specific brightness changes) of the presence of small satellites in a number of asteroids gravitationally bound to them, which probably also arose during impact events as fragments of colliding bodies. This fact, although it caused heated debate among scientists in the past, was convincingly confirmed by the example of the asteroid 243 Ida. With the help of the Galileo spacecraft, it was possible to obtain images of this asteroid along with its satellite (which was later named Dactyl), which are shown in Figures 2 and 3.

9. About what we don't know yet

Much remains unclear and even mysterious in the studies of asteroids. First, these are general problems related to the origin and evolution of solid matter in the main and other asteroid belts and associated with the emergence of the entire solar system. Their solution is important not only for the correct understanding of our system, but also for understanding the causes and patterns of the emergence of planetary systems in the vicinity of other stars. Thanks to the capabilities of modern observational technology, it was possible to establish that a number of neighboring stars have large planets like Jupiter. Next in line is the discovery of smaller terrestrial planets in these and other stars. There are also questions that can only be answered by a detailed study of individual minor planets. In essence, each of these bodies is unique, as it has its own, sometimes specific, history. For example, asteroids members of some dynamical families (for example, Themis, Flora, Gilda, Eos, and others), which, as mentioned, have a common origin, can differ markedly in optical characteristics, which indicates some of their features. On the other hand, it is obvious that a detailed study of all sufficiently large asteroids only in the main belt will require a lot of time and effort. And yet, probably, only by collecting and accumulating detailed and accurate information about each of the asteroids, and then with the help of its generalization, is it possible to gradually refine the understanding of the nature of these bodies and the basic laws of their evolution.

BIBLIOGRAPHY:

1. Threat from the sky: rock or accident? (Under the editorship of A.A. Boyarchuk). M: "Kosmosinform", 1999, 218 p.

2. Fleischer M. Dictionary of mineral species. M: "Mir", 1990, 204 p.

Asteroids are celestial bodies that were formed due to the mutual attraction of dense gas and dust orbiting our Sun at an early stage of its formation. Some of these objects, like an asteroid, have reached enough mass to form a molten core. At the moment Jupiter reaches its mass, most of the planetosimals (future protoplanets) were split and ejected from the original asteroid belt between Mars and. During this epoch, part of the asteroids was formed due to the collision of massive bodies within the influence of the gravitational field of Jupiter.

Orbit classification

Asteroids are classified according to features such as visible reflections of sunlight and characteristics of their orbits.

According to the characteristics of the orbits, asteroids are combined into groups, among which families can be distinguished. A group of asteroids is considered to be a certain number of such bodies whose orbital characteristics are similar, that is, semiaxis, eccentricity and orbital inclination. A family of asteroids should be considered a group of asteroids that do not just move in close orbits, but are probably fragments of one large body, and were formed as a result of its split.

The largest of the known families may contain several hundred asteroids, while the most compact families may contain up to ten. Approximately 34% of asteroid bodies are members of asteroid families.

As a result of the formation of most groups of asteroids in the solar system, their parent body was destroyed, however, there are also such groups whose parent body survived (for example).

Classification by spectrum

The spectral classification is based on the spectrum of electromagnetic radiation, which is the result of the asteroid reflecting sunlight. Registration and processing of this spectrum makes it possible to study the composition of a celestial body and assign an asteroid to one of the following classes:

• Group of carbon asteroids or C-group. Representatives of this group consist mostly of carbon, as well as elements that were part of the protoplanetary disk of our solar system in the early stages of its formation. Hydrogen and helium, as well as other volatile elements, are practically absent in carbonaceous asteroids, however, the presence of various minerals is possible. Another distinguishing feature of such bodies is their low albedo - reflectivity, which requires the use of more powerful observation tools than in the study of asteroids of other groups. More than 75% of the asteroids in the solar system are representatives of the C-group. The most famous bodies of this group are Hygiea, Pallas, and once - Ceres.
• A group of silicon asteroids or S-group. Asteroids of this type are composed mainly of iron, magnesium and some other rocky minerals. For this reason, silicon asteroids are also called stony asteroids. Such bodies have a fairly high albedo, which allows you to observe some of them (for example, Irida) simply with binoculars. The number of silicon asteroids in the solar system is 17% of the total, and they are most common at a distance of up to 3 astronomical units from the Sun. The largest representatives of the S-group: Juno, Amphitrite and Herculina.