The bulk of asteroids discovered by scientists (about 98%) is located between the planetary orbits of Jupiter and Mars. Their distance from the star fluctuates between 2.06-4.30 AU. That is, for circulation periods, fluctuations have the following range - 2.9-8.92 years. In the group of minor planets, there are those that have unique orbits. These asteroids usually get masculine names. The most popular are the names of the heroes of Greek mythology - Eros, Icarus, Adonis, Hermes. These minor planets move outside the asteroid belt. Their distance from the Earth fluctuates, asteroids can approach it at 6 - 23 million km. A unique approach to the Earth occurred in 1937. The minor planet Hermes approached it by 580 thousand km. This distance is 1.5 times the distance of the Moon from the Earth.
The brightest of the known asteroids is Vesta (about 6m). A large mass of minor planets has intense brilliance during opposition (7m - 16m).
The calculation of the diameters of asteroids is carried out by brightness, ability to reflect visible and infrared rays.
Of the 3.5 thousand list, only 14 asteroids have a transverse size exceeding 250 km. The rest are much more modest, there are even asteroids with a diameter of 0.7 km. The largest known asteroids- Ceres, Pallas, Vesta and Hygia (from 1000 to 450 km). Small asteroids do not have a spheroid shape, they are more like shapeless blocks.
The masses of asteroids also fluctuate. The largest mass is determined for Ceres, it is 4000 times smaller than the size of the planet Earth. The mass of all asteroids is also less than the mass of our planet and is one thousandth of it.
All small planets do not have an atmosphere. Some of them have axial rotation, which is established by a regularly recorded change in brightness. So, Pallas has a rotation period of 7.9 hours, and Icarus revolves in just 2 hours and 16 minutes.
According to the reflectivity of asteroids, they were combined into 3 groups - metallic, light and dark. The last group includes asteroids, the surface of which is capable of reflecting no more than 5% of the incident light from the Sun. Their surface is formed by rocks similar to carbonaceous and black basalt. That is why dark asteroids are called carbonaceous.
The reflectivity of light asteroids is the highest (10-25%). These celestial bodies have a surface similar to silicon compounds. They are called stone asteroids. Metallic asteroids are the least common. They are similar to light, the surface of these bodies is more reminiscent of alloys of iron and nickel.
The correctness of such a classification is confirmed by the chemical composition of meteorites falling on the Earth's surface. A small group of asteroids is singled out that cannot be classified according to this feature. The percentage ratio of the 3 given groups of asteroids is as follows: dark (type C) - 75%, light (type S) - 15% and 10% metallic (type M).
The minimum reflectivity of asteroids is 3-4%, and the maximum reaches 40% of the total amount of incident light. Asteroids of small size rotate most rapidly, they are very diverse in shape. Presumably they consist of the substance that formed the solar system. This assumption is confirmed by the change in the dominant type of asteroids belonging to the asteroid belt with distance from the Sun.
In their movement, asteroids inevitably collide with each other, while scattering into small pieces.
The pressure inside the asteroids is not high, so there is no heating of them. Their surface may slightly heat up under the action of sunlight, but this heat is not retained and goes into space. Estimated asteroid surface temperatures fluctuate from -120 °С to -100 °С. A significant increase in temperature, for example, up to +730 °C (Icarus), can be recorded only at the moments of approach to the Sun. After the removal of the asteroid from it, a sharp cooling occurs.
Nathan Eismont,
Candidate of Physical and Mathematical Sciences, Leading Researcher (Space Research Institute of the Russian Academy of Sciences)
Anton Ledkov,
Researcher (Space Research Institute RAS)
"Science and Life" No. 1, 2015, No. 2, 2015
The solar system is usually perceived as an empty space in which eight planets circle, some with their satellites. Someone will remember several small planets, to which Pluto was recently attributed, about the asteroid belt, about meteorites that sometimes fall to Earth, and about comets that occasionally decorate the sky. This idea is quite correct: not one of the many spacecraft has suffered from a collision with an asteroid or a comet - space is quite spacious.
Nevertheless, the huge volume of the solar system contains not hundreds of thousands and not tens of millions, but quadrillions (ones with fifteen zeros) of cosmic bodies of various sizes and masses. All of them move and interact according to the laws of physics and celestial mechanics. Some of them were formed in the very early Universe and consist of its primordial matter, and these are the most interesting objects of astrophysical research. But there are also very dangerous bodies - large asteroids, the collision of which with the Earth can destroy life on it. Tracking and eliminating the asteroid hazard is an equally important and exciting area of work for astrophysicists.
History of the discovery of asteroids
The first asteroid was discovered in 1801 by Giuseppe Piasi, director of the observatory in Palermo (Sicily). He named it Ceres and at first considered it a minor planet. The term "asteroid", translated from ancient Greek - "like a star", was proposed by astronomer William Herschel (see "Science and Life" No. 7, 2012, article "The Tale of the Musician William Herschel, Who Doubled the Cosmos"). Ceres and similar objects (Pallas, Juno and Vesta) discovered in the next six years were seen as points, not as disks in the case of the planets; at the same time, unlike the fixed stars, they moved like planets. It should be noted that the observations that led to the discovery of these asteroids were carried out purposefully in an attempt to find the “missing” planet. The fact is that already discovered planets were located in orbits spaced from the Sun at distances corresponding to Bode's law. In accordance with it, there should have been a planet between Mars and Jupiter. As you know, no planets were found in such an orbit, but an asteroid belt, called the main one, was later discovered approximately in this area. In addition, the Bode law, as it turned out, does not have any physical justification and is now considered simply as a kind of random combination of numbers. Moreover, discovered later (1848) Neptune was in an orbit that was not consistent with it.
After the discovery of the four mentioned asteroids, further observations for eight years did not lead to success. They were stopped due to the Napoleonic Wars, during which the town of Lilienthal near Bremen burned down, where meetings of astronomers - asteroid hunters were held. Observations resumed in 1830, but success came only in 1845 with the discovery of the asteroid Astrea. Since that time, asteroids have been discovered with a frequency of at least one per year. Most of them belong to the main asteroid belt, between Mars and Jupiter. By 1868, there were already about a hundred discovered asteroids, by 1981 - 10,000, and by 2000 - more than 100,000.
Chemical composition, shape, size and orbits of asteroids
If asteroids are classified according to their distance from the Sun, then the first group includes vulcanoids - a kind of hypothetical belt of small planets between the Sun and Mercury. Not a single object from this belt has yet been discovered, and although numerous impact craters formed by the fall of asteroids are observed on the surface of Mercury, this cannot serve as proof of the existence of this belt. Previously, the presence of asteroids there tried to explain the anomalies in the motion of Mercury, but then they were explained on the basis of relativistic effects. So the final answer to the question of the possible presence of Vulcanoids has not yet been received. This is followed by near-Earth asteroids belonging to four groups.
Main belt asteroids move in orbits located between the orbits of Mars and Jupiter, that is, at distances from 2.1 to 3.3 astronomical units (AU) from the Sun. The planes of their orbits are near the ecliptic, their inclination to the ecliptic lies mainly up to 20 degrees, reaching up to 35 degrees for some, eccentricities - from zero to 0.35. Obviously, the largest and brightest asteroids were the first to be discovered: the average diameters of Ceres, Pallas and Vesta are 952, 544 and 525 kilometers, respectively. The smaller the size of the asteroids, the more there are: only 140 of the 100,000 main belt asteroids have an average diameter of more than 120 kilometers. The total mass of all its asteroids is relatively small, accounting for only about 4% of the mass of the Moon. The largest asteroid - Ceres - has a mass of 946·10 15 tons. The value itself seems very large, but it is only 1.3% of the mass of the Moon (735 10 17 tons). As a first approximation, the size of an asteroid can be determined by its brightness and by its distance from the Sun. But we must also take into account the reflective characteristics of the asteroid - its albedo. If the surface of the asteroid is dark, it glows weaker. It is for these reasons that in the list of ten asteroids, located in the figure in the order of their discovery, the third largest asteroid Hygiea is in last place.
Drawings illustrating the main asteroid belt tend to show many boulders moving fairly close together. In fact, the picture is very far from reality, since, generally speaking, a small total mass of the belt is distributed over its large volume, so that space is rather empty. All spacecraft launched to date beyond the orbit of Jupiter have passed through the asteroid belt without any appreciable risk of colliding with an asteroid. However, by the standards of astronomical time, collisions of asteroids with each other and with planets no longer look so unlikely, as can be judged by the number of craters on their surfaces.
Trojans- asteroids moving along the orbits of the planets, the first of which was discovered in 1906 by the German astronomer Max Wolf. The asteroid moves around the Sun in the orbit of Jupiter, ahead of it by an average of 60 degrees. Further, a whole group of celestial bodies was discovered moving ahead of Jupiter.
Initially, they received names in honor of the heroes of the legend of the Trojan War, who fought on the side of the Greeks besieging Troy. In addition to the asteroids leading Jupiter, there is a group of asteroids lagging behind it by about the same angle; they were named Trojans after the defenders of Troy. Currently, asteroids of both groups are called Trojans, and they move in the vicinity of the Lagrange points L 4 and L 5 , points of stable motion in the three-body problem. Celestial bodies that have fallen into their vicinity make an oscillatory motion without going too far. For reasons that have not yet been explained, there are about 40% more asteroids ahead of Jupiter than lagging behind. This was confirmed by recent measurements made by the American satellite NEOWISE using a 40-cm telescope equipped with detectors operating in the infrared range. Measurements in the infrared range significantly expand the possibilities of studying asteroids in comparison with those that give visible light. Their effectiveness can be judged by the number of asteroids and comets in the solar system cataloged using NEOWISE. There are more than 158,000 of them, and the mission of the apparatus continues. Interestingly, the Trojans are markedly different from most of the main belt asteroids. They have a matte surface, a reddish-brown color, and belong mainly to the so-called D-class. These are asteroids with a very low albedo, that is, with a weakly reflective surface. Similar to them can be found only in the outer regions of the main belt.
It's not just Jupiter that has Trojans; other planets of the solar system, including the Earth (but not Venus and Mercury), also accompany the Trojans, grouping in the vicinity of their Lagrange points L 4 , L 5 . The Earth Trojan asteroid 2010 TK7 was discovered with the help of the NEOWISE telescope quite recently - in 2010. It moves ahead of the Earth, while the amplitude of its oscillations near the point L 4 is very large: the asteroid reaches a point opposite to the Earth in motion around the Sun, and unusually far out of the plane of the ecliptic.
Such a large amplitude of oscillations leads to its possible approach to the Earth up to 20 million kilometers. However, a collision with the Earth, at least in the next 20,000 years, is completely excluded. The motion of the terrestrial Trojan is very different from the motion of the Jupiter Trojans, which do not leave their Lagrange points for such significant angular distances. This nature of the motion makes it difficult for spacecraft to reach it, because due to the significant inclination of the Trojan’s orbit to the plane of the ecliptic, reaching the asteroid from the Earth and landing on it requires too high a characteristic velocity and, consequently, high fuel consumption.
Kuiper belt lies outside the orbit of Neptune and extends up to 120 AU. from the sun. It is close to the plane of the ecliptic, inhabited by a huge number of objects that include water ice and frozen gases, and serves as a source of so-called short-period comets. The first object from this region was discovered in 1992, and to date, more than 1300 of them have been discovered. Since the celestial bodies of the Kuiper belt are located very far from the Sun, it is difficult to determine their size. This is done on the basis of measurements of the brightness of the light they reflect, and the accuracy of the calculation depends on how well we know the value of their albedo. Measurements in the infrared range are much more reliable, since they give the levels of self-radiation of objects. Such data were obtained by the Spitzer space telescope for the largest Kuiper belt objects.
One of the most interesting objects of the belt is Haumea, named after the Hawaiian goddess of fertility and childbearing; it is part of a family formed as a result of collisions. This object appears to have collided with another one half the size. The impact caused large chunks of ice to scatter and caused Haumea to rotate with a period of about four hours. Such a fast spin gave it the shape of an American football or melon. Haumea is accompanied by two satellites - Hi'iaka (Hi'iaka) and Namaka (Namaka).
According to currently accepted theories, about 90% of Kuiper belt objects move in distant circular orbits beyond the orbit of Neptune - where they formed. Several dozen objects of this belt (they are called centaurs, because, depending on the distance to the Sun, they manifest themselves either as asteroids or as comets), possibly formed in regions closer to the Sun, and then the gravitational influence of Uranus and Neptune transferred them to high elliptical orbits with aphelions up to 200 AU and great inclinations. They formed a disk 10 AU thick, but the actual outer edge of the Kuiper belt has not yet been determined. More recently, Pluto and Charon were considered as the only examples of the largest objects of icy worlds in the outer part of the solar system. But in 2005, another planetary body was discovered - Eris (named after the Greek goddess of discord), whose diameter is slightly smaller than the diameter of Pluto (initially it was assumed that it was 10% larger). Eris moves in an orbit with a perihelion of 38 AU. and aphelion 98 a.u. She has a small satellite - Dysnomia (Dysnomia). At first, Eris was planned to be considered the tenth (after Pluto) planet in the solar system, but then instead the International Astronomical Union excluded Pluto from the list of planets, forming a new class called dwarf planets, which included Pluto, Eris and Ceres. It is assumed that in the Kuiper belt there are hundreds of thousands of icy bodies with a diameter of 100 kilometers and at least a trillion comets. However, these objects are mostly relatively small—10–50 kilometers across—and not very bright. The period of their revolution around the Sun is hundreds of years, which greatly complicates their detection. If we agree with the assumption that only about 35,000 Kuiper belt objects have a diameter of more than 100 kilometers, then their total mass is several hundred times greater than the mass of bodies of this size from the main asteroid belt. In August 2006, it was reported that eclipses by small objects were found in the X-ray data archive of the neutron star Scorpio X-1. This gave grounds to assert that the number of Kuiper belt objects with sizes of about 100 meters or more is approximately a quadrillion (10 15). Initially, in the earlier stages of the evolution of the solar system, the mass of Kuiper belt objects was much larger than now, from 10 to 50 Earth masses. At present, the total mass of all the bodies of the Kuiper belt, as well as the Oort cloud located even further from the Sun, is much less than the mass of the Moon. As computer simulations show, almost all of the mass of the primordial disk beyond 70 AU. was lost due to collisions caused by Neptune, which led to the grinding of belt objects into dust, which was swept into interstellar space by the solar wind. All of these bodies are of great interest, since it is assumed that they have been preserved in their original form since the formation of the solar system.
Oort cloud contains the most distant objects in the solar system. It is a spherical region that extends over distances from 5,000 to 100,000 AU. from the Sun and is considered as a source of long-period comets reaching the inner region of the solar system. The cloud itself was not instrumentally observed until 2003. In March 2004, a team of astronomers announced the discovery of a planet-like object that orbits the Sun at a record distance, meaning it has a uniquely cold temperature.
This object (2003VB12), named Sedna after the Eskimo goddess who gives life to the inhabitants of the Arctic sea depths, approaches the Sun for a very short time, moving in a highly elongated elliptical orbit with a period of 10,500 years. But even during the approach to the Sun, Sedna does not reach the outer border of the Kuiper belt, which is located at 55 AU. from the Sun: its orbit lies between 76 (perihelion) and 1000 (aphelion) AU. This allowed the discoverers of Sedna to attribute it to the first observed celestial body from the Oort cloud, constantly located outside the Kuiper belt.
According to spectral characteristics, the simplest classification divides asteroids into three groups:
C - carbon (75% known),
S - silicon (17% known),
U - not included in the first two groups.
At present, the above classification is increasingly expanding and detailing, including new groups. By 2002, their number increased to 24. An example of a new group is the M-class of mostly metallic asteroids. However, it should be taken into account that the classification of asteroids according to the spectral characteristics of their surface is a very difficult task. Asteroids of the same class do not necessarily have identical chemical compositions.
Space missions to asteroids
Asteroids are too small for detailed study with ground-based telescopes. They can be imaged using radar, but for this they must fly close enough to the Earth. A rather interesting method for determining the size of asteroids is the observation of occultations of stars by asteroids from several points along the path on a direct star - asteroid - point on the Earth's surface. The method consists in the fact that according to the known trajectory of the asteroid, the points of intersection of the star-asteroid direction with the Earth are calculated, and along this path at some distances from it, determined by the estimated size of the asteroid, telescopes are installed that track the star. At some point, the asteroid obscures the star, it disappears for the observer, and then reappears. From the duration of the shading time and the known speed of the asteroid, its diameter is determined, and with a sufficient number of observers, the silhouette of the asteroid can also be obtained. There is now a community of amateur astronomers who are successfully making coordinated measurements.
Flights of spacecraft to asteroids open up incomparably more opportunities for their study. The first asteroid (951 Gaspra) was photographed by the Galileo spacecraft in 1991 on its way to Jupiter, then in 1993 it took asteroid 243 Ida and its satellite Dactyl. But it was done, so to speak, incidentally.
The first spacecraft specifically designed for asteroid exploration was NEAR Shoemaker, which photographed the asteroid 253 Matilda and then went into orbit around 433 Eros with a landing on its surface in 2001. I must say that the landing was not originally planned, but after the successful study of this asteroid from the orbit of its satellite, they decided to try to make a soft landing. Although the device was not equipped with landing devices and its control system did not provide for such operations, the commands from the Earth managed to land the device, and its systems continued to function on the surface. In addition, the flyby of Matilda made it possible not only to obtain a series of images, but also to determine the mass of the asteroid from the perturbation of the trajectory of the apparatus.
As an incidental task (during the execution of the main one), the Deep Space apparatus explored the asteroid 9969 Braille in 1999 and the Stardust apparatus, the asteroid 5535 Annafranc.
With the help of the Japanese Hayabus apparatus (translated as “hawk”) in June 2010, it was possible to return soil samples to Earth from the surface of asteroid 25 143 Itokawa, which belongs to near-Earth asteroids (Apollos) of spectral class S (silicon). The photo of the asteroid shows rugged terrain with many boulders and cobblestones, of which more than 1000 have a diameter of more than 5 meters, and some are up to 50 meters in size. We will return to this feature of Itokawa later.
The Rosetta spacecraft, launched by the European Space Agency in 2004 to the Churyumov-Gerasimenko comet, successfully landed the Philae module on its nucleus on November 12, 2014. Along the way, the spacecraft flew around asteroids 2867 Steins in 2008 and 21 Lutetia in 2010. The device got its name from the name of the stone (Rosetta) found in Egypt by Napoleonic soldiers near the ancient city of Rosetta on the Nile island of Philae, which gave the lander its name. Texts in two languages are carved on the stone: ancient Egyptian and ancient Greek, which gave the key to revealing the secrets of the civilization of the ancient Egyptians - deciphering hieroglyphs. Choosing historical names, the project developers emphasized the purpose of the mission - to uncover the secrets of the origin and evolution of the solar system.
The mission is interesting because at the time of landing of the Philae module on the surface of the comet's nucleus, it was far from the Sun and therefore was inactive. As it approaches the Sun, the surface of the core heats up and the emission of gases and dust begins. The development of all these processes can be observed, being in the center of events.
Very interesting is the ongoing mission Dawn (Dawn), carried out under the NASA program. The device was launched in 2007, reached the asteroid Vesta in July 2011, then transferred to its satellite orbit and conducted research there until September 2012. Currently, the device is on its way to the largest asteroid - Ceres. On it is an electric rocket ion thruster. Its efficiency, determined by the speed of the expiration of the working fluid (xenon), is almost an order of magnitude higher than the efficiency of traditional chemical engines (see "Science and Life" No. 9, 1999, article "Space electric locomotive"). This made it possible to fly from the orbit of the satellite of one asteroid to the orbit of the satellite of another. Although the asteroids Vesta and Ceres move in fairly close orbits of the main asteroid belt and are the largest in it, they differ greatly in physical characteristics. If Vesta is a “dry” asteroid, then Ceres, according to ground-based observations, has water, seasonal polar caps of water ice, and even a very thin layer of the atmosphere.
The Chinese also contributed to asteroid exploration by sending their Chang'e spacecraft to asteroid 4179 Tautatis. He took a series of photographs of its surface, while the minimum flight distance was only 3.2 kilometers; however, the best shot was taken at a distance of 47 kilometers. The images show that the asteroid has an irregular elongated shape - 4.6 kilometers in length and 2.1 kilometers in diameter. The mass of the asteroid is 50 billion tons, its very curious feature is its very uneven density. One part of the asteroid's volume has a density of 1.95 g/cm 3 , the other one - 2.25 g/cm 3 . In this regard, it has been suggested that Tautatis was formed as a result of the union of two asteroids.
As for asteroid missions in the near future, one could start with the Japanese Aerospace Agency, which plans to continue its research program with the launch of the Hyabus-2 spacecraft in 2015, with the goal of returning soil samples from asteroid 1999 JU3 to Earth in 2020. The asteroid belongs to the spectral class C, is in an orbit that crosses the orbit of the Earth, its aphelion almost reaches the orbit of Mars.
A year later, that is, in 2016, the NASA OSIRIS-Rex project starts, the purpose of which is to return soil from the surface of the near-Earth asteroid 1999 RQ36, recently named Bennu and assigned to spectral class C. It is planned that the device will reach the asteroid in 2018 and in 2023 will deliver 59 grams of its rock to Earth.
Having listed all these projects, it is impossible not to mention an asteroid weighing about 13,000 tons, which fell near Chelyabinsk on February 15, 2013, as if confirming the statement of the famous American specialist on the asteroid problem Donald Yeomans: “If we do not fly to asteroids, then they fly to us ". This emphasized the importance of yet another aspect of the study of asteroids - the asteroid hazard and the solution of problems related to the possibility of asteroids colliding with the Earth.
A very unexpected way to study asteroids was proposed by the Asteroid Redirect Mission, or, as it is called, the Keck project. Its concept was developed by the Keck Institute for Space Research in Pasadena (California). William Myron Keck is a well-known American philanthropist who founded the US Scientific Research Foundation in 1954. In the project, it was assumed as an initial condition that the task of exploring the asteroid is solved with the participation of a person, in other words, the mission to the asteroid must be manned. But in this case, the duration of the entire flight with the return to Earth will inevitably be at least several months. And what is most unpleasant for a manned expedition, in the event of an emergency, this time cannot be reduced to acceptable limits. Therefore, it was proposed, instead of flying to the asteroid, to do the opposite: to deliver, using unmanned vehicles, the asteroid to the Earth. But not to the surface, as it happened with the Chelyabinsk asteroid, but to an orbit similar to the lunar one, and send a manned spacecraft to the asteroid that has become close. This ship will approach it, capture it, and the astronauts will study it, take rock samples and deliver them to Earth. And in an emergency, astronauts will be able to return to Earth within a week. As the main candidate for the role of an asteroid moved in this way, NASA has already chosen the near-Earth asteroid 2011 MD, which belongs to the cupids. Its diameter is from 7 to 15 meters, density is 1 g/cm 3 , that is, it can look like a loose pile of rubble weighing about 500 tons. Its orbit is very close to the orbit of the Earth, inclined to the ecliptic by 2.5 degrees, and the period is 396.5 days, which corresponds to a semi-major axis of 1.056 AU. It is interesting to note that the asteroid was discovered on June 22, 2011, and on June 27 it flew very close to the Earth - only 12,000 kilometers.
A mission to capture an asteroid into Earth satellite orbit is planned for the early 2020s. The spacecraft, designed to capture the asteroid and transfer it to a new orbit, will be equipped with xenon electric thrusters. The operations to change the asteroid's orbit also include a gravitational maneuver near the Moon. The essence of this maneuver is to control the movement with the help of electric rocket engines, which will ensure the passage of the vicinity of the Moon. At the same time, due to the influence of its gravitational field, the speed of the asteroid changes from the initial hyperbolic (that is, leading to the departure from the Earth's gravitational field) to the speed of the Earth's satellite.
Formation and evolution of asteroids
As already mentioned in the section on the history of the discovery of asteroids, the first of them were discovered during the search for a hypothetical planet, which, in accordance with Bode's law (now recognized as erroneous), should have been in orbit between Mars and Jupiter. It turned out that there is an asteroid belt near the orbit of the never discovered planet. This served as the basis for constructing a hypothesis, according to which this belt was formed as a result of its destruction.
The planet was named Phaeton after the son of the ancient Greek sun god Helios. Calculations simulating the process of Phaeton's destruction did not confirm this hypothesis in all its varieties, starting from the planet being torn apart by the gravity of Jupiter and Mars and ending with a collision with another celestial body.
The formation and evolution of asteroids can only be considered as a component of the processes of the emergence of the solar system as a whole. At present, the generally accepted theory suggests that the solar system arose from a primordial accumulation of gas and dust. A disk was formed from the cluster, the inhomogeneities of which led to the emergence of planets and small bodies of the solar system. This hypothesis is supported by modern astronomical observations, which make it possible to detect the development of planetary systems of young stars in their early stages. Computer modeling also confirms it, constructing pictures that are surprisingly similar to pictures of planetary systems at certain phases of their development.
At the initial stage of the formation of the planets, the so-called planetesimals arose - the "embryos" of the planets, on which dust then adhered due to the gravitational influence. As an example of such an initial phase of planetary formation, the asteroid Lutetia is pointed out. This rather large asteroid, reaching 130 kilometers in diameter, consists of a solid part and a thick (up to a kilometer) layer of dust adhering, as well as boulders scattered over the surface. As the mass of the protoplanets increased, the force of attraction and, as a result, the force of compression of the forming celestial body increased. There was a heating of the substance and its melting, leading to the stratification of the protoplanet according to the density of its materials, and the transition of the body to a spherical shape. Most researchers are inclined to the hypothesis that during the initial phases of the evolution of the solar system, many more protoplanets were formed than the planets and small celestial bodies observed today. At that time, the formed gas giants - Jupiter and Saturn - migrated into the system, closer to the Sun. This introduced significant disorder into the movement of the emerging bodies of the solar system and caused the development of a process called the period of heavy bombardment. As a result of resonant influences from mainly Jupiter, part of the resulting celestial bodies was ejected to the outskirts of the system, and part was thrown onto the Sun. This process went on from 4.1 to 3.8 billion years ago. Traces of the period, which is called the late stage of heavy bombardment, remained in the form of many impact craters on the Moon and Mercury. The same thing happened with the formation of bodies between Mars and Jupiter: the frequency of collisions between them was high enough to prevent them from turning into objects larger and more regular than we see today. It is assumed that among them there are fragments of bodies that went through certain phases of evolution, and then split during collisions, as well as objects that did not have time to become parts of larger bodies and, thus, represent samples of more ancient formations. As mentioned above, the Lutetia asteroid is just such a sample. This was confirmed by the studies of the asteroid carried out by the Rosetta spacecraft, including shooting during a close flyby in July 2010.
Thus, Jupiter plays a significant role in the evolution of the main asteroid belt. Due to its gravitational influence, we have obtained the currently observed picture of the distribution of asteroids within the main belt. As for the Kuiper belt, the influence of Neptune is added to the role of Jupiter, leading to the ejection of celestial objects into this remote region of the solar system. It is assumed that the influence of the giant planets extends to an even more distant Oort cloud, which, however, formed closer to the Sun than it is now. In the early phases of the evolution of approaching the giant planets, the primordial objects (planetesimals) in their natural motion performed what we call gravitational maneuvers, replenishing the space attributed to the Oort cloud. Being at such great distances from the Sun, they are also subject to the influence of the stars of our Galaxy - the Milky Way, which leads to their chaotic transition to the return trajectory to the close region of the circumsolar space. We observe these planetesimals as long period comets. As an example, one can point to the brightest comet of the 20th century - Comet Hale-Bopp, discovered on July 23, 1995 and reached perihelion in 1997. The period of its revolution around the Sun is 2534 years, and the aphelion is at a distance of 185 AU. from the sun.
Asteroid-comet hazard
Numerous craters on the surface of the Moon, Mercury and other bodies of the solar system are often mentioned as an illustration of the level of asteroid-comet hazard for the Earth. But such a reference is not entirely correct, since the vast majority of these craters were formed during the "period of heavy bombardment." Nevertheless, with the help of modern technologies, including the analysis of satellite imagery, it is possible to detect traces of collisions with asteroids on the Earth's surface, which belong to much later periods of the evolution of the solar system. The largest and oldest known crater, Vredefort, is located in South Africa. Its diameter is about 250 kilometers, its age is estimated at two billion years.
The Chicxulub crater on the coast of the Yucatan Peninsula in Mexico was formed after an asteroid impact 65 million years ago, equivalent to the energy of an explosion of 100 teraton (10 12 tons) of TNT. It is now believed that the extinction of the dinosaurs was the result of this catastrophic event, which caused tsunamis, earthquakes, volcanic eruptions and climate changes due to the dust layer formed in the atmosphere that covered the Sun. One of the youngest - Barringer Crater - is located in the desert of Arizona, USA. Its diameter is 1200 meters, depth is 175 meters. It arose 50 thousand years ago as a result of the impact of an iron meteorite with a diameter of about 50 meters and a mass of several hundred thousand tons.
In total, there are now about 170 impact craters formed by the fall of celestial bodies. The event near Chelyabinsk attracted the most attention, when on February 15, 2013, an asteroid entered the atmosphere in this area, the size of which was estimated at about 17 meters and a mass of 13,000 tons. It exploded in the air at an altitude of 20 kilometers, its largest part weighing 600 kilograms fell into Lake Chebarkul.
Its fall did not lead to casualties, the destruction was noticeable, but not catastrophic: glass was broken on a fairly large area, the roof of the Chelyabinsk zinc plant collapsed, about 1,500 people were injured by glass fragments. It is believed that the catastrophe did not happen due to the element of luck: the trajectory of the fall of the meteorite was gentle, otherwise the consequences would have been much more difficult. The energy of the explosion is equivalent to 0.5 megatons of TNT, which corresponds to 30 bombs dropped on Hiroshima. The Chelyabinsk asteroid became the most detailed event of this magnitude after the explosion of the Tunguska meteorite on June 17 (30), 1908. According to modern estimates, the fall of celestial bodies, like Chelyabinsk, around the world occurs about once every 100 years. As for the Tunguska event, when trees were burned and felled over an area of 50 kilometers in diameter as a result of an explosion at an altitude of 18 kilometers with an energy of 10–15 megatons of TNT, such disasters happen about once every 300 years. However, there are cases when smaller bodies, colliding with the Earth more often than those mentioned, caused noticeable damage. An example is a four-meter asteroid that fell in Sikhote-Alin northeast of Vladivostok on February 12, 1947. Although the asteroid was small, it was composed almost entirely of iron and turned out to be the largest iron meteorites ever observed on the surface of the Earth. At an altitude of 5 kilometers, it exploded, and the flash was brighter than the Sun. The territory of the epicenter of the explosion (its projection on the earth's surface) was uninhabited, but on an area with a diameter of 2 kilometers, the forest was damaged and more than a hundred craters with a diameter of up to 26 meters were formed. If such an object fell on a large city, hundreds and even thousands of people would die.
At the same time, it is quite obvious that the probability of death of a particular person as a result of an asteroid fall is very low. This does not exclude the possibility that hundreds of years may pass without significant casualties, and then the fall of a large asteroid will lead to the death of millions of people. In table. 1 shows the probabilities of an asteroid impact, correlated with the mortality rate from other events.
It is not known when the next asteroid impact will occur, comparable or more severe in its consequences to the Chelyabinsk event. It may fall in 20 years, and in several centuries, but it may also tomorrow. Getting early warning of an event like the Chelyabinsk event is not only desirable - it is necessary to effectively deflect potentially dangerous objects larger than, say, 50 meters. As for collisions with the Earth of smaller asteroids, these events happen more often than we think: about once every two weeks. This is illustrated by the above map of the fall of asteroids measuring a meter or more over the past twenty years, prepared by NASA.
.Methods for deflecting potentially hazardous near-Earth objects
The discovery in 2004 of the asteroid Apophis, whose probability of collision with the Earth in 2036 was then considered quite high, led to a significant increase in interest in the problem of asteroid-comet defense. Work was launched to detect and catalog dangerous celestial objects, and research programs were launched to solve the problem of preventing their collisions with the Earth. As a result, the number of asteroids and comets found has increased dramatically, so that by now there are more of them discovered than was known before the start of work on the program. Various methods have also been proposed for deflecting asteroids from impact trajectories with the Earth, including rather exotic ones. For example, coating the surfaces of dangerous asteroids with paint that will change their reflective characteristics, leading to the required deflection of the asteroid's trajectory due to the pressure of sunlight. Research continued on ways to change the trajectories of dangerous objects by colliding spacecraft with them. The latter methods seem to be quite promising and do not require the use of technologies that go beyond the capabilities of modern rocket and space technology. However, their effectiveness is limited by the mass of the homing spacecraft. For the most powerful Russian carrier Proton-M, it cannot exceed 5-6 tons.
Let us estimate the change in speed, for example, of Apophis, whose mass is about 40 million tons: a collision with it by a spacecraft weighing 5 tons at a relative speed of 10 km / s will give 1.25 millimeters per second. If the strike is delivered long before the expected collision, it is possible to create the required deflection, but this “long time” will be many decades. It is currently impossible to predict the asteroid's trajectory so far with acceptable accuracy, especially considering that there is uncertainty in knowing the parameters of the impact dynamics and, consequently, in estimating the expected change in the asteroid's velocity vector. Thus, in order to deflect a dangerous asteroid from a collision with the Earth, it is required to find an opportunity to direct a more massive projectile at it. As such, we can offer another asteroid with a mass significantly exceeding the mass of the spacecraft, say 1500 tons. But to control the movement of such an asteroid, too much fuel would be needed to put the idea into practice. Therefore, for the required change in the trajectory of the asteroid projectile, it was proposed to use the so-called gravitational maneuver, which does not require any fuel consumption in itself.
A gravitational maneuver is understood as a flight by a space object (in our case, an asteroid-projectile) of a sufficiently massive body - the Earth, Venus, other planets of the solar system, as well as their satellites. The meaning of the maneuver lies in such a choice of the parameters of the trajectory relative to the flyby body (height, initial position and velocity vector), which will allow, due to its gravitational influence, to change the orbit of an object (in our case, an asteroid) around the Sun so that it will be on the collision trajectory. In other words, instead of imparting a speed impulse to a controlled object with the help of a rocket engine, we receive this impulse due to the attraction of the planet, or, as it is also called, the sling effect. Moreover, the magnitude of the impulse can be significant - 5 km / s or more. To create it with a standard rocket engine, it is necessary to spend an amount of fuel that is 3.5 times the mass of the apparatus. And for the gravitational maneuver method, fuel is needed only to bring the device to the calculated maneuver trajectory, which reduces its consumption by two orders of magnitude. It should be noted that this method of changing the orbits of spacecraft is not new: it was proposed in the early thirties of the last century by the pioneer of Soviet rocket technology F.A. Zander. At present, this technique is widely used in the practice of space flights. Suffice it to mention once again, for example, the European spacecraft Rosetta: in the course of a ten-year mission, it performed three gravitational maneuvers near the Earth and one near Mars. One can recall the Soviet spacecraft Vega-1 and Vega-2, which first circled Halley's comet - on the way to it they performed gravitational maneuvers using the gravitational field of Venus. To reach Pluto in 2015, NASA's New Horizons spacecraft used a maneuver in Jupiter's field. The list of missions using gravity assist is far from exhaustive with these examples.
The use of a gravitational maneuver to guide relatively small near-Earth asteroids to dangerous celestial objects to deviate from the trajectory of a collision with the Earth was proposed by the staff of the Space Research Institute of the Russian Academy of Sciences at an international conference on the problem of asteroid hazard, organized in Malta in 2009. And the following year, a journal publication appeared outlining this concept and justifying it.
To confirm the feasibility of the concept, the asteroid Apophis was chosen as an example of a dangerous celestial object.
Initially, they accepted the condition that the danger of an asteroid is established approximately ten years before its alleged collision with the Earth. Accordingly, the scenario of the asteroid's deviation from the trajectory passing through it was built. First of all, from the list of near-Earth asteroids whose orbits are known, one was chosen, which will be transferred to the vicinity of the Earth into an orbit suitable for performing a gravitational maneuver that ensures that the asteroid hits Apophis no later than 2035. As a selection criterion, we took the magnitude of the velocity impulse that must be communicated to the asteroid in order to transfer it to such a trajectory. The maximum allowable impulse was 20 m/s. Next, a numerical analysis of possible operations to guide the asteroid to Apophis was carried out in accordance with the following flight scenario.
After launching the head unit of the Proton-M launch vehicle into low Earth orbit with the help of the Breeze-M booster unit, the spacecraft is transferred to the flight path to the projectile asteroid with subsequent landing on its surface. The device is fixed on the surface and moves along with the asteroid to the point where it turns on the engine, imparting an impulse to the asteroid, transferring it to the calculated trajectory of the gravitational maneuver - flying around the Earth. In the process of motion, the necessary measurements are taken to determine the motion parameters of both the target asteroid and the projectile asteroid. Based on the measurement results, the projectile trajectory is calculated and corrected. With the help of the propulsion system of the apparatus, the asteroid is given velocity impulses that correct errors in the parameters of the trajectory of movement towards the target. The same operations are performed on the trajectory of the spacecraft's flight to the projectile asteroid. The key parameter in developing and optimizing the scenario is the velocity impulse that must be imparted to the projectile asteroid. For candidates for this role, the dates of the message of the impulse, the arrival of the asteroid to the Earth and the impact with a dangerous object are determined. These parameters are selected in such a way that the momentum imparted to the projectile asteroid is minimal. In the process of research, the entire list of asteroids was analyzed as candidates, the orbital parameters of which are currently known - there are about 11,000 of them.
As a result of calculations, five asteroids were found, the characteristics of which, including sizes, are given in Table. 2. It was hit by asteroids, the dimensions of which significantly exceed the values corresponding to the maximum allowable mass: 1500–2000 tons. In this connection, two remarks must be made. First, a far from complete list of near-Earth asteroids (11,000) was used for the analysis, while, according to modern estimates, there are at least 100,000 of them. boulders on its surface, the mass of which fits within the indicated limits (we can recall the asteroid Itokawa). Note that it is precisely this approach that is assessed as realistic in the American project for the delivery of a small asteroid to the lunar orbit. From Table. 2 it can be seen that the smallest velocity impulse - only 2.38 m/s - is necessary if the asteroid 2006 XV4 is used as a projectile. True, he himself is too big and exceeds the estimated limit of 1500 tons. But if you use its fragment or boulder on the surface with such a mass (if any), then the indicated impulse will create a standard rocket engine with a gas exhaust velocity of 3200 m/s, spending 1.2 tons of fuel. Calculations have shown that a device with a total mass of more than 4.5 tons can be landed on the surface of this asteroid, so the delivery of fuel will not create problems. And the use of an electric rocket engine will reduce fuel consumption (more precisely, the working fluid) to 110 kilograms.
However, it should be taken into account that the data given in the table on the required speed impulses refer to the ideal case, when the required change in the speed vector is realized absolutely exactly. In fact, this is not the case, and, as already noted, it is necessary to have a supply of working fluid for orbit corrections. With the accuracies achieved so far, the correction may require a total of up to 30 m/s, which exceeds the nominal values of the magnitude of the change in speed to solve the problem of intercepting a dangerous object.
In our case, when the controlled object has a mass three orders of magnitude larger, a different solution is required. It exists - this is the use of an electric rocket engine, which makes it possible to reduce the consumption of the working fluid by a factor of ten for the same corrective impulse. In addition, to improve the accuracy of guidance, it is proposed to use a navigation system that includes a small apparatus equipped with a transceiver, which is placed in advance on the surface of a dangerous asteroid, and two sub-satellites accompanying the main apparatus. With the help of transceivers, the distance between the devices and their relative speeds are measured. Such a system makes it possible to ensure that the asteroid-projectile hits the target with a deviation within 50 meters, provided that a small chemical engine with a thrust of several tens of kilograms is used in the last phase of the approach to the target, producing a speed impulse within 2 m/s.
Of the issues that arise when discussing the feasibility of the concept of using small asteroids to deflect dangerous objects, the question of the risk of an asteroid colliding with the Earth, transferred to the trajectory of a gravitational maneuver around it, is essential. In table. 2 shows the distances of asteroids from the center of the Earth at perigee when performing a gravitational maneuver. For four, they exceed 15,000 kilometers, and for asteroid 1994, GV is 7427.54 kilometers (the average radius of the Earth is 6371 kilometers). The distances look safe, but there is still no guarantee that there is no risk if the size of the asteroid is such that it can reach the Earth's surface without burning up in the atmosphere. As the maximum allowable size, a diameter of 8–10 meters is considered, provided that the asteroid is not iron. A radical way to solve the problem is to use Mars or Venus to maneuver.
Capturing asteroids for research
The basic idea of the Asteroid Redirect Mission (ARM) project is to transfer an asteroid to another orbit, more convenient for research with direct human participation. As such, an orbit close to the lunar one was proposed. As another option for changing the asteroid orbit, IKI RAS considered methods for controlling the movement of asteroids using gravity maneuvers near the Earth, similar to those that were developed to guide small asteroids to dangerous near-Earth objects.
The goal of such maneuvers is to transfer asteroids to orbits that are resonant with the orbital motion of the Earth, in particular, with the ratio of the periods of the asteroid and the Earth 1:1. Among the near-Earth asteroids, there are thirteen that can be transferred to resonant orbits in the indicated ratio and at the lower permissible limit of the perigee radius - 6700 kilometers. To do this, it is enough for any of them to report a speed impulse not exceeding 20 m/s. Their list is presented in Table. 3, where the magnitudes of the velocity impulses are indicated, transferring the asteroid to the trajectory of the gravitational maneuver near the Earth, as a result of which the period of its orbit becomes equal to the earth, that is, one year. The maximum and minimum achievable speeds of the asteroid in its heliocentric motion are also given there. It is interesting to note that the maximum speeds can be very high, allowing the maneuver to throw the asteroid quite far from the Sun. For example, the asteroid 2012 VE77 can be sent into an orbit with an aphelion at a distance from the orbit of Saturn, and the rest - beyond the orbit of Mars.
The advantage of resonant asteroids is that they return to the vicinity of the Earth every year. This makes it possible at least every year to send a spacecraft to land on an asteroid and deliver soil samples to Earth, and almost no fuel is required to return the descent vehicle to Earth. In this regard, an asteroid in a resonant orbit has advantages over an asteroid in a lunar orbit, as planned in the Keck project, since it requires a noticeable fuel consumption to return. For unmanned missions, this can be decisive, but for manned flights, when it is necessary to ensure that the device returns to Earth as quickly as possible in an emergency (within a week or even earlier), the advantage may be on the side of the ARM project.
On the other hand, the annual return of resonant asteroids to the Earth allows periodic gravitational maneuvers, each time changing their orbit to optimize research conditions. In this case, the orbit must remain resonant, which is easy to implement by performing multiple gravity maneuvers. Using this approach, it is possible to transfer the asteroid to an orbit identical to the Earth, but slightly inclined to its plane (to the ecliptic). Then the asteroid will approach the Earth twice a year. The family of orbits resulting from a sequence of gravity maneuvers includes an orbit whose plane lies in the ecliptic, but has a very large eccentricity and, like the asteroid 2012 VE77, reaches the orbit of Mars.
If we further develop the technology of gravitational maneuvers for planets, including the construction of resonant orbits, then the idea arises to use the Moon. The fact is that the planet's gravitational maneuver in its pure form does not allow capturing an object into the satellite's orbit, since the energy of its relative motion does not change when flying around the planet. If at the same time it flies around the natural satellite of the planet (the Moon), then its energy can be reduced. The problem is that the reduction should be sufficient to transfer to the satellite's orbit, that is, the initial velocity relative to the planet should be small. If this requirement is not met, the object will leave the vicinity of the Earth forever. But if you choose the geometry of the combined maneuver so that as a result the asteroid remains in a resonant orbit, then in a year you can repeat the maneuver. Thus, it is possible to capture an asteroid into the orbit of the Earth's satellite by applying gravity maneuvers near the Earth while maintaining the resonance condition and coordinated flyby of the Moon.
Obviously, individual examples confirming the possibility of implementing the concept of controlling the motion of asteroids using gravitational maneuvers do not guarantee a solution to the problem of asteroid-comet hazard for any celestial object that threatens to collide with the Earth. It may happen that in a particular case there is no suitable asteroid that can be directed at it. But, as shown by the latest results of calculations carried out taking into account the "fresh" cataloged asteroids, with the maximum allowable velocity impulse required to transfer an asteroid to the planet's vicinity, equal to 40 m/s, the number of suitable asteroids is 29, 193 and 72 for Venus, Earth and Mars respectively. They are included in the list of celestial bodies, the movement of which can be controlled by means of modern rocket and space technology. The list is rapidly growing, as two to five asteroids are currently discovered on average per day. So, for the period from November 1 to November 21, 2014, 58 near-Earth asteroids were discovered. Until now, we could not influence the movement of natural celestial bodies, but a new phase in the development of civilization is beginning, when this becomes possible.
Glossary for the article
Bode's law(the Titius-Bode rule, established in 1766 by the German mathematician Johann Titius and reformulated in 1772 by the German astronomer Johann Bode) describes the distances between the orbits of the planets of the solar system and the Sun, as well as between the planets and the orbits of its natural satellites. One of his mathematical formulations: R i = (D i + 4)/10, where D i = 0, 3, 6, 12 ... n, 2n, and R i is the average radius of the planet's orbit in astronomical units (a. e.).
This empirical law is valid for most planets with an accuracy of 3%, but it seems that it has no physical meaning. There is, however, an assumption that at the stage of formation of the solar system, as a result of gravitational perturbations, a regular ring structure of regions arose in which the orbits of protoplanets turned out to be stable. Later studies of the solar system showed that Bode's law, generally speaking, is far from always being fulfilled: the orbits of Neptune and Pluto, for example, are much closer to the Sun than he predicts (see table).
(L-points, or libration points, from lat. Libration- swinging) - points in the system of two massive bodies, for example, the Sun and a planet or a planet and its natural satellite. A body of significantly smaller mass - an asteroid or a space laboratory - will remain at any of the Lagrange points, oscillating with a small amplitude, provided that only gravitational forces act on it.
The Lagrange points lie in the plane of the orbit of both bodies and are designated by indices from 1 to 5. The first three - collinear - lie on a straight line connecting the centers of massive bodies. Point L 1 is located between massive bodies, L 2 - behind the less massive, L 3 - behind the more massive. The position of the asteroid at these points is the least stable. Points L 4 and L 5 - triangular, or Trojan - are in orbit on both sides of the line connecting the bodies of large mass, at angles of 60 about from the line connecting them (for example, the Sun and the Earth).
Point L 1 of the Earth-Moon system is a convenient place for placing a manned orbital station that allows astronauts to get to the Moon with minimal fuel costs, or an observatory for observing the Sun, which at this point is never obscured by either the Earth or the Moon.
Point L 2 of the Sun-Earth system is convenient for building space observatories and telescopes. The object at this point retains its orientation relative to the Earth and the Sun indefinitely. It already houses the American laboratories Planck, Herschel, WMAP, Gaia and others.
At the point L 3, on the other side of the Sun, science fiction writers have repeatedly placed a certain planet - the Counter-Earth, which either arrived from afar, or was created simultaneously with the Earth. Modern observations have not detected it.
Eccentricity(Fig. 1) - a number characterizing the shape of a second-order curve (ellipse, parabola and hyperbola). Mathematically, it is equal to the ratio of the distance of any point of the curve to its focus to the distance from this point to the straight line, called the directrix. Ellipses - the orbits of asteroids and most other celestial bodies - have two directrixes. Their equations are: x = ±(a/e), where a is the semi-major axis of the ellipse; e - eccentricity - a constant value for any given curve. The eccentricity of the ellipse is less than 1 (for a parabola, e \u003d 1, for a hyperbola, e\u003e 1); when e > 0, the shape of the ellipse approaches a circle; when e > 1, the ellipse becomes more and more elongated and compressed, degenerating into a segment in the limit - its own major axis 2a. Another, simpler and more visual definition of the eccentricity of an ellipse is the ratio of the difference between its maximum and minimum distances to the focus to their sum, that is, the length of the major axis of the ellipse. For circumsolar orbits, this is the ratio of the difference in the distance of a celestial body from the Sun at aphelion and perihelion to their sum (major axis of the orbit).
sunny wind- a constant flow of plasma of the solar corona, that is, charged particles (protons, electrons, helium nuclei, oxygen ions, silicon, iron, sulfur) in radial directions from the Sun. It occupies a spherical volume with a radius of at least 100 AU. That is, the boundary of the volume is determined by the equality of the dynamic pressure of the solar wind and the pressure of interstellar gas, the magnetic field of the Galaxy and galactic cosmic rays.
Ecliptic(from Greek. ekleipsis- eclipse) - a large circle of the celestial sphere, along which the apparent annual movement of the Sun occurs. In reality, since the Earth moves around the Sun, the ecliptic is a section of the celestial sphere by the plane of the Earth's orbit. The ecliptic line runs through the 12 constellations of the zodiac. Its Greek name is due to the fact that it has been known since antiquity: solar and lunar eclipses occur when the Moon is near the point of intersection of its orbit with the ecliptic.
The population of the asteroid belt is very diverse. But all these differences fade before the variety of asteroid orbits. All the planets in the solar system move in the same plane in almost circular orbits. And asteroids, subject to the influence of the Sun and planets, move along a wide variety of trajectories. The main conductor of their movement is, of course, the giant Jupiter. Most of the minor planets are removed from the Sun, on average, by 2.2-3.6 AU, that is, they are located between the orbits of Mars and Jupiter, and are completely subject to the influence of this giant.
The orbital eccentricity of most asteroids is less than 0.3 (between 0.1 and 0.8) and the inclination is less than 16°.
Among the asteroids there are groups that move in the orbit of Jupiter around the Sun, like his retinue. The Greek group (Achilles, Ajax, Odysseus and others) is ahead of Jupiter by 60 °. The Trojan group (Priam, Aeneas, Troilus and others) is 60° behind Jupiter. It is currently believed that the latter group contains about 700 asteroids.
Asteroids "prefer" to meet Jupiter less often, avoiding those orbits in which such encounters can occur regularly. Therefore, some areas of the asteroid belt are almost uninhabited - these are the so-called Kirkwood hatches. Avoiding encounters with Jupiter, some asteroids move in resonance with it, keeping their orbital periods in a simple ratio with the period of revolution of the giant planet. The simplest case of such a resonance with a period ratio of 1:1 is the Trojans. In 1866, the American astronomer Kirkwood discovered the existence of gaps in the distribution of the rotation periods of asteroids and in the distribution of the semi-major axes of their orbits. Kirkwood found that asteroids avoid those periods that are in a simple integer ratio with the period of Jupiter's revolution around the Sun, for example, 1:2, 1:3, 2:5, etc. Due to the gravitational influence of Jupiter, the asteroids change their orbit and leave this region of space.
However, asteroids are not only located between the orbits of Jupiter and Mars - some of them are scattered throughout the solar system, and each planet probably has its own group of asteroids.
A study of the unnamed asteroid 3753, conducted by the Canadian astronomer Wigert, showed that this asteroid surprisingly accompanies the Earth: the average radius of its orbit is almost equal to that of the Earth, and therefore the periods of their revolution around the Sun almost coincide. Slowly, slowly, the asteroid is approaching the Earth, and having approached, slightly changes its orbit under the influence of the forces of Earth's gravity. If an asteroid lags behind the Earth, then it approaches it from the front, and the gravity of the Earth slows it down. From this, the size of the asteroid's orbit and the period of revolution along it are reduced, and it begins to outstrip the Earth, ending up behind it. Now the Earth's gravity causes the asteroid to move to a higher orbit with a longer period, and the original situation is repeated. If the orbit of asteroid 3753 were close to circular, its trajectory relative to the Earth would resemble a horseshoe. But the large eccentricity (e = 0.515) and inclination (i = 20°) of the asteroid's orbit make its motion even more intricate. Being influenced not only by the Sun and the Earth, but also by all other planets, it cannot move steadily along a horseshoe-shaped orbit. Calculations show that 2500 years ago asteroid 3753 crossed the orbit of Mars, and about 8000 years ago it should cross the orbit of Venus; in this case, a transition under the influence of its gravity to a new orbit and even a collision with the planet is quite possible.
It is important for the inhabitants of the Earth to know the asteroids whose orbits are close to it. There are three families of asteroids (according to their typical representatives):
1221 Cupid; the orbit at perihelion almost touches the Earth;
1862 Apollo; the orbit at perihelion goes beyond the Earth's orbit;
2962 Aten; family cross the earth's orbit.
Some asteroids move in resonance with several planets at once. This was first seen in the motion of the asteroid Toro. It makes 5 orbital revolutions in approximately the same time as the Earth - 8, Venus - 13. The perihelion of the asteroid Toro is located between the orbits of Venus and the Earth. Another asteroid, Amur, moves in resonance with Venus, Earth, Mars and Jupiter, making 3 of its revolutions in the same time during which Venus makes 13 revolutions, the Earth - 8 revolutions; resonance with Mars 12:17 and with Jupiter 9:2. Obviously, such a movement protects the asteroids from being captured by the planet's gravitational field and prolongs their life.
Many asteroids are beyond the orbit of Jupiter. In 1977, the asteroid 2060 Chiron was discovered, the orbit of which is as follows: perihelion inside the orbit of Saturn 8.51 AU, aphelion near the orbit of Uranus 19.9 AU. Chiron's orbital eccentricity is 0.384.
Near perihelion, Chiron develops a coma and a tail. However, the size and mass of Chiron is much larger than the size of ordinary comets. In ancient Greek mythology, Chiron is half-man, half-horse; cosmic Chiron is either an asteroid or a comet. Now such objects are called centaurs.
In 1992, even more distant objects were discovered, larger than 200 km, far beyond the orbits of Neptune and Pluto. The total number of bodies in the Kuiper belt, according to experts, is several times greater than the number of asteroids between the orbits of Mars and Jupiter.
In 1993, the Galileo interplanetary probe, flying past the asteroid 243 Ida, discovered a small satellite with a diameter of 1.5 km, called Dactyl, which revolves around 243 Ida at a distance of about 100 km. This was the first time a satellite was discovered around an asteroid. Then came a message from the Southern European Observatory in La Silla (Chile) about finding a second satellite, this time around the asteroid 3671 Dionysus. Currently, 7 asteroids are known to have small satellites.
Dionysus was included in the list of candidates for research, because it belongs to a special group of asteroids that periodically cross the Earth's orbit and have a chance to collide with our planet. The asteroid 1862 Apollo, discovered in 1934, became the prototype of this group, so all asteroids with such orbits are now referred to the Apollo group. Dionysus comes close to Earth once every 13 years. This is exactly what happened on July 6, 1997, when it passed at a distance of 17 million km from the Earth. According to the thermal radiation of Dionysus, astronomers determined that its surface is very bright, well reflecting the sun's rays, and its diameter is about 1 km. Recall that the asteroid Ida, in which a satellite was first discovered, has a diameter of 50 km.
In 1992, the asteroid Tutatis passed at a distance of only 2.5 million km from the Earth. It turned out to be formed, as it were, by two blocks, the dimensions of which are 2 km and 3 km. Since then, the term has appeared: contact-double asteroids.
It is too early to talk about the origin of double, and possibly more complex asteroids. It is necessary to accumulate observational data. But one thing is clear: the more complex the space system, the more valuable information it carries about its origin and evolution.
Astronomers have already found more than a thousand asteroids crossing the Earth's orbit. Perhaps in the future, scientists will have to work hard to prevent a collision of any of them with our planet.
As you know, all the planets of our solar system move in the same plane, along almost circular trajectories. And individual celestial bodies are asteroids, they are subject to the influence of the Sun and planets in the system and move in different orbits.
Huge Jupiter has a huge impact on the orbits of asteroids. Many minor planets are at a distance of 2.2-3.6 AU from the Sun, and these minor planets are located between the orbits of Mars and Jupiter, which means they are under the influence of the planet Jupiter. The eccentricity of the asteroids' trajectory is less than 0.3 (0.1-0.8), and the inclination itself is less than 16 degrees. Among the moving asteroids, there are groups that make a trajectory around the Sun in the orbit of the planet Jupiter.
There are such groups as the "Greeks" - "Achilles", "Odysseus", "Ajax" and many others who are 60 degrees ahead of the planet Jupiter in their movement. And a group called "Trojans" - "Aeneas", "Priam", "Troilus" and many others, on the contrary, lag behind the planet Jupiter by 60 degrees in their movement.
At the moment, according to the latest studies, there are about 700 asteroids in the last group. These asteroids are much less likely to hit the planet Jupiter, avoiding those trajectories on which such encounters can occur regularly. Kirkwood hatches are precisely those places in the asteroid belt that are almost uninhabited. Some asteroids, not meeting the planet Jupiter, make their movement in resonance with it. The most striking example of this movement is the "Trojans", they make movements in the ratio of the period of one to one. The American astronomer Kirkwood in 1866 made a discovery in the field of astronomy - the existence of gaps in the division of the periods of revolution of asteroids and in the distribution of the largest semi-axes of their trajectories. This scientist found out that asteroids do not make periods that are located in an elementary ratio with the period of rotation of the planet Jupiter around the Sun, for example, in a ratio of one to two, one to three, two to five, etc. Under the influence of the gravitational influence of the planet Jupiter, the asteroids change their trajectory and disappear from this outer space. Not all asteroids are located between the orbits of the planets Mars and Jupiter, some of the asteroids are scattered throughout the solar system, and any planet in this system theoretically has its own "retinue" of asteroids. Canadian astronomer Wigert conducted a study of an asteroid that does not have its own name, but has the assigned code 3753, and found out that this asteroid always accompanies our planet: the approximate radius of the orbit of this asteroid is almost equal to the radius of the orbit of our planet, and their periods of rotation around the Sun are almost the same . The asteroid itself is slowly approaching our planet, and having approached it, it changes its trajectory under the influence of the gravitational attraction of our planet. And if an asteroid begins to lag behind our planet, then it makes its approach from the front, and the very gravity of our planet slows down this process. And because of this, the very circumference of the asteroid's orbit and the period of rotation along it are shortened, and after that it begins to bypass the planet Earth, ending up behind our planet.
The very gravitational attraction of our planet creates the transition of the asteroid to a more extensive trajectory, and the final situation is repeated. Theoretically, if the trajectory of an asteroid codenamed 3753 were circular, then its orbit relative to our planet would be identical to the shape of a horseshoe. The huge eccentricity, which is equal to e = 0.515, and the inclination itself, which equals i = 20 degrees, makes the asteroid's trajectory itself more bizarre. This asteroid, which is influenced not only by our planet and the Sun, but also by many other planets, cannot have a constant horseshoe trajectory. These studies suggest that 2500 thousand years ago an asteroid, codenamed "3453" crossed the orbit of the planet Mars, and in 8000 it should cross the trajectory of the planet Venus. At the same time, there is a theory that this asteroid, under the influence of the gravity of Venus, can switch to a new trajectory of motion, and there is also a potential danger of a collision with the planet.
Earthlings always need to know all the asteroids that are close to our planet. There are three types of classifications of asteroids (according to their characteristic representatives): the Amur asteroid, codenamed "1221"; its orbit at perihelion almost reaches our planet; asteroid "Apollo", codenamed "1862"; its orbit at perihelion wraps around the orbit of our planet; asteroid "Aton", codenamed "2962"; family crossing the orbit of our planet. A small number of asteroids make their trajectory in resonance with several planets at the same time. This was first discovered in the trajectory of the asteroid Toro. This asteroid makes five orbits, almost the same amount of time that Earth takes about eight and Venus about thirteen.
The points of the orbit of the asteroid "Toro" are located between the trajectories of the planets Venus and Earth. And another celestial body - the Amur asteroid, makes its movement in resonance with the planets Earth, Mars, Venus and Jupiter, making three revolutions, at the same time when the Earth makes eight revolutions; and the resonance with the planet Mars is 12:17 and with the planet Jupiter 9:2. Such trajectories of movements of asteroids protect them from the influence of the gravitational field of the planets, and this increases their life expectancy. As we already know, a large number of asteroids are located behind the trajectory of the planet Jupiter. When the asteroid Chiron was discovered in 1977, the following was discovered: the points of the orbit of this asteroid were inside the orbit of Saturn (8.51 AU), and the aphelion itself was located near the trajectory of the planet Uranus (19.9 AU). ).
The eccentricity of the orbit of the asteroid "Chiron" is 0.384, near the perihelion, the asteroid "Chiron" has a tail and a coma. But in terms of parameters, the Chiron astroid far exceeds many ordinary comets. If we draw an analog with ancient Greek mythology, that is, with something to compare, in the myths Chiron is a character who was half-man, half-horse, at the same time, the Chiron asteroid is half a comet-asteroid, there is no exact definition of it. At the moment, such celestial bodies are called centaurs. Far beyond the orbits of the planets Neptune and Pluto, in 1992, even more distant celestial bodies were discovered, which in their size reached more than 200 kilometers. The number of celestial bodies in the Kuiper belt, according to scientists, is much greater than the number of celestial bodies that are located between the trajectories of the planets Mars and Jupiter. The interplanetary spacecraft "Galileo", in 1993, moving past the asteroid "Ida", codenamed "243", discovered a small satellite, which reached a diameter of about 1.5 kilometers. This orbiting satellite around the asteroid "Ida" at a distance of 100 kilometers was called "Dactyl". This satellite was the very first satellite that became known to science. But soon a notification was received from Chile, the city of La Silla from the Southern European Observatory that they had discovered a satellite near the asteroid "Dionysus" with the code name "3671".
At the moment, science knows about seven asteroids that have their own satellites. The asteroid "Dionysus" was included in the list of those candidates that require more detailed study, since it belongs to the group of asteroids that intersect the orbit of our planet with repeated periods and have the potential danger of colliding with the Earth.
The Apollo asteroid discovered in 1934 with the code name 1862 became an analogue of this group, and after that, all discovered asteroids with similar orbits began to be attributed to the Apollo group. The asteroid "Dionysus" approaches the Earth once every thirteen years, and this was 07/06/1997, when it passed at a distance of about 17 million kilometers from planet Earth. Scientists-astronomers on the thermal radiation of the asteroid "Dionysus" were able to calculate that its surface is very bright and reflects the sun's rays well, and the diameter of the asteroid itself reaches about one kilometer. It should be recalled that the asteroid Ida, which was one of the first to have a satellite discovered, reaches about 50 kilometers in diameter. The asteroid "Tutatis", making its usual trajectory, in 1992 passed at a distance of 2.5 million kilometers from our planet. Later, it turned out that this asteroid was formed with the help of two blocks, the dimensions of which reached two and three kilometers. After that, the term "contact-double" asteroids appeared. But it is still too early to talk about this type of asteroids, since more information about this type of asteroids is needed. But it becomes clear that the more complex the Universe, the more valuable information it brings about its origin and evolution.
At the moment, astronomers have already identified about 1000 asteroids that crossed the very orbit of our planet. And theoretically, scientists will have to work hard to prevent the potential threat from asteroids.
asteroid orbits
Asteroids of the Main Belt move in stable orbits, close to circular or slightly eccentric. They are in the "safe" zone, where the gravitational influence of the large planets on them is minimal, in the first place, Jupiter. It is believed that it is Jupiter that is “to blame” for the fact that a large planet could not form on the site of the Main Asteroid Belt during the youth of the solar system.
However, at the beginning of the 20th century many scientists believed that between Jupiter and Mars there used to be a large planet, which for some reason collapsed. Olbers was the first to express this hypothesis, immediately after his discovery of Pallas. He also suggested calling the hypothetical planet Phaeton. However, modern cosmogony has abandoned the idea of the destruction of a large planet: the asteroid belt probably always contained many small bodies, which were prevented from uniting by the influence of Jupiter.
This giant still continues to play a primary role in the evolution of asteroid orbits. Its long-term (more than 4 billion years) gravitational influence on the asteroids of the Main Belt led to the emergence of a number of "forbidden" orbits and even zones in which there are practically no small bodies, and if they get there, they cannot stay there for a long time . These zones are called Kirkwood gaps (or hatches) after Daniel Kirkwood (1814-1895), who first discovered them in the distribution of the orbital periods of just a few dozen asteroids.
Orbits in Kirkwood hatches are called resonant, since the asteroids moving along them experience regular gravitational perturbation from Jupiter at the same points of their orbit. The periods of revolution in these orbits are in simple relation to the period of revolution of Jupiter (for example, 1:2, 3:7, 2:5, 1:3). If any asteroid, for example, as a result of a collision with another body, falls into a resonant orbit, then its eccentricity and semi-major axis rapidly change under the influence of Jupiter's gravitational field. The asteroid is leaving its resonant orbit and may even leave the Main Belt. This is Kirkwood's permanent "clean-up" mechanism.
However, we note that if we depict the instantaneous distribution of all the asteroids of the Main Belt, then we will not see any “gaps”. At any given time, the asteroids fill the belt quite evenly, because, moving in elliptical orbits, they often cross the “forbidden zones”.
There is another, opposite, example of the gravitational influence of Jupiter: at the outer boundary of the Main Asteroid Belt there are two narrow "zones" containing an excess number of asteroids. The periods of revolution in them are in proportions of 2:3 and 1:1 with the period of revolution of Jupiter. It is clear that the 1:1 resonance means that the asteroids are moving almost in the orbit of Jupiter. But they do not approach the giant planet, but keep a distance, on average, equal to the radius of Jupiter's orbit. These asteroids were named after the heroes of the Trojan War. Those of them that are ahead of Jupiter in their orbit are called "Greeks", and the lagging group is called "Trojans" (both groups together are often called "Trojans"). The movement of these small bodies takes place in the vicinity of the “triangular Lagrange points”, where gravitational and centrifugal forces are equalized during circular motion. It is important that with a small deviation from the equilibrium position, forces arise that tend to return the object to its place, i.e. its movement is steady.
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