Ceres This is a rather large celestial body (diameter 975 * 909 km) who has not been since the discovery: both a full-fledged planet of the solar system, and an asteroid, and since 2006 it has acquired a new status - dwarf planet. last name 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 in 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 Size 582 * 556 kilometers, and it is also part of the asteroid belt. The angle of the axis of rotation of Pallas 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 once 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 did Vesta survive the collision with such large object and retained its integrity remains a mystery. Its core consists of heavy metal, and the bark is from 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.
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 hallmark Such bodies have a 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.
In astronomy, an asteroid is a small celestial body that rotates in an independent elliptical orbit around the Sun. Chemical composition asteroids is varied. Most of these celestial bodies are carbonaceous objects. However, there are also a considerable number of silicon and metal asteroids in the solar system.
asteroid belt
In the solar system between the orbits of the planets Mars and Jupiter is located great amount asteroids of various sizes and shapes. This cluster of celestial bodies is called the asteroid belt. It is here that the largest asteroids of our system are located: Vesta, Ceres, Hygiea and Pallas. It is worth noting that the history of observation and study of asteroids began with the discovery of Ceres.
The largest asteroids
Vesta
It is the heaviest asteroid and one of the largest (second largest). The celestial body was discovered in 1807 by Heinrich Olbers. Interestingly, Vesta can be observed with the naked eye. The asteroid was named by Carl Gauss in honor of the ancient Roman goddess, the patroness of the family hearth.
Ceres
Ceres, named after the ancient Roman goddess of fertility, was discovered in 1801 by Giuseppe Piazzi. Initially, scientists believed that they had discovered another planet, but later found that Ceres is an asteroid. The diameter of this celestial body is 960 km, which makes the asteroid the largest in the belt.
Hygiea
Credit for the discovery of Hygiea belongs to Annibale de Gasparis. In 1849, he discovered a large celestial body in the asteroid belt, which later received the name of the ancient Greek goddess of health and well-being.
Pallas
This asteroid was discovered a year after the discovery of Ceres, thanks to the observations of the German astronomer Heinrich Olbers. Pallas was named after the sister of the ancient Greek goddess of war, Athena.
Earth collision hazard
Note that in the past, our planet took on the impact of 6 asteroids, with a diameter of at least 10 km. This is evidenced by huge craters on the surface of the Earth in various countries. The oldest crater is 2 billion years old, the youngest is 50 thousand years old. Thus, the potential danger of an asteroid colliding with the Earth always exists.
Scientists fear something similar could happen in 2029, when the giant asteroid Apophis, named after the ancient Egyptian god of destruction, passes close to our planet. However, time will tell whether the asteroid will collide with the Earth or safely pass it.
All asteroids discovered so far have direct motion: they move around the Sun in the same direction as the large planets (i
The boundaries of the ring are somewhat arbitrary: the spatial density of asteroids (the number of asteroids per unit volume) decreases with distance from the central part. If, as the asteroid moves along its orbit, the mentioned plane zr is rotated (around an axis perpendicular to the ecliptic plane and passing through the Sun) following the asteroid (so that it remains in this plane all the time), then the asteroid in one revolution will describe a certain loop in this plane .
Most of these loops lie within the shaded area, like Ceres and Vesta, moving in slightly eccentric and slightly inclined orbits. In few asteroids, due to the significant eccentricity and inclination of the orbit, the loop, like that of Pallas (i=35o), goes beyond this region or even lies entirely outside it, like that of the Atenians. Therefore, asteroids are also found far outside the ring.
The volume of space occupied by the ring-torus, where 98% of all asteroids move, is huge - about 1.6 1026 km3. For comparison, we point out that the volume of the Earth is only 1012 km e. Asteroids move in orbits with a linear (heliocentric) speed of about 20 km / s, spending from 3 to 9 years for one revolution around the Sun.
Their average daily motion is within 400-1200. The eccentricity of these orbits is small - from 0 to 0.2 and rarely exceeds 0.4. But even with a very small eccentricity, only 0.1, the heliocentric distance of the asteroid during its orbit changes by several tenths of an astronomical unit, and at e = 0.4 by 1.5 - 3 AU. That is, depending on the size of the orbit, the inclination of the orbits to the plane of the ecliptic is usually from 5 ° to 10 °.
But with an inclination of 10°, the asteroid can deviate from the plane of the ecliptic by about 0.5 AU. That is, at an inclination of 30 °, move away from it by 1.5 AU According to the average daily movement, asteroids are usually divided into five groups. Groups I, II, and III, numerous in composition, include asteroids moving, respectively, in the outer (most distant from the Sun), central, and inner zones of the ring.
The central zone is dominated by asteroids of the spherical subsystem, while in the inner zone 3/4 of the asteroids are members of the flat system. As we move from the inner zone to the outer one, there are more and more circular orbits: in group III, the eccentricity e
Only bodies in less eccentric orbits, unattainable for this giant of the solar system, have survived. All asteroids of the ring are, so to speak, in a safe zone. But even they are constantly experiencing perturbations from the planets. The strongest influence on them is, of course, Jupiter. Therefore, their orbits are constantly changing. To be completely strict, it must be said that the path of the asteroid in space is not ellipses, but open quasi-elliptical coils that fit next to each other. Only occasionally - when approaching a planet - do the coils noticeably deviate from one another. The planets, of course, disturb the movement not only of asteroids, but also of each other. However, the perturbations experienced by the planets themselves are small and do not change the structure of the solar system.
They cannot cause the planets to collide with each other. With asteroids, the situation is different. Due to the large eccentricities and inclinations of the orbits of asteroids under the influence of planetary perturbations, they change quite strongly even if there are no approaches to the planets. Asteroids deviate from their path in one direction or the other. The farther, the greater these deviations become: after all, the planets continuously “pull” the asteroid, each towards itself, but Jupiter is stronger than all.
Observations of asteroids cover still too short time intervals to reveal significant changes in the orbits of most asteroids, with the exception of some rare cases. Therefore, our ideas about the evolution of their orbits are based on theoretical considerations. Briefly, they boil down to the following The orbit of each asteroid oscillates around its average position, spending several tens or hundreds of years on each oscillation. Its semi-axis, eccentricity and inclination change synchronously with a small amplitude. Perihelion and aphelion either approach the Sun or move away from it. These fluctuations are included as an integral part in fluctuations of a larger period - thousands or tens of thousands of years.
They have a slightly different character. The semi-major axis does not experience additional changes. On the other hand, the amplitudes of oscillations of the eccentricity and inclination can be much larger. With such time scales, one can no longer consider the instantaneous positions of the planets in their orbits: as in an accelerated film, an asteroid and a planet appear to be smeared in their orbits, as it were.
It becomes reasonable to consider them as gravitating rings. The inclination of the asteroid ring to the plane of the ecliptic, where the planetary rings are located - the source of perturbing forces - leads to the fact that the asteroid ring behaves like a top or a gyroscope. Only the picture is more complicated, because the asteroid's orbit is not rigid and its shape changes over time. The asteroid orbit rotates in such a way that the normal to its plane, restored at the focus where the Sun is located, describes a cone. In this case, the line of nodes rotates in the ecliptic plane with a more or less constant speed clockwise. During one revolution, the inclination, eccentricity, perihelion and aphelion distances experience two oscillations.
When the line of knots coincides with the line aspid (and this happens twice in one revolution), the slope is maximum and the eccentricity is minimum. The shape of the orbit becomes closer to circular, the minor semiaxis of the orbit increases, the perihelion is maximally moved away from the Sun, and the aphelion is close to it (since q+q’=2a=const). Then the line of nodes shifts, the inclination decreases, the perihelion moves towards the Sun, the aphelion moves away from it, the eccentricity increases, and the orbit's minor semiaxis shortens. Extreme values are reached when the line of nodes is perpendicular to the line of the slate. Now the perihelion is closest to the Sun, the aphelion is the farthest from it, and both of these points deviate the most from the ecliptic.
Studies of the evolution of orbits over long periods of time show that the described changes are included in changes of an even longer period, occurring with even greater amplitudes of elemental oscillations, and the aspid line is also included in the movement. So, each orbit continuously pulsates, and besides, it also rotates. For small e and i, their oscillations occur with small amplitudes. Almost circular orbits, which, moreover, lie near the plane of the ecliptic, change hardly noticeably.
For them, it all comes down to a slight deformation and a slight deviation of one or the other part of the orbit from the plane of the ecliptic. But the greater the eccentricity and inclination of the orbit, the stronger the perturbations are manifested over long time intervals. Thus, planetary perturbations lead to continuous mixing of asteroid orbits, and therefore, to mixing of objects moving along them. This makes it possible for asteroids to collide with each other. Over the past 4.5 billion years, since the existence of asteroids, they have experienced many collisions with each other. The inclinations and eccentricities of the orbits lead to the non-parallelism of their mutual motions, and the speed with which the asteroids pass one another (the chaotic velocity component) averages about 5 km/s. Collisions with such speeds lead to the destruction of bodies.
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 around the circumsolar elliptical orbits. 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 speed 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 vast majority of them varies over 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. It is obvious that such shapes 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.
| N | Asteroid Name Rus./Lat. |
Diameter (km) |
Weight (10 15 kg) |
Period rotation (hour) |
Orbital. period (years) |
Range. Class |
Big p / axis orb. (a.u.) |
Eccentricity orbits |
|---|---|---|---|---|---|---|---|---|
| 1 | Ceres/ Ceres |
960 x 932 | 87000 | 9,1 | 4,6 | With | 2,766 | 0,078 |
| 2 | Pallas/ Pallas |
570 x 525 x 482 | 318000 | 7,8 | 4,6 | U | 2,776 | 0,231 |
| 3 | Juno/ Juno |
240 | 20000 | 7,2 | 4,4 | S | 2,669 | 0,258 |
| 4 | Vesta/ Vesta |
530 | 300000 | 5,3 | 3,6 | U | 2,361 | 0,090 |
| 8 | Flora/ Flora |
141 | 13,6 | 3,3 | S | 0,141 | ||
| 243 | Ida | 58 x 23 | 100 | 4,6 | 4,8 | S | 2,861 | 0,045 |
| 253 | Matilda/ Mathilde |
66 x 48 x 46 | 103 | 417,7 | 4,3 | C | 2,646 | 0,266 |
| 433 | Eros/Eros | 33 x 13 x 13 | 7 | 5,3 | 1,7 | S | 1,458 | 0,223 |
| 951 | Gaspra/ Gaspra |
19 x 12 x 11 | 10 | 7,0 | 3,3 | S | 2,209 | 0,174 |
| 1566 | Icarus/ Icarus |
1,4 | 0,001 | 2,3 | 1,1 | U | 1,078 | 0,827 |
| 1620 | Geographer/ geographos |
2,0 | 0,004 | 5,2 | 1,4 | S | 1,246 | 0,335 |
| 1862 | Apollo/ Apollo |
1,6 | 0,002 | 3,1 | 1,8 | S | 1,471 | 0,560 |
| 2060 | Chiron/ Chiron |
180 | 4000 | 5,9 | 50,7 | B | 13,633 | 0,380 |
| 4179 | Toutatis/ Toutatis |
4.6 x 2.4 x 1.9 | 0,05 | 130 | 1,1 | S | 2,512 | 0,634 |
| 4769 | Castalia/ Castalia |
1.8 x 0.8 | 0,0005 | 0,4 | 1,063 | 0,483 |
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. The 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 her in some kind of collision, or a foreign object captured by her. 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 snapshot, spacecraft NEAR
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 leading role in the fact that in the place of the main asteroid belt during the period of the origin of the solar system, another planet could not arise. 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 that were not captured by proto-Jupiter for any reason, but were in the sphere of its gravitational influence, were effectively scattered into different sides. 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. Moreover, 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 on 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. AT recent times scientists are 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 main belt bodies, but rather rapidly evolve under the influence of the gravitational fields not only of Jupiter, but also of the planets. terrestrial group. 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 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 vicinity, 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 in 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 prevent the detection 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 (with 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 good examples the absence of a sharp boundary between asteroids and comets in terms of the 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 estimate 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 general characteristics 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 asteroids are compared with similar values for terrestrial rocks and minerals, as well as those meteorites, samples of which are available in terrestrial collections. Chemical and mineral composition The samples that 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 plagioclases (with 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 on asteroids there are 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. 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 having 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 matter. 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 same spectral characteristics has the mineral enstatite (this is a high-temperature variety of pyroxene) or other silicates containing iron in a free (non-oxidized) state, which, therefore, can be included in the composition of E-type asteroids. 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 interesting fact, as the presence of hydrated silicates on a significant part of these bodies. Although the cause of such an 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 surface matter 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 different types in space, a clear connection was found between their supposed chemical-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 with mathematical modeling and numerical calculations, the melting point of such a silicate substance could be in the range of 500–1000°C. Thus, after differentiation and cooling, the parent bodies of asteroids experienced numerous collisions not only between themselves and their fragments, but also with Jupiter, Saturn and the farther 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 speeds 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 into small fragments, first of all, the least durable external silica 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 solid in the main and other asteroid belts and associated with the emergence of the entire solar system. Their solution is importance 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 major planets Jupiter type. 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 for a detailed study of all, it is enough large asteroids only in the main belt it will take 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, it is 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.
