One of the asteroids in the solar system. Threat from orbit

Astronomers have been studying asteroids for a long time, but the world community became interested in them only 10 years ago, after mass media reported a risk of collision with the celestial body Apophis. This catastrophe would be the death of a quarter of the world's population. Later, when scientists recalculated the asteroid’s trajectory, the panic passed, but interest in celestial pebbles and astronomy remained. Which asteroids are the most “noticeable”?

The size of this asteroid reaches almost 950 kilometers. The discovery of this celestial body occurred in 1801, and since then it has not been named. First a full-fledged planet, then an asteroid, and in 2006 it was recognized dwarf planet, since Ceres is the most powerful object in the asteroid belt. Ceres looks like a sphere, which is a bit surprising for an asteroid, and its rocky core and crust are made of minerals and frozen water. Earthlings need not be afraid of a collision with Ceres in the coming millennia, since the point of its orbit is as much as 263 million kilometers away from Earth.

It also boasts of its size – 532 kilometers. Pallas is also part of the asteroid belt, and there are high hopes for it because of the silicon in which it is rich. Perhaps someday Pallas will be a valuable source of silicon for us.

The diameter of this asteroid is 530 kilometers. But, even with its smaller size, Vesta has the lead in the “heavy weight”. The core of the asteroid is heavy metal, and the crust is rock. Because of its differences, Vesta is able to reflect four times more sunlight than other asteroids. It is because of this that it can sometimes be seen from Earth. This event occurs once every three to four years.

This asteroid cannot be called small; its diameter is about 400 kilometers. But Hygiea is very dim, so it was discovered later than its colleagues. Hygea is absolutely typical of the most common type of asteroid and has carbonaceous content. When Hygeia is closest to planet Earth, it can be seen with binoculars.

An asteroid with a diameter of 326 kilometers, although considered quite large, is still a little-studied celestial object to this day. And the reason is that Interamnia is celestial body a rare spectral class F. Modern scientists still have not figured out what astronomical objects of this class are made of, and are also in the dark about their internal structure. What can I say, even the form of Interamnia still remains a mystery! Here is such a little-known celestial object.

This asteroid was discovered a long time ago; more than one hundred and fifty years have passed since then. Its diameter is about 302 kilometers. Europa is distinguished by its elongated orbit, which is why the distance from the asteroid to the Sun fluctuates all the time. If life existed on Europa, it would be inhabited by mutants with increased adaptability. Europa's density is almost the same as water, so the surface of the asteroid is porous. Imagine a flying pumice stone spinning in the Great Asteroid Ring.

The diameter of this celestial object is estimated differently; it ranges from 270-326 kilometers. Davida owes its name to its discoverer, Raymond Dugan, who dedicated the asteroid to astronomy professor David Todd. Subsequently, “David” was changed into a female name, since at that time it was customary to give celestial bodies only female names, and tried to take them from Greek mythology.

A celestial body with a diameter of 232 kilometers. Silvia, like Europa, is porous, but for different reasons. The asteroid is made of rubble, held together only by gravity. Sylvia is also famous for being the first triple asteroid to have as many as two moons.

An amazing asteroid that looks like either peanuts or dumbbells. Thanks to its strange shape, it has caused controversy about its origin, some consider it man-made, other scientists prove it natural origin. Hector has his own, as yet unnamed, moon. Hector is also interesting because of its origin, it contains rocks and ice. This composition is found among asteroids in the Kuiper belt, which means that Hector came from there.

A celestial body with a diameter of 248-270 kilometers rotates very quickly. Its density is very high, but scientists explain this by the “largeness” of Euphrosyne. By the way, asteroids never cease to amaze the public! Quite recently, the celestial body UW-158 passed at a distance of 2.4 million kilometers from our planet. Surprisingly, its core contains almost 100 million tons of platinum! It’s even a little sad that such wealth literally floated away from us!

The shape and surface of the asteroid Ida.
North is at the top.
The animation was done by Typhoon Oner.
(Copyrighted © 1997 by A. Tayfun Oner).

1. General ideas

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, so they are also called minor planets. The diameters of asteroids range from several tens of meters (conventionally) to 1000 km (the size of the largest asteroid Ceres). The term "asteroid" (or "star-like") was coined by the famous 18th-century astronomer William Herschel to describe 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 were measured using the "star occultation" method, at those fortunate moments when they were in the same line of sight with sufficiently bright stars. In most cases, their sizes are estimated using special astrophysical measurements and calculations. The bulk of currently known asteroids moves between the orbits of Mars and Jupiter at distances from the Sun of 2.2-3.2 astronomical units (hereinafter - AU). In total, approximately 20,000 asteroids have been discovered to date, of which about 10,000 are registered, that is, they are assigned numbers or even proper names, and the orbits are calculated with great accuracy. Proper names for asteroids are usually assigned by their discoverers, but in accordance with established international rules. At first, when little was known about the minor planets, their names were taken, as for other planets, from ancient Greek mythology. The annular region of space that these bodies occupy is called the main asteroid belt. With an average linear orbital speed of about 20 km/s, main belt asteroids spend one revolution around the Sun from 3 to 9 Earth years, depending on the distance from it. The inclinations of the planes of their orbits relative to the ecliptic plane sometimes reach 70°, but are generally in the range of 5-10°. On this basis, all known main belt asteroids are divided approximately equally into flat (with orbital inclinations of up to 8°) and spherical subsystems.

During telescopic observations of asteroids, it was discovered that the brightness of the absolute majority of them changes over time. short time(from several hours to several days). Astronomers have long assumed that these changes in the brightness of asteroids are related to their rotation and are determined primarily by their irregular shape. The very first photographs of asteroids obtained using spacecraft confirmed this and also showed that the surfaces of these bodies are pitted with craters or craters different sizes. Figures 1-3 show the first space images of asteroids obtained using different spacecraft. It is obvious that such forms and surfaces of small planets were formed during their numerous collisions with other solid celestial bodies. In general, when the shape of an asteroid observed from 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.

Table 1. Information about some asteroids.

N	Asteroid Name Russian/Lat.	Diameter (km)	Weight (10 15 kg)	Period rotation (hour)	Orbital. period (years)	Spectrum. Class	Big p/axis orb. (au)	Eccentricity orbits
1	Ceres/ Ceres	960 x 932	87000	9,1	4,6	WITH	2,766	0,078
2	Pallas/ Pallas	570 x 525x 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/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	Ikarus/ 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 that was discovered first. It was discovered by 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 discovered. This was done by the German astronomer Heinrich Olbers on March 28, 1802.

3 Juno - discovered by K. Harding in 1804.

4 Vesta is the third largest asteroid, also discovered by G. Olbers in 1807. This body has observational evidence of the presence of a basaltic crust covering an olivine mantle, which may be a consequence of the 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, operating in low-Earth orbit.

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

243 Ida is a main belt asteroid imaged by the Galileo spacecraft on August 28, 1993. These images revealed a small moon of Ida, later named Dactyl. (See Figures 2 and 3).

253 Matilda is an asteroid, images of which were obtained using the NIAR spacecraft in June 1997 (See Fig. 4).

433 Eros is a near-Earth asteroid, images of which were obtained using the NIAR spacecraft in February 1999.

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

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

1620 Geograph is a near-Earth asteroid that is either a binary 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 exhibiting periodic 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 included in the asteroid), moving along an eccentric trajectory (eccentricity 0.3801) between orbits of Saturn and Uranus.

4179 Toutatis is a binary asteroid whose components are likely in contact and has dimensions of approximately 2.5 km and 1.5 km. Images of this asteroid were obtained using radars located at 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 at Arecibo.

Image of asteroid 951 Gaspra

Rice. 1. Image of asteroid 951 Gaspra, obtained using the Galileo spacecraft, in pseudo-color, that is, as a combination of images through violet, green and red filters. The resulting colors are specifically enhanced to highlight subtle differences in surface detail. Exposure areas have a bluish tint rocks, while areas covered with regolith (crushed material) have a reddish color. 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 on the right.
NASA GAL-09 image.

Image of asteroid 243 Idas

Rice. 2 False-color image of asteroid 243 Ida and its small moon Dactyl taken by the Galileo spacecraft. The source images used to obtain the image shown in the figure were obtained from approximately 10,500 km. Color differences may indicate variations in surfactant composition. The bright blue areas may be coated with a substance consisting of iron-containing minerals. Ida's length is 58 km, and its rotation axis is oriented vertically with a slight tilt to the right.
NASA GAL-11 image.

Rice. 3. Image of Dactyl, the small satellite of 243 Ida. It is not yet known whether he is a piece of Ida, broken off from her during 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 approximately 4000 km, 4 minutes before the closest approach to the asteroid. The dimensions of Dactyl are approximately 1.2 x 1.4 x 1.6 km. NASA GAL-04 image

Asteroid 253 Matilda

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

2. How could the main asteroid belt arise?

The orbits of bodies concentrated in the main belt are stable and have a close to circular or slightly eccentric shape. Here they move in a “safe” zone, where the gravitational influence on them of large planets, and primarily Jupiter, is minimal. The scientific facts available today show that it was Jupiter who played main role is that another planet could not arise in place of the main asteroid belt during the birth of the Solar System. But even at the beginning of our century, many scientists were still confident 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 for this hypothetical planet - Phaeton. Let's do it small retreat and describe one episode from the history of the Solar system - that history that is based on modern scientific facts. This is necessary, in particular, for understanding the origin of 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 on the border of condensation of volatile compounds (H 2, H 2 O, NH 3, CO 2, CH 4, etc.), which flowed from a zone of 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. When it reached a sufficiently large mass, it began to capture with its gravitational field previously condensed matter located closer to the Sun, in the zone of the parent bodies of asteroids, and thus slow down the growth of the latter. On the other hand, smaller bodies that were not captured by the proto-Jupiter for any reason, but were within the sphere of its gravitational influence, were effectively scattered in different directions. In a similar way, there was probably an ejection of bodies from the formation zone of Saturn, although not so intensely. These bodies also penetrated the belt of the parent bodies of asteroids or planetesimals that arose earlier between the orbits of Mars and Jupiter, “sweeping” them out of this zone or subjecting them to fragmentation. Moreover, before this, the gradual growth of the parent bodies of asteroids was possible due to their low relative speeds (up to about 0.5 km/s), when collisions of any objects ended in their union, and not fragmentation. 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 asteroid parent bodies was replaced by the process of their fragmentation during mutual collisions, and the potential possibility of forming a sufficiently large planet at a given distance from the Sun disappeared forever.

3. Asteroid orbits

Returning to current state 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, named after Daniel Kirkwood, the scientist who first discovered them. Such orbits are resonant, since asteroids moving along them experience strong gravitational influence from Jupiter. The orbital periods corresponding to these orbits are in simple relationships with the orbital period of Jupiter (for example, 1:2; 3:7; 2:5; 1:3, etc.). If an asteroid or its fragment, as a result of a collision with another body, falls into a resonant or close to it orbit, then the semimajor axis and eccentricity of its orbit change quite quickly under the influence of the Jovian gravitational field. It all ends with the asteroid either leaving the resonant orbit and may even leave the main asteroid belt, or it is doomed to new collisions with neighboring bodies. This clears the corresponding Kirkwood space of any objects. However, it should be emphasized that in the main asteroid belt there are no gaps or empty spaces if we imagine the instantaneous distribution of all the bodies included in it. All asteroids, at any given time, fairly evenly fill the asteroid belt, since, moving along elliptical orbits, they spend most of their time in the “alien” 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 the orbits of asteroids, the orbital periods of which are in proportions of 2:3 and 1:1 in relation to the orbital period Jupiter. Obviously, asteroids with an orbital period corresponding to the 1:1 ratio are located directly in the orbit of Jupiter. But they move at a distance from it equal to the radius of the Jupiterian orbit, either ahead or behind. Those asteroids that are ahead of Jupiter in their movement are called “Greeks”, and those that follow it are called “Trojans” (so 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 general name for this group of asteroids is “Trojans”. Unlike Trojans, which could gradually accumulate in the vicinity of Lagrange points during the long-term 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 corresponding parent bodies. This is, for example, the Flora asteroid family, 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 thus estimate the original number of their parent bodies.

4. Near-Earth asteroids

Up close inner edge In the main asteroid belt, there are other groups of bodies whose orbits extend far beyond the main belt and can even intersect with the orbits of Mars, Earth, Venus and even Mercury. First of all, these are the groups of asteroids Amur, Apollo and Aten (by the names of the largest representatives included in these groups). The orbits of such asteroids are no longer as stable as those of main-belt bodies, but evolve relatively quickly under the influence of the gravitational fields of not only Jupiter, but also the terrestrial planets. For this reason, such asteroids can move from one group to another, and the very division of asteroids into the above groups is conditional, based on data on the modern orbits of asteroids. In particular, the Amurians move in elliptical orbits, the perihelion distance (minimum distance to the Sun) of which does not exceed 1.3 AU. The Apollons move in orbits with a perihelion distance of less than 1 AU. (remember 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 semi-major axis 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 within the Earth’s orbit. It is obvious that the Apollons and Atonians, 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 significantly 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 cosmic bodies that are located or may end up in the terrestrial environs is widely discussed in scientific and public circles. More details about this, as well as about the measures proposed to protect our planet, can be found in the recently published book edited by A.A. Boyarchuk.

5. About other asteroid belts

Asteroid-like bodies also exist 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 early 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 of approximately 100 to 800 km have been discovered beyond the orbit of Neptune. The collection of these bodies was called the “Kuiper belt,” although they are not yet enough to form a “full-fledged” belt. However, according to some estimates, the number of bodies in it may be no less (if not more) than in the main asteroid belt. Based on their orbital parameters, 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 (semi-major axes about 39 AU; eccentricities 0.11-0.35; orbital inclinations to the ecliptic 0-20 degrees), similar to the orbit of Pluto, where they originated the name of this class. Currently, there are even discussions among scientists about whether Pluto should be considered a full-fledged planet or just one of the objects of the above-mentioned class. However, Pluto's status will most likely not change, since its average diameter (2390 km) is significantly 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 inclinations (0-40°). So far, great distances and relatively small sizes have prevented the discovery of new similar bodies at a faster rate, although the largest telescopes and the most modern technology are used for this. Based on a comparison of these bodies with known asteroids based on their optical characteristics, it is now believed that the former are the most primitive in our planetary system. This means that their matter, from the moment of its condensation from the protoplanetary nebula, experienced completely minor changes compared, for example, with the substance of the terrestrial planets. In fact, the absolute majority of these bodies in their composition can be the nuclei of comets, which will also be discussed in the “Comets” section.

A number of asteroid bodies have been discovered (this number is likely to increase over time) between the Kuiper belt and the main asteroid belt - this is the "Centaur class" - 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 gas atmosphere (coma) and tail. They are formed from volatile compounds that make up the substance of this body as it passes through the perihelion portions of its orbit. Chiron is one of the illustrative examples the absence of a sharp boundary between asteroids and comets in terms of the composition of matter and, possibly, in origin. It is about 200 km in size and its orbit overlaps with the orbits of Saturn and Uranus. Another name for objects of this class is the “Kazimirchak-Polonskaya belt” - named after E.I. Polonskaya, who proved the existence of asteroid bodies between giant planets.

6. A little about asteroid research methods

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 to be fragments of asteroids, comet nuclei and surfaces of terrestrial planets. But we still obtain the largest amount of information about small planets using ground-based telescopic measurements. Therefore, asteroids are divided into so-called "spectral types" or classes according, first of all, to their observable optical characteristics. First of all, this is albedo (the proportion of light reflected by a body from the amount of sunlight incident on it per unit time, if we consider the directions of incident and reflected rays to be the same) and the general shape of the body’s reflection spectrum in the visible and near-infrared ranges (which is obtained by simply dividing at each the light 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 substance that composes asteroids. Sometimes additional data (if any) are taken into account, for example, about the radar reflectivity of the asteroid, 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, there has already been a need to introduce subclasses and smaller divisions of the spectral types of asteroids to characterize some general features of their individual groups. Before you give general characteristics asteroids of different spectral types, we will explain how the composition of asteroid matter can be assessed using remote measurements. As already noted, it is believed that asteroids of a particular type have approximately the same albedo values and reflectance spectra that are similar in shape, which can be replaced by average (for a given type) values or characteristics. These average values for a given type of asteroid are compared with similar values for terrestrial rocks and minerals, as well as those meteorites from which samples are available in terrestrial collections. Chemical and mineral composition s samples, which are called “analog samples,” along with their spectral and other physical properties, as a rule, have already been well studied in terrestrial laboratories. Based on such a comparison and selection of analogue samples, a certain average chemical and mineral composition of matter for asteroids of this type is determined to a 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 asteroidal matter was involved throughout the history of the Solar System were not as diverse and complex as on the terrestrial planets. If about 4,000 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 more high 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 gas or liquid oxidizing environment, the main components of which there was oxygen and water. Their long and constant interaction with primary minerals and rocks of the earth's crust led to the wealth of minerals that we observe. Returning to asteroids, it should be noted that, according to remote sensing data, they mainly consist of simpler silicate compounds. First of all, these are anhydrous silicates, such as pyroxenes (their general formula is ABZ 2 O 6, where positions “A” and “B” are occupied by cations different metals, and “Z” - Al or Si), olivines (A 2+ 2 SiO 4, where A 2+ = 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. Another type of silicate compound commonly found on asteroids is hydrosilicates or layered silicates. These include serpentines (with the general formula A 3 Si 2 O 5? (OH), where A = Mg, Fe 2+, Ni), chlorites (A 4-6 Z 4 O 10 (OH,O) 8, where A and Z are mainly cations of various metals) and a number of other minerals that contain hydroxyl (OH). It can be assumed that asteroids contain not only simple oxides, compounds (for example, sulfur dioxide) and alloys of iron and other metals (in particular FeNi), carbon (organic) compounds, but even metals and carbon in free state. This is evidenced by the results of a study of meteorite matter that constantly falls on the Earth (see section “Meteorites”).

7. Spectral types of asteroids

To date, the following main spectral classes or types of small planets have been identified, designated 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 match the spectral characteristics of these asteroids. In contrast, 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 a reflectance spectrum that drops off sharply 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 have been classified as 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 circumsolar 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-changed bodies of the asteroid belt. Minor E-type planets have the highest albedo values (their surface material can reflect up to 50% of the light falling on them) and are slightly reddish in color. The mineral enstatite (this is a high-temperature variety of pyroxene) or other silicates containing iron in a free (unoxidized) state, which, therefore, can be part of E-type asteroids, have the same spectral characteristics. Asteroids that are similar in reflection spectra to P- and E-type bodies, but are between them in albedo value, are classified as M-type. It turned out that the optical properties of these objects are very similar to the properties of metals in a 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 as the presence of hydrated silicates on a significant part of these bodies has recently been established. Although the reason for the emergence of such an unusual combination of high-temperature and low-temperature materials has not yet been fully established, it can be assumed that hydrosilicates could have been introduced to M-type asteroids during their collisions with more primitive bodies. Of the remaining spectral classes in albedo and general form Reflectance spectra in the visible range of Q-, R-, S- and V-type asteroids are quite similar: they have a relatively high albedo (S-type bodies are slightly lower) and a reddish color. The differences between them boil down to the fact that the wide absorption band of about 1 micron present in their reflection spectra in the near-infrared range has different depths. This absorption band is characteristic of a mixture of pyroxenes and olivines, and the position of its center and depth depend on the fractional 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 the diffusely reflected (that is, transmitted through the substance and information-carrying about its composition) light. For these asteroids, the depth of the absorption band at 1 μm increases from S- to Q-, R- and V-types. In accordance with the above, bodies of the listed types (except V) can consist of a mixture of olivines, pyroxenes and metals. The substance of V-type asteroids can include, along with pyroxenes, feldspars, and in composition be similar to terrestrial basalts. And finally, the last, T-type, includes asteroids that have a low albedo and a reddish reflectance spectrum, which is similar to the spectra of P- and D-type bodies, but occupying an intermediate position between their spectra in terms of inclination. Therefore, the mineralogical composition of T-, P- and D-type asteroids is considered to be approximately the same and corresponds to silicates rich in carbon or organic compounds.

When studying the distribution of asteroids of different types in space, a clear connection was discovered 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 away they are, as a rule, located. 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 inner part 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 asteroids of these three types are located within the main belt. The maximum of the total distribution of E- and R-type asteroids extends somewhat 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 extends 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.

To conclude the review of minor planets, we will briefly outline the meaning of the general hypothesis about the origin of asteroids of various classes, which is finding more and more confirmation.

8. On the origin of minor planets

At the dawn of the formation of the Solar System, about 4.5 billion years ago, from the gas-dust disk surrounding the Sun, as a result of turbulent and other non-stationary phenomena, clumps of matter arose, which, through 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 overall chemical composition changed. The annular zone of the protoplanetary disk, from which the main asteroid belt was 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 matter 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 decreased sharply, and 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 disturbances 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 planetesimals of preplanetary size (about 500-1000 km) had time to form, which then began to fragment during collisions due to rapid growth their relative speeds (from 0.1 to 5 km/s). However, during this period, some asteroid parent bodies, or at least those that contained a high proportion of silicate compounds and were located closer to the Sun, were already heated up or even experienced gravitational differentiation. Two possible mechanisms for heating the interior of such proto-asteroids are now being considered: as a consequence of the decay of radioactive isotopes, or as a result of the action of induction currents induced in the matter of these bodies by powerful flows of charged particles from the young and active Sun. The parent bodies of asteroids, which for some reason have survived to this day, according to scientists, are the largest asteroids 1 Ceres and 4 Vesta, basic 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 matter, metal cores and other lighter silicate shells were released, and in some cases even basaltic crust (for example, 4 Vesta), like the terrestrial planets . But still, since the substance in the asteroid zone contained a significant amount of volatile compounds, it average temperature melting 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. So, after differentiation and cooling, the parent bodies of the asteroids experienced numerous collisions not only with each other and their fragments, but also with bodies that invaded the asteroid belt from the zones Jupiter, Saturn and the more distant periphery of the Solar system. As a result of long-term impact evolution, proto-asteroids were fragmented into a huge number of smaller bodies, now observed as asteroids. At relative speeds At about several kilometers per second, collisions of bodies consisting of several silicate shells with different mechanical strengths (the more metals a solid contains, the more durable it is), led to the “tearing off” of them and crushing into small fragments, primarily the least strong external 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 represent the entire nuclei of their parent bodies (such as the S-asteroid 15 Eunomia and the M-asteroid 16 Psyche with diameters of about 270 km) or their fragments due to their high metal content . Asteroids of A- and R-spectral types can be fragments of intermediate silicate shells, and E- and V-types can be the outer shells of such parent bodies. Based on the analysis of the spatial distributions of E-, V-, R-, A-, M- and S-type asteroids, we can also conclude that they have undergone the most intense thermal and impact processing. 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 asteroids of these types. 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 fragmentation, but this process was probably not so intense as to significantly affect their observed characteristics and, accordingly, their chemical and mineral composition. (This issue will also be discussed in the “Meteorites” section). However, as numerical modeling of collisions of silicate bodies of asteroid sizes shows, many of the currently existing asteroids could reaccumulate after mutual collisions (that is, combine from the remaining fragments) and therefore are not monolithic bodies, but moving “piles of cobblestones.” There is numerous observational evidence (based on specific changes in brightness) of the presence of small satellites of a number of asteroids gravitationally associated with them, which probably also arose during impact events as fragments of colliding bodies. This fact, although hotly debated among scientists in the past, was convincingly confirmed by the example of the asteroid 243 Ida. Using the Galileo spacecraft, it was possible to obtain images of this asteroid along with its satellite (which was later named Dactyl), which are presented in Figures 2 and 3.

9. What we don't know yet

There is still much that is unclear and even mysterious in asteroid research. Firstly, this common 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 correct ideas about our system, but also for understanding the reasons 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 around these and other stars. There are also questions that can only be answered through a detailed study of individual minor planets. Essentially, each of these bodies is unique, as it has its own, sometimes specific, history. For example, asteroids that are members of some dynamic families (for example, Themis, Flora, Gilda, Eos and others), having, as stated, common origin, may differ noticeably 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 using its generalization, is it possible to gradually clarify the understanding of the nature of these bodies and the basic patterns of their evolution.

REFERENCES:

1. Threat from the sky: fate or chance? (Ed. A.A. Boyarchuk). M: "Cosmosinform", 1999, 218 p.

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

Asteroids are small, rocky worlds orbiting outer space around our Sun. They are too small to be called planets. They are also known as planetoids or small planets. In total, the mass of all asteroids is less than the mass of Earth's Moon. However, their size and relatively small mass do not make them safe space objects. Many of them have fallen to the surface of the Earth in the past and will fall in the future. This is one of the reasons why astronomers study asteroids and are ready to learn their orbits and physical characteristics.

Most asteroids are located in a huge ring between the orbits of Mars and Jupiter. This place is more widely known as the Main Asteroid Belt. Scientists estimate that the asteroid belt contains about 200 asteroids larger than 100 kilometers in diameter, more than 75,000 asteroids larger than 1 kilometer in diameter, and millions of smaller bodies.

Approximate number of asteroids N with diameter greater than D

D	100 m	300 m	1 km	3 km	10 km	30 km	50 km	100 km	300 km	500 km	900 km
N	25 000 000	4 000 000	750 000	200 000	10 000	1100	600	200	5	3	1

However, not all objects in the main asteroid belt are asroids - recently comets were discovered there, and in addition there is Ceres, an asteroid that, due to its size, was raised to the status of a dwarf planet.

The location, as well as the size of the asteroids, may also vary. For example, asteroids called Trojans are found along the orbital path of Jupiter. Asteroids from the Amur and Apollo groups, due to their close location to the center of the solar system, can cross the Earth’s orbit.

How are asteroids formed?

Asteroids are leftover material from the formation of our solar system about 4.6 billion years ago.

The process of their formation is similar to the process of formation of planets, but until Jupiter has gained its current mass. After this, more than 99% of the total mass of the formed asteroids was thrown out of the main belt by the gravitational influence of Jupiter. The remaining 1% is what we see in the main asteroid belt.

How are asteroids classified?

Asteroids are classified depending on the location of their orbit and the elements of which they are composed. Currently, three main classes of asteroids have been precisely identified depending on their chemical composition.

C - class: More than 75% of known asteroids belong to this class. In their composition in large quantities carbon and its compounds are present. This type of asteroid is widespread in the outer region of the Main Asteroid Belt;

S - class: This type of asteroid accounts for about 17% of known asteroids, which are mainly located in the inner region of the asteroid belt. Their basis is rocky rock.

M - class: This type of asteroid consists mainly of metallic compounds and occupies the remainder of the known asteroids.

I would like to note that the above classification covers most asteroids. But there are other quite rare species.

Features of asteroids.

Asteroids can vary greatly in size. Ceres, the largest member of the main asteroid belt, measures about 940 kilometers in diameter. One of the smallest representatives of the belt, called 1991 BA, was found in 1991 and is only 6 meters in diameter.

10 first discovered asteroids

Almost all asteroids have an irregular shape. Only the largest ones are approximately spherical in shape. Most often, their surface is completely covered with craters - for example, on Vesta there is a crater with a diameter of about 460 kilometers. The surface of most asteroids is covered with a deep layer of cosmic dust.

Most asteroids quietly rotate in elliptical orbits around the Sun, but this does not prevent individual representatives from creating more chaotic trajectories of their movement. Currently, astronomers know about 150 asteroids that have small satellites. There are also binary or double asteroids of approximately the same size rotating around the center of mass they created. Scientists also know the existence of triple asteroid systems.

According to scientists, many asteroids during the formation of the solar system were captured by the gravitational attraction of other planets. So, as an example, we can cite the moons of Mars - Deimos and Phobos, which in the distant past most likely were asteroids. The same story could happen to most of the small moons located in orbit around the gas giants - Jupiter, Saturn, Uranus and Neptune.

The temperature on the surface of most asteroids does not exceed -73 degrees Celsius. Asteroids for the most part remained untouched by cosmic bodies for billions of years. This fact allows scientists, through their research, to understand and study the process of formation and evolution of the Solar system.

Are asteroids dangerous for Earth?

Since the Earth was formed 4.5 billion years ago, asteroids have constantly fallen onto its surface. However, the fall of large objects is a rather rare event.

The fall of asteroids with a size of about 400 meters in diameter can lead to a global catastrophe on Earth. Researchers estimate that the impact of an asteroid of this size could raise enough dust into the atmosphere to create a “nuclear winter” on Earth. The fall of such objects occurs on average once every 100,000 years.

Small asteroids, which can destroy, for example, a city or cause a huge tsunami but will not lead to a global catastrophe, fall to Earth a little more often, approximately every 1000 - 10,000 years.

Last a shining example, is the fall of an asteroid with a diameter of about 20 meters in Chelyabinsk region. The impact created a shock wave across its surface, which injured more than 1,600 people, most from broken glass. The total power of the explosion, according to various estimates, was about 100 - 200 kilotons of TNT.

Useful articles that will answer most interesting questions about asteroids.

Deep space objects

Asteroids have been known to astronomers for a long time, but the world community started talking about them seriously only after 2004, when information appeared in the media that this could have been a disaster, destroying about 25% of life on the planet. Then the trajectory of the asteroid was recalculated, everyone calmed down, but interest in asteroids and others remained. So, ?
1

Diameter is about 950 km. What this celestial body has been since its discovery (which happened, for a moment, in 1801!): a full-fledged planet, an asteroid, and since 2006 it has been considered a dwarf planet - for being the largest in the asteroid belt. Ceres is spherical in shape, which is completely uncharacteristic of asteroids; the core consists of rock, and the crust is made of minerals and water ice. The closest point of its orbit is at a distance of 263 million km from Earth, so it is unlikely that a collision should be expected - at least in the next few thousand years.

Its diameter is 532 km. It also forms part of the asteroid belt and is very rich in silicon - in the future it may become a source of minerals for earthlings.

530 km in diameter. Even though Vesta is smaller in size than previous asteroids, it is the heaviest asteroid. Its core consists of heavy metal, its crust is made of rock. Due to the characteristics of this rock, Vesta reflects 4 times more sunlight than the leader of our top - Ceres, so sometimes, once every 3-4 years, Vesta’s movements can be observed from Earth with the naked eye.

Its diameter is considerable - 407 km, but this asteroid is so dim that it was discovered later than the others. Hygea is a typical representative of the most common type of asteroid - with carbonaceous content. At the moment of its maximum approach to the Earth, this celestial body can be observed not through a telescope, but through binoculars.

Diameter – 326 km. Despite the fact that Interamnia is a very large asteroid, it still remains a very little-studied celestial body. First of all, because they belong to asteroids of the rare spectral class F - neither their exact composition nor internal structure modern science unknown. As for Interamnia, even its exact form is unknown! Complete mysteries...

The diameter of this asteroid is 302.5 km, and it was discovered a long time ago - in 1858. It has a very elongated orbit, so the distance from Europa to the Sun can change very significantly (if there was life here, it would be some super-adaptive mutants!). Its density index is only slightly greater than that of water, which means that the surface of this celestial body is porous. It's like a giant pumice stone rotating in the Great Asteroid Ring.

Its diameter, according to various estimates, ranges from 270 to 326 km. Where does such a strange name come from? The discoverer of this asteroid, Raymond Dugan, named the celestial body he discovered after astronomy professor David Todd, but the name was remade into a “female” version - “David”, since at that time only female names were given to asteroids (and, as you may have already note, most are from Greek mythology).

Diameter – 232 km. This asteroid, like Europa, has a large porosity - essentially, it is a pile of rubble that is held together by gravity. Sylvia is the first triple asteroid known to us, because it has at least 2 satellites!

A very strange space object with dimensions of 370 × 195 × 205 and a shape that looks like either a peanut or a dumbbell, and in addition to everything, it also has its own (as yet unnamed) moon. Its origin is interesting: the fact is that Hector consists of a mixture of rock and ice. The Kuiper belt objects Pluto and its satellite Triton have this composition. This means that Hector arrived from the Kuiper Belt (the region of space beyond Pluto), most likely at the dawn of the formation of the Solar System, when the planets were actively migrating.

Size – according to various sources, from 248 to 270 km – is a large and rapidly rotating asteroid. It has a very high density, but this is due to its large size.
And just recently - on July 19 - asteroid UW-158 with a core containing about 100 million tons of platinum passed very close to Earth (2.4 million km, nothing for space)! Such wealth is gone... So asteroids continue to surprise us!

Composite image (to scale) of asteroids taken in high resolution. As of 2011, these were, from largest to smallest: (4) Vesta, (21) Lutetia, (253) Matilda, (243) Ida and his companion Dactyl, (433) Eros, (951) Gaspra, (2867) Steins, (25143) Itokawa

Asteroid (a synonym common until 2006 - minor planet) is a relatively small celestial body moving in orbit around. Asteroids are significantly inferior in mass and size, have an irregular shape and do not have, although they may also have.

Definitions

Comparative sizes of asteroid (4) Vesta, dwarf planet Ceres and the Moon. Resolution 20 km per pixel

The term asteroid (from ancient Greek ἀστεροειδής - “like a star”, from ἀστήρ - “star” and εἶδος - “appearance, appearance, quality”) was coined by the composer Charles Burney and introduced by William Herschel on the basis that these objects observed as points - in contrast to the planets, which when observed through a telescope look like disks. Exact definition The term "asteroid" is still not established. Until 2006, asteroids were also called minor planets.

The main parameter by which classification is carried out is body size. Asteroids are considered bodies with a diameter of more than 30 m; smaller bodies are called .

In 2006, the International Astronomical Union classified most asteroids as .

Asteroids in the Solar System

Main asteroid belt ( white) and Trojan asteroids of Jupiter (green)

Currently, hundreds of thousands of asteroids have been discovered in the Solar System. As of January 11, 2015, there were 670,474 objects in the database, of which 422,636 had accurately determined orbits and assigned an official number, more than 19,000 of them had officially approved names. It is estimated that there may be from 1.1 to 1.9 million objects in the Solar System that are larger than 1 km. Most known on at the moment asteroids are concentrated within, located between the orbits and.

The largest asteroid in the Solar System was considered to be approximately 975 × 909 km in size, but since August 24, 2006 it received the status. The other two largest asteroids are (2) Pallas and have a diameter of ~500 km. (4) Vesta is the only asteroid belt object that can be observed naked eye. Asteroids moving in other orbits can also be observed during close passages (for example, (99942) Apophis).

The total mass of all main belt asteroids is estimated at 3.0-3.6 10 21 kg, which is only about 4% of the mass. The mass of Ceres is 9.5 10 20 kg, that is, about 32% of the total, and together with the three largest asteroids (4) Vesta (9%), (2) Pallas (7%), (10) Hygiea (3% ) - 51%, that is, the vast majority of asteroids have an insignificant mass by astronomical standards.

Asteroid exploration

The study of asteroids began after the discovery of the planet in 1781 by William Herschel. Its average heliocentric distance turned out to correspond to the Titius-Bode rule.

At the end of the 18th century, Franz Xaver organized a group of 24 astronomers. Since 1789, this group has been searching for a planet that, according to the Titius-Bode rule, should be located at a distance of about 2.8 astronomical units from the Sun - between the orbits of Mars and Jupiter. The task was to describe the coordinates of all stars in the area of zodiacal constellations at a certain moment. On subsequent nights, the coordinates were checked and objects that had moved greater distances were identified. The estimated displacement of the desired planet should have been about 30 arcseconds per hour, which should have been easy to notice.

Ironically, the first asteroid, Ceres, was discovered by accident by the Italian Piazzi, who was not involved in this project, in 1801, on the first night of the century. Three others - (2) Pallas, (3) Juno and (4) Vesta - were discovered over the next few years - the last, Vesta, in 1807. After another 8 years of fruitless searches, most astronomers decided that there was nothing more there and stopped research.

However, Karl Ludwig Henke persisted, and in 1830 he resumed the search for new asteroids. Fifteen years later, he discovered Astraea, the first new asteroid in 38 years. He also discovered Hebe less than two years later. After this, other astronomers joined the search, and then at least one new asteroid was discovered per year (with the exception of 1945).

In 1891, Max Wolf was the first to use the astrophotography method to search for asteroids, in which asteroids left short light lines in photographs with a long exposure period. This method greatly accelerated the discovery of new asteroids compared to previously used visual observation methods: Max Wolf single-handedly discovered 248 asteroids, starting with (323) Brusius, while little more than 300 had been discovered before him. Now, a century later, 385 thousand asteroids have official number, and 18 thousand of them are also a name.

In 2010, two independent teams of astronomers from the United States, Spain and Brazil announced that they had simultaneously discovered water ice on the surface of one of the largest main belt asteroids, Themis. This discovery provides insight into the origins of water on Earth. At the beginning of its existence, the Earth was too hot to hold enough water. This substance was supposed to arrive later. It was assumed that comets could have brought water to Earth, but the isotopic composition of terrestrial water and water in comets does not match. Therefore, it can be assumed that water was brought to Earth during its collision with asteroids. Researchers also discovered complex hydrocarbons on Themis, including molecules that are precursors to life.

Asteroid naming

At first, asteroids were given the names of heroes of Roman and Greek mythology, later discoverers received the right to call them whatever they wanted - for example, by their own name. At first, asteroids were given predominantly female names; only asteroids with unusual orbits (for example, Icarus, approaching closer to the Sun) received male names. Later, this rule was no longer observed.

Not any asteroid can receive a name, but only one whose orbit has been more or less reliably calculated. There have been cases when an asteroid received a name decades after its discovery. Until the orbit is calculated, the asteroid is given a temporary designation reflecting the date of its discovery, for example, 1950 DA. The numbers indicate the year, the first letter is the number of the crescent in the year in which the asteroid was discovered (in the example given, this is the second half of February). The second letter indicates the serial number of the asteroid in the specified crescent; in our example, the asteroid was discovered first. Since there are 24 crescents and 26 English letters, two letters are not used in the designation: I (due to the similarity with the unit) and Z. If the number of asteroids discovered during the crescent exceeds 24, they again return to the beginning of the alphabet, assigning the second the letter index is 2, the next time it returns - 3, etc.

After receiving a name, the official naming of the asteroid consists of the number ( serial number) and names - (1) Ceres, (8) Flora, etc.

Determining the shape and size of an asteroid

Asteroid (951) Gaspra. One of the first images of an asteroid obtained from a spacecraft. Transmitted by the Galileo space probe during its flyby of Gaspra in 1991 (colors enhanced)

The first attempts to measure the diameters of asteroids using the method direct measurement visible disks using a filament micrometer were undertaken by William Herschel in 1802 and Johann Schröter in 1805. After them, in the 19th century, other astronomers measured the brightest asteroids in a similar way. The main disadvantage of this method was the significant discrepancies in the results (for example, the minimum and maximum sizes of Ceres obtained by different scientists differed tenfold).

Modern methods for determining the size of asteroids include methods of polarimetry, radar, speckle interferometry, transit and thermal radiometry.

One of the simplest and highest quality is the transit method. As an asteroid moves relative to Earth, it sometimes passes against the background of a distant star; this phenomenon is called asteroid occultation. By measuring the duration of the decrease in the brightness of a given star and knowing the distance to the asteroid, you can quite accurately determine its size. This method makes it possible to fairly accurately determine the size of large asteroids, like Pallas.

The polarimetry method involves determining the size based on the brightness of the asteroid. The larger the asteroid, the more sunlight it reflects. However, the brightness of an asteroid strongly depends on the albedo of the asteroid's surface, which in turn is determined by the composition of its constituent rocks. For example, the asteroid Vesta, due to the high albedo of its surface, reflects 4 times more light than Ceres and is the most visible asteroid in the sky, which can sometimes be observed with the naked eye.

However, the albedo itself can also be determined quite easily. The fact is that the lower the brightness of the asteroid, that is, the less it reflects solar radiation in the visible range, the more it absorbs it and, heating up, then emits it in the form of heat in the infrared range.

The polarimetry method can also be used to determine the shape of an asteroid, by recording changes in its brightness during rotation, and to determine the period of this rotation, as well as to identify large structures on the surface. In addition, results obtained from infrared telescopes are used to determine dimensions using thermal radiometry.

Asteroid classification

The general classification of asteroids is based on the characteristics of their orbits and a description of the visible spectrum of sunlight reflected by their surface.

Orbit groups and families

Asteroids are grouped into groups and families based on the characteristics of their orbits. Usually the group is named after the first asteroid that was discovered in a given orbit. Groups are relatively loose formations, while families are denser, formed in the past during the destruction of large asteroids from collisions with other objects.

Spectral classes

In 1975, Clark R. Chapman, David Morrison, and Ben Zellner developed a system for classifying asteroids based on color, albedo, and characteristics of the spectrum of reflected sunlight. Initially, this classification defined only three types of asteroids:

Class C - carbon, 75% of known asteroids.
Class S - silicate, 17% of known asteroids.
Class M - metal, most others.

This list was later expanded and the number of types continues to grow as more asteroids are studied in detail:

Class A - characterized by a fairly high albedo (between 0.17 and 0.35) and a reddish color in the visible part of the spectrum.
Class B - in general, they belong to class C asteroids, but they almost do not absorb waves below 0.5 microns, and their spectrum is slightly bluish. The albedo is generally higher than that of other carbon asteroids.
Class D - characterized by a very low albedo (0.02−0.05) and a smooth reddish spectrum without clear absorption lines.
Class E - the surface of these asteroids contains a mineral such as enstatite and may be similar to achondrites.
Class F - generally similar to class B asteroids, but without traces of “water”.
Class G - characterized by a low albedo and an almost flat (and colorless) reflectance spectrum in the visible range, indicating strong ultraviolet absorption.
Class P - like class D asteroids, they are characterized by a rather low albedo (0.02−0.07) and a smooth reddish spectrum without clear absorption lines.
Class Q - at a wavelength of 1 micron, the spectrum of these asteroids contains bright and broad lines of olivine and pyroxene and, in addition, features indicating the presence of metal.
Class R - characterized by a relatively high albedo and a reddish reflectance spectrum at a length of 0.7 µm.
Class T - characterized by a low albedo and a reddish spectrum (with moderate absorption at a wavelength of 0.85 μm), which is similar to the spectrum of P- and D-class asteroids, but occupying an intermediate position in inclination.
Class V - asteroids of this class are moderately bright and quite close to the more general S class, which are also mainly composed of rock, silicates and iron (chondrites), but are distinguished by their higher pyroxene content.
Class J is a class of asteroids believed to have formed from the interior of Vesta. Their spectra are close to those of class V asteroids, but they are distinguished by particularly strong absorption lines at a wavelength of 1 μm.

It should be borne in mind that the number of known asteroids classified as a particular type does not necessarily correspond to reality. Some types are quite difficult to determine, and the type of a given asteroid may change with more careful research.

Problems of spectral classification

Initially, spectral classification was based on three types material that makes up asteroids:

Class C - carbon (carbonates).
Class S - silicon (silicates).
Class M - metal.

However, there are doubts that such a classification unambiguously determines the composition of the asteroid. While the different spectral class of asteroids indicates their different composition, there is no evidence that asteroids of the same spectral class are composed of the same materials. As a result, scientists did not accept new system, and the implementation of spectral classification stopped.

Size distribution

The number of asteroids decreases noticeably as their size increases. Although this generally follows a power law, there are peaks at 5 km and 100 km where there are more asteroids than would be expected from a logarithmic distribution.

Asteroid formation

In July 2015, the Victor Blanco Telescope's DECam camera was reported to have discovered Neptune's 11th and 12th Trojans, 2014 QO441 and 2014 QP441. This increased the number of Trojans at Neptune's L4 point to 9. This survey also discovered 20 other objects designated as the Minor Planet Center, including 2013 RF98, which has one of the longest orbital periods.

Objects in this group are given the names of centaurs of ancient mythology.

The first centaur to be discovered was Chiron (1977). As it approaches perihelion, it exhibits a coma characteristic of comets, so Chiron is classified as both a comet (95P/Chiron) and an asteroid (2060 Chiron), although it is significantly larger than a typical comet.