What types of asteroids are there? Asteroid – Magazine "All about Space". What is the probability that the Earth will be destroyed by an asteroid collision?
Asteroids are relatively small celestial bodies moving in orbit around the Sun. They are significantly smaller in size and mass than planets, have an irregular shape and do not have an atmosphere.
In this section of the site, everyone can learn many interesting facts about asteroids. You may already be familiar with some, others will be new to you. Asteroids are an interesting spectrum of the Cosmos, and we invite you to familiarize yourself with them in as much detail as possible.
The term "asteroid" was first coined by the famous composer Charles Burney and used by William Herschel based on the fact that these objects, when viewed through a telescope, appear as points of stars, while planets appear as disks.
There is still no precise definition of the term “asteroid”. Until 2006, asteroids were usually called minor planets.
The main parameter by which they are classified is body size. Asteroids include bodies with a diameter greater than 30 m, and bodies with a smaller size are called meteorites.
In 2006, the International Astronomical Union classified most asteroids as small bodies in our solar system.
To date, hundreds of thousands of asteroids have been identified in the Solar System. As of January 11, 2015, the database included 670,474 objects, of which 422,636 had orbits determined, they had an official number, more than 19 thousand of them had official names. According to scientists, there may be from 1.1 to 1.9 million objects in the solar system larger than 1 km. Most of the asteroids currently known are located within the asteroid belt, located between the orbits of Jupiter and Mars.
The largest asteroid in the Solar System is Ceres, measuring approximately 975x909 km, but since August 24, 2006 it has been classified as a dwarf planet. The remaining two large asteroids (4) Vesta and (2) Pallas have a diameter of about 500 km. Moreover, (4) Vesta is the only object in the asteroid belt that is visible to the naked eye. All asteroids that move in other orbits can be tracked during their passage near our planet.
As for the total weight of all main belt asteroids, it is estimated at 3.0 - 3.6 1021 kg, which is approximately 4% of the weight of the Moon. However, the mass of Ceres accounts for about 32% of the total mass (9.5 1020 kg), and together with three other large asteroids - (10) Hygiea, (2) Pallas, (4) Vesta - 51%, that is, most asteroids are different an insignificant mass by astronomical standards.
Asteroid exploration
After William Herschel discovered the planet Uranus in 1781, the first discoveries of asteroids began. The average heliocentric distance of asteroids follows the Titius-Bode rule.
Franz Xaver created a group of twenty-four astronomers at the end of the 18th century. Beginning in 1789, this group specialized in searching for a planet that, according to the Titius-Bode rule, should be located at a distance of approximately 2.8 astronomical units (AU) from the Sun, namely between the orbits of Jupiter and Mars. The main task was to describe the coordinates of stars located in the area of zodiacal constellations at a specific moment. The coordinates were checked on subsequent nights, and objects moving over long distances were identified. According to their assumption, the displacement of the desired planet should be about thirty arcseconds per hour, which would be very noticeable.
The first asteroid, Ceres, was discovered by the Italian Piazii, who was not involved in this project, completely by accident, on the first night of the century - 1801. The three others—(2) Pallas, (4) Vesta, and (3) Juno—were discovered over the next few years. The most recent (in 1807) was Vesta. After another eight years of pointless searching, many astronomers decided that there was nothing more to look for there and abandoned all attempts.
But Karl Ludwig Henke showed persistence and in 1830 he again began searching for new asteroids. 15 years later he discovered Astraea, which was the first asteroid in 38 years. And after 2 years he discovered Hebe. After this, other astronomers joined the work, and then at least one new asteroid was discovered per year (except 1945).
The astrophotography method for searching for asteroids was first used by Max Wolf in 1891, according to which asteroids left short light lines in photographs with a long exposure period. This method significantly accelerated the identification of new asteroids compared to visual observation methods used previously. Alone, Max Wolf managed to discover 248 asteroids, while few before him managed to find more than 300. Nowadays, 385,000 asteroids have an official number, and 18,000 of them also have a name.
Five years ago, two independent teams of astronomers from Brazil, Spain and the United States announced that they had simultaneously identified water ice on the surface of Themis, one of the largest asteroids. Their discovery made it possible to find out the origin of water on our planet. At the beginning of its existence, it was too hot, unable to hold large amounts of water. This substance appeared later. Scientists have suggested that comets brought water to Earth, but the isotopic compositions of water in comets and terrestrial water do not match. Therefore, we can assume that it fell on Earth during its collision with asteroids. At the same time, scientists discovered complex hydrocarbons on Themis, incl. molecules are the precursors of life.
Name of asteroids
Initially, asteroids were given the names of heroes of Greek and Roman mythology; later discoverers could call them whatever they wanted, even their own name. At first, asteroids were almost always given female names, while only those asteroids that had unusual orbits received male names. Over time, this rule was no longer observed.
It is also worth noting that not any asteroid can receive a name, but only one whose orbit has been reliably calculated. There have often been cases when an asteroid was named many years after its discovery. Until the orbit was calculated, the asteroid was given only a temporary designation reflecting the date of its discovery, for example, 1950 DA. The first letter means the number of the crescent in the year (in the example, as you can see, this is the second half of February), respectively, the second indicates its serial number in the specified crescent (as you can see, this asteroid was discovered first). The numbers, as you might guess, indicate the year. Since there are 26 English letters, and 24 crescents, two letters have never been used in the designation: Z and I. In the event that the number of asteroids discovered during a crescent is more than 24, scientists returned to the beginning of the alphabet, namely, writing the second letter - 2, respectively, on the next return - 3, etc.
The name of the asteroid after receiving the name consists of a serial number (number) and name - (8) Flora, (1) Ceres, etc.
Determining the size and shape of asteroids
The first attempts to measure the diameters of asteroids using the method of directly measuring visible disks with a filament micrometer were made by Johann Schröter and William Herschel in 1805. Then, in the 19th century, other astronomers used exactly the same method to measure the brightest asteroids. The main disadvantage of this method is significant discrepancies in the results (for example, the maximum and minimum sizes of Ceres, which were obtained by astronomers, differed by 10 times).
Modern methods for determining the size of asteroids consist of polarimetry, thermal and transit radiometry, speckle interferometry, and radar methods.
One of the highest quality and simplest is the transit method. When an asteroid moves relative to the Earth, it can pass against the background of a separated star. This phenomenon is called “coating of stars by asteroids.” By measuring the duration of the star's brightness decline and having data on the distance to the asteroid, it is possible to accurately determine its size. Thanks to this method, it is possible to accurately calculate the sizes of large asteroids, like Pallas.
The polarimetry method itself consists of determining the size based on the brightness of the asteroid. The size of the asteroid determines the amount of sunlight it reflects. But in many ways, the brightness of an asteroid depends on the albedo of the asteroid, which is determined by the composition of which the asteroid's surface is made. For example, due to its high albedo, the asteroid Vesta reflects four times more light compared to Ceres and is considered the most visible asteroid, which can often be seen even with the naked eye.
However, the albedo itself is also very easy to determine. The lower the brightness of an asteroid, that is, the less it reflects solar radiation in the visible range, the more it absorbs it; after it heats up, it emits it as heat in the infrared range.
It can also be used to calculate 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 the largest structures on the surface. In addition, the results obtained from infrared telescopes are used for sizing through thermal radiometry.
Asteroids and their classification
The general classification of asteroids is based on the characteristics of their orbits, as well as a description of the visible spectrum of sunlight that is reflected by their surface.
Asteroids are usually grouped into groups and families based on the characteristics of their orbits. Most often, a group of asteroids is named after the very first asteroid discovered in a given orbit. Groups are a relatively loose formation, while families are denser, formed in the past during the destruction of large asteroids as a result of collisions with other objects.
Spectral classes
Ben Zellner, David Morrison, and Clark R. Champain developed a general system for classifying asteroids in 1975, which was based on albedo, color, and characteristics of the spectrum of reflected sunlight. At the very beginning, this classification defined exclusively 3 types of asteroids, namely:
Class C – carbon (most known asteroids).
Class S – silicate (about 17% of known asteroids).
Class M - metal.
This list was expanded as more and more asteroids were studied. The following classes have appeared:
Class A - characterized by a high albedo and a reddish color in the visible part of the spectrum.
Class B - belong to class C asteroids, but they do not absorb waves below 0.5 microns, and their spectrum is slightly bluish. In general, the albedo is higher compared to other carbon asteroids.
Class D - have a low albedo and a smooth reddish spectrum.
Class E - the surface of these asteroids contains enstatite and is similar to achondrites.
Class F - similar to Class B asteroids, but do not have traces of “water”.
Class G - have a low albedo and an almost flat reflectance spectrum in the visible range, which indicates strong UV absorption.
Class P - just like D-class asteroids, they are distinguished by a low albedo and a smooth reddish spectrum that does not have clear absorption lines.
Class Q - have broad and bright lines of pyroxene and olivine at a wavelength of 1 micron and features indicating the presence of metal.
Class R - characterized by a relatively high albedo and at a length of 0.7 microns have a reddish reflection spectrum.
Class T - characterized by a reddish spectrum and low albedo. The spectrum is similar to D and P class asteroids, but is intermediate in inclination.
Class V - characterized by moderate brightness and similar to the more general S-class, which are also largely composed of silicates, stone and iron, but are characterized by a high pyroxene content.
Class J is a class of asteroids that are believed to have formed from the interior of Vesta. Despite the fact that their spectra are close to those of class V asteroids, at wavelengths of 1 micron they are distinguished by strong absorption lines.
It is worth considering that the number of known asteroids that belong to a certain type does not necessarily correspond to reality. Many types are difficult to determine; the type of an asteroid may change with more detailed studies.
Asteroid size distribution
As the size of asteroids grew, their number noticeably decreased. Although this generally follows a power law, there are peaks at 5 and 100 kilometers where there are more asteroids than predicted by the logarithmic distribution.
How asteroids were formed
Scientists believe that planetesimals in the asteroid belt evolved in the same way as in other regions of the solar nebula until the planet Jupiter reached its current mass, after which, as a result of orbital resonances with Jupiter, 99% of the planetesimals were thrown out of the belt. Modeling and jumps in spectral properties and rotation rate distributions indicate that asteroids larger than 120 kilometers in diameter formed by accretion during this early era, while smaller bodies represent debris from collisions between different asteroids after or during the dispersal of the primordial belt by Jupiter's gravity . Vesti and Ceres acquired an overall size for gravitational differentiation, during which heavy metals sank to the core, and a crust formed from relatively rocky rocks. As for the Nice model, many Kuiper belt objects formed in the outer asteroid belt, at a distance of more than 2.6 astronomical units. Moreover, later most of them were thrown out by Jupiter’s gravity, but those that survived may belong to class D asteroids, including Ceres.
Threat and danger from asteroids
Despite the fact that our planet is significantly larger than all asteroids, a collision with a body larger than 3 kilometers in size could cause the destruction of civilization. If the size is smaller, but more than 50 m in diameter, then it can lead to enormous economic damage, including numerous casualties.
The heavier and larger the asteroid, the more dangerous it poses, but in this case it is much easier to identify it. At the moment, the most dangerous asteroid is Apophis, whose diameter is about 300 meters; a collision with it can destroy an entire city. But, according to scientists, in general it does not pose any threat to humanity in a collision with the Earth.
Asteroid 1998 QE2 approached the planet on June 1, 2013 at its closest distance (5.8 million km) in the last two hundred years.
Asteroids Asteroid In Greek it means like a star.- small cosmic bodies of irregular shape, encircling the Sun in different orbits. These bodies are more than 30 meters in diameter and do not have their own atmosphere.The bulk of them are located in the belt, which stretches between the orbits of Jupiter and. The belt has the shape of a torus, and its density decreases beyond a distance of 3.2 AU.
Until August 24, 2006, Ceres was considered the largest asteroid (975x909 km), but they decided to change its status, assigning it the title of a dwarf planet. And the total mass of all objects of the main belt is small - 3.0 - 3.6.1021 kg, which is 25 times less than the mass.
Photo of the dwarf planet Ceres
Sensitive photometers make it possible to study changes in the brightness of cosmic bodies. The result is a light curve, from the shape of which you can determine the rotation period of the asteroid and the location of its rotation axis. The frequency ranges from several hours to several hundred hours. The light curve can also help determine asteroid shapes. Only the largest objects approach the shape of a ball; the rest have an irregular shape.
Based on the nature of the change in brightness, it can be assumed that some asteroids have satellites, while others are binary systems or bodies that roll over each other’s surfaces.
The orbits of asteroids change under the powerful influence of the planets, and Jupiter has a particularly strong influence on their orbits. It has led to the fact that there are entire zones where small planets are absent, and if they manage to get there, it is only for a very short time. Such zones, called hatches or Kirkwood gaps, alternate with areas filled with cosmic bodies that form families. The main part of asteroids is divided into families, which were most likely formed fromcrushing larger bodies. These clusters are named after their largest member.
At a distance after 3.2 AU. Two flocks of asteroids – the Trojans and the Greeks – are circling in Jupiter’s orbit. One flock (Greeks) overtakes the gas giant, while the other (Trojans) lags behind. These groups move quite steadily because they are located at “Lagrange points”, where the gravitational forces acting on them are equal. Their divergence angle is the same - 60°. The Trojans were able to accumulate over a long period of time after the evolution of the collisions of various asteroids. But there are other families with very close orbits, formed by recent decays of their parent bodies. Such an object is the Flora family, which has about 60 members.
Interaction with the Earth
Not far from the inner edge of the main belt there are groups of bodies whose orbits can intersect with the orbits of the Earth and the terrestrial planets. The main objects include the Apollo, Amur, and Aten groups. Their orbits are unstable, depending on the influence of Jupiter and other planets. The division of such asteroids into groups is quite arbitrary, because they can move from group to group. Such objects cross the Earth's orbit, creating a potential threat. The Earth's orbit is periodically crossed by about 2000 objects whose size is more than 1 km.
They are either fragments of larger asteroids, or cometary nuclei from which all the ice has evaporated. In 10 - 100 million years, these bodies will definitely fall on the planet that attracts them, or on the Sun.
Asteroids in Earth's past
The most famous event of this kind was the fall of an asteroid 65 million years ago, when half of everything living on the planet died. It is believed that the size of the fallen body was about 10 km, and the epicenter was the Gulf of Mexico. Traces of a hundred-kilometer crater were also discovered on Taimyr (in the bend of the Popigai River). On the surface of the planet there are about 230 astroblemes - large impact ring formations.
Compound
Asteroids can be classified according to their chemical composition and morphology. Determining the size of such a small body as an asteroid in the vast Solar System, which also does not emit light, is extremely difficult. This helps to implement the photometric method - measuring the brightness of a celestial body. The properties of asteroids are judged by the properties and nature of the reflected light. So, using this method, all asteroids were divided into three groups:
- Carbon– type C. There are the most of them – 75%. They reflect light poorly and are located on the outside of the belt.
- Sandy– type S. These bodies reflect light more strongly and are located in the inner zone.
- Metal– type M. Their reflectivity is similar to bodies of group S, and they are located in the central zone of the belt.
The composition of asteroids is similar, because the latter are actually their fragments. Their mineralogical composition is not diverse. Only about 150 minerals have been identified, while there are more than 1000 on Earth.
Other asteroid belts
Similar space objects exist outside of orbit. There are quite a lot of them in the peripheral areas of the solar system. Beyond the orbit of Neptune is the Kuiper Belt, which contains hundreds of objects with sizes ranging from 100 to 800 km.
Between the Kuiper belt and the main asteroid belt there is another collection of similar objects belonging to the “Centaur class”. Their main representative was the asteroid Chiron, which sometimes pretends to be a comet, becoming covered in a coma and spreading its tail. This two-faced type has a strand size of 200 km and is proof that comets and asteroids have a lot in common.
Origin hypotheses
What is an asteroid - a fragment of another planet or proto-matter? This is still a mystery that people have been trying to solve for a long time. Here are two main hypotheses:
Explosion of the planet. The most romantic version is the exploding mythical planet Phaeton. It was supposedly inhabited by intelligent beings who had reached a high standard of living. But a nuclear war broke out, ultimately destroying the planet. But the study of the structure and composition of meteorites revealed that the substance of just one planet is not enough for such diversity. And the age of the meteorites - from a million to hundreds of millions of years - shows that the fragmentation of asteroids was prolonged. And the planet Phaeton is just a beautiful fairy tale.
Collisions of protoplanetary bodies. This hypothesis prevails. It explains the origin of asteroids quite reliably. The planets formed from a cloud of gas and dust. But in the regions between Jupiter and Mars, the process culminated in the creation of protoplanetary bodies, from the collision of which asteroids were born. There is a version that the largest of the small planets are precisely the embryos of a planet that failed to form. Such objects include Ceres, Vesta, Pallas.
Largest asteroids
Ceres. It is the largest object in the asteroid belt, with a diameter of 950 km. Its mass is almost a third of the total mass of all bodies in the belt. Ceres consists of a rocky core surrounded by an icy mantle. It is assumed that liquid water is present under the ice. The dwarf planet orbits the Sun every 4.6 years at a speed of 18 km/sec. Its rotation period is 9.15 hours, and its average density is 2 g/cm 3 .
Pallas. The second largest object in the asteroid belt, but with the transfer of Ceres to the status of a dwarf planet, it became the largest asteroid. Its parameters are 582x556x500 km. The flyby of the star takes 4 years at a speed of 17 km/sec. A day on Pallas is 8 hours long, and the surface temperature is 164° K.
Vesta. This asteroid became the brightest and the only one that can be seen without the use of optics. The dimensions of the body are 578x560x458 km, and only the asymmetrical shape does not allow Vesta to be classified as a dwarf planet. Inside it is an iron-nickel core, and around it is a stone mantle.
Vesta has many large craters, the largest of which is 460 km across and is located near the south pole. The depth of this formation reaches 13 km, and its edges rise above the surrounding plain by 4–12 km.
Evgenia. This is a fairly large asteroid with a diameter of 215 km. It is interesting because it has two satellites. They were The Little Prince (13 km) and S/2004 (6 km). They are 1200 and 700 km away from Evgenia, respectively.
Studying
The detailed study of asteroids began with the Pioneer spacecraft. But the Galileo apparatus was the first to take pictures of the Gaspra and Ida objects in 1991. A detailed examination was also carried out by NEAR Shoemaker and Hayabusa devices. Their targets were Eros, Matilda and Itokawa. Soil particles were even delivered from the latter. In 2007, the Dawn station set off for Vesta and Ceres, reaching Vesta on July 16, 2011. This year the station should arrive at Ceres, and then it will try to reach Pallas.
It is unlikely that any life will be found on asteroids, but there is certainly a lot of interesting things there. You can expect a lot from these objects, but you don’t want just one thing: their unexpected arrival to visit us.
What is an asteroid? Sooner or later, every person interested in space exploration begins to ask this question. Wanting to find detailed information on this topic, people often stumble upon various scientific sites designed for an adult audience. On such portals, as a rule, almost all articles are replete with a huge number of scientific terms and concepts that are very difficult for ordinary people to understand. But what should schoolchildren or students do, for example, who need to prepare a report on the topic of space and formulate in their own words what an asteroid is? If you are concerned about this problem, then we recommend that you read our publication. In this article you will find all the necessary information on this topic and get an answer to the question of what an asteroid is, in simple and understandable language. Interested? Then we wish you pleasant reading!
Origin of the word "asteroid"
Before we move on to the main topic of the article, let's first take a look at the history. Many people are interested in the translation of the word “asteroid”, and we could not ignore this issue. This concept comes from the Greek words aster and idos. The first is translated as "star", and the second - "view".
What is an asteroid
Asteroids are small cosmic bodies moving in orbit around the main body of our galaxy - the Sun. Unlike planets, they do not have a regular shape, large size, or atmosphere. The total mass of one such body does not exceed 0.001 the mass of the globe. Despite this, some asteroids have their own moons.
The first person to call such space objects with the word “asteroid” was William Herschel. Among specialists, there is a special classification according to which only those bodies whose diameter reaches 30 meters can be considered asteroids.
The largest asteroids in the Solar System
The largest cosmic body of this type is considered to be an asteroid called Ceres. Its dimensions are so large (975 × 909 kilometers) that in 2006 it was officially assigned the status of a dwarf planet. In second place are the objects Pallas and Vesta, whose diameter is approximately 500 kilometers. Vesta is located in the asteroid belt (which will be discussed below) and can be seen from our home planet with the naked eye.
History of research
What is an asteroid? We think we've already figured this out. And now we once again invite you to plunge into the wilds of our history in order to find out who was at the origins of the study of the celestial bodies discussed in the article.
It all started at the end of the 18th century, when Franz Xaver, with the participation of more than 20 astronomers, began searching for a planet that should be located between the orbit of Jupiter and the orbit of Mars. Xaver had a goal to study absolutely all the bodies of the zodiacal constellations known at that time. Some time later, the coordinates began to be refined, and researchers began to pay attention to shifting objects.
The asteroid Ceres is believed to have been accidentally discovered on January 1, 1801 by Italian astronomer Piazzi. In fact, the orbit of this celestial object was calculated much earlier by Xavier astronomers. A few years later, researchers also found Juno, Palada and Vesta.
Carl Ludwig Henke made a special contribution to the study of asteroids. In 1845 he discovered Astraea, and in 1847 - Hebe. Henke's merits gave impetus to the development of astronomy, and after his research, new asteroids began to be found almost every year.
In 1891, Max Wolf invented the method of astrophotography, thanks to which he was able to recognize about 250 such space objects.
To date, several thousand asteroids have been discovered. These celestial bodies are allowed to give any names, but on the condition that their orbit is accurately and accurately calculated.
Asteroid belt
Almost all space objects of this type are located within one large ring called the asteroid belt. According to scientists' research, it contains about 200 small planets, the average size of which exceeds 100 kilometers. If we talk about bodies that do not exceed a kilometer in size, then there are even more of them: from 1 to 2 million!
Due to frequent collisions, many asteroids located in this belt are fragments of other similar cosmic bodies. This explains the fact that there are too few objects in the belt that have their own satellites. But collisions are not the only reason large asteroids lack their own satellites. A special role in these processes is played by changes in gravity caused by the formation of new objects after direct impacts, and the uneven distribution of the rotation axes of celestial asteroids. The only bodies that have direct rotation are the previously mentioned Ceres, Pallas and Vesta. They were able to maintain this position only thanks to their impressive dimensions, which provide them with large angular momentum.
Asteroid and meteoroid. What is the difference
Talking about what the word “asteroid” means, we cannot ignore this issue. A meteoroid is a solid celestial object that moves in interplanetary space. The main parameter by which a meteoroid and an asteroid are distinguished is their size. As mentioned earlier, only a cosmic body whose diameter reaches (or exceeds) 30 meters can be considered an asteroid. Meteoroids, on the contrary, are much more modest in size.
Another important factor is that asteroids and meteoroids are, in fact, completely different space objects. The fact is that the laws according to which they move in outer space are very different.
Asteroid Apophis
What is the asteroid Apophis? We think that among those reading this article there are people interested in this issue. Apophis is a celestial object that is constantly approaching the Earth. This cosmic body was discovered in 2004 by scientists at the Kitt Peak Observatory, located in Arizona. Its discoverers are Roy Tucker, David Tolenomi and Fabrizio Bernardi.
Apophis has a diameter of 270 meters, an average orbital speed of 30.728 kilometers per second, and a weight of more than one ton.
The asteroid was originally called 2004 MN4, but in 2005 it was renamed after the evil demon Apep from ancient Egyptian mythology. According to the beliefs of the inhabitants of Ancient Egypt, Apep is a huge beast that lives underground. In the minds of the Egyptians, he was the real embodiment of evil and the main opponent of the god Ra. Every night, while traveling along the Nile River, Ra entered into mortal combat with Apep. The Sun God always won, and therefore a new day came.
Apep's threat to Earth
After the discovery of this celestial object, ordinary people immediately began to ask one single question: is Apophis dangerous for the inhabitants of the Earth? Experts' forecasts differ depending on what time period of rapprochement with our world we are talking about. For example, in 2013, this celestial object flew at a distance of 14.46 million kilometers from Earth, but already in 2029, according to scientists, it will approach our planet by 29.4 thousand kilometers. For comparison, this is below the altitude at which geostationary satellites are located.
Despite such a close distance, many researchers convince us that we have nothing to fear. Initially, the probability that Apophis would fall to Earth in 2029 was estimated at almost 3%, but now such a probability is not considered at all. In the future, the asteroid will be visible to the naked eye. Visually, it will resemble a rapidly moving luminous point.
Scientists also said there is a small possibility that in 2029 this cosmic body may fall into a region of space in space in which the gravitational field of our planet can change the orbit of Apophis. In February 2013, researchers from NASA made a statement that an asteroid could fall to Earth in 2068. According to research results, after 2029 this object may fall into 20 such gravitational areas. But here, too, scientists reassure ordinary citizens: the likelihood of a collision in 2068 is extremely low.
Despite such positive forecasts, researchers say that there is no point in relaxing. The study of Apophis will continue to determine the risks to all humanity.
We think we have figured out what the asteroid Apophis is. Now let's take a more global look at the topic of a potential collision between the Earth and some space object.
What is the probability that the Earth will be destroyed by an asteroid collision?
There is an opinion among ordinary people that absolutely all asteroids pose a great danger to our planet. In fact, research by scientists shows that at the moment there is no such asteroid that could destroy the Earth.
Only those asteroids whose diameter exceeds 10 kilometers pose a serious danger to our planet. Fortunately, today all of them are known to modern astronomy, their trajectories have been determined and nothing threatens the Earth.
Now you know about the meaning of the word “asteroid”, the history of the study of these space objects, as well as the danger they pose to the planets. We hope that the information provided in the article was interesting to you.
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 empty space in which eight planets revolve, some with their satellites. Someone will remember several small planets to which Pluto was recently assigned, the asteroid belt, meteorites that sometimes fall to Earth, and comets that occasionally grace the sky. This idea is quite fair: none of the numerous spacecraft was damaged by a collision with an asteroid or comet - space is quite spacious.
And yet, the enormous volume of the Solar System contains not hundreds of thousands or tens of millions, but quadrillions (ones followed by fifteen zeros) of cosmic bodies of various sizes and masses. They all 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 asteroid danger 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 small planet. The term “asteroid,” translated from ancient Greek as “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 Space”). Ceres and similar objects (Pallas, Juno and Vesta), discovered in the next six years, were visible as points, rather than as disks in the case of planets; at the same time, unlike the fixed stars, they moved like planets. It should be noted that the observations that resulted in the discovery of these asteroids were carried out purposefully in attempts to discover the “missing” planet. The fact is that the already discovered planets were located in orbits separated 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 is known, no planet was found in such an orbit, but an asteroid belt, called the main one, was later discovered approximately in this area. In addition, Bode’s law, as it turned out, does not have any physical basis and is currently considered simply as some kind of random combination of numbers. Moreover, Neptune, discovered later (1848), found itself in an orbit that was inconsistent 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, where meetings of astronomers and asteroid hunters were held, burned down. Observations resumed in 1830, but success came only in 1845 with the discovery of the asteroid Astrea. Since that time, asteroids began to be 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 we classify asteroids by their distance from the Sun, then the first group includes vulcanoids - a certain hypothetical belt of minor 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 evidence of the existence of this belt. Previously, they tried to explain the anomalies in the movement of Mercury by the presence of asteroids there, but then they were explained on the basis of taking into account relativistic effects. So the final answer to the question about the possible presence of Vulcanoids has not yet been received. Next come 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 located near the ecliptic, their inclination to the ecliptic is 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 discovered first: 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 of them: only 140 out of 100,000 main belt asteroids have an average diameter of more than 120 kilometers. The total mass of all its asteroids is relatively small, amounting to only about 4% of the Moon's mass. 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). To a first approximation, the size of an asteroid can be determined by its brightness and distance from the Sun. But we must also take into account the reflective characteristics of the asteroid - its albedo. If the surface of an asteroid is dark, it glows less. It is for these reasons that in the list of ten asteroids, arranged in the figure in the order of their discovery, the third largest asteroid, Hygiea, is in last place.
Pictures of the main asteroid belt typically show a lot of rocks moving quite close to each other. In fact, the picture is very far from reality, since, generally speaking, the small total mass of the belt is distributed over its large volume, so that the space is quite empty. All spacecraft launched to date beyond the orbit of Jupiter have flown through the asteroid belt without any significant risk of collision 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 planets, the first of which was discovered in 1906 by the German astronomer Max Wulf. The asteroid moves around the Sun in the orbit of Jupiter, ahead of it by an average of 60 degrees. Next, 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 ahead of Jupiter, there is a group of asteroids lagging behind it by approximately 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 fall into their vicinity perform an oscillatory motion without going too far. For reasons that have not yet been explained, there are approximately 40% more asteroids ahead of Jupiter than lagging ones. This was confirmed by measurements recently carried out by the American NEOWISE satellite using a 40-centimeter telescope equipped with detectors operating in the infrared range. Measurements in the infrared range significantly expand the possibilities of studying asteroids compared to those provided by 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 device continues. Interestingly, the Trojans are noticeably different from most of the main belt asteroids. They have a matte surface, reddish-brownish color and belong mainly to the so-called D-class. These asteroids have a very low albedo, that is, with a weakly reflective surface. Similar ones can only be found in the outer regions of the main belt.
It's not just Jupiter that has Trojans; other planets of the Solar System, including Earth (but not Venus and Mercury), are also accompanied by Trojans, grouping in the vicinity of their Lagrange points L 4, L 5. The Earth Trojan asteroid 2010 TK7 was discovered using the NEOWISE telescope quite recently - in 2010. It moves ahead of the Earth, while the amplitude of its oscillations around point L 4 is very large: the asteroid reaches a point opposite the Earth in its movement around the Sun, and goes unusually far out of the ecliptic plane.
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 movement of the Earth's Trojan is very different from the movement of the Jupiter Trojans, which do not leave their Lagrange points at such significant angular distances. This nature of the motion makes it difficult for spacecraft missions to it, since due to the significant inclination of the Trojan’s orbit to the ecliptic plane, reaching the asteroid from Earth and landing on it requires too high a characteristic speed and, therefore, high fuel consumption.
Kuiper Belt lies beyond the orbit of Neptune and extends up to 120 AU. from the sun. It is close to the ecliptic plane, inhabited by a huge number of objects, including 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 1,300 of them have been discovered. Since the celestial bodies of the Kuiper belt are located very far from the Sun, their sizes are difficult to determine. This is done based on 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 provide the levels of the objects’ own radiation. 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 childbirth; he represents part of a family formed as a result of collisions. This object apparently collided with another one half the size. The impact scattered large chunks of ice and caused Haumea to rotate with a period of about four hours. This rapid rotation gave it the shape of an American football or a melon. Haumea is accompanied by two companions - Hi'iaka and 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 from the Sun they manifest themselves either as asteroids or comets), may have 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 large inclinations. They formed a disk 10 AU thick, but the actual outer edge of the Kuiper Belt is still not defined. Until recently, Pluto and Charon were considered the only examples of the largest objects on icy worlds in the outer solar system. But in 2005, another planetary body was discovered - Eris (named after the Greek goddess of discord), whose diameter is slightly less than the diameter of Pluto (it was initially assumed that it was 10% larger). Eris moves in an orbit with a perihelion of 38 AU. and aphelion 98 au. She has a small companion - Dysnomia. At first, Eris was planned to be considered the tenth (following Pluto) planet of 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 the Kuiper belt contains 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. Their orbital period around the Sun is hundreds of years, which makes their detection very difficult. If we accept the assumption that only about 35,000 Kuiper Belt objects have a diameter greater 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 in the archive of data on the measurement of X-ray radiation from the neutron star Scorpius X-1, its eclipses by small objects were discovered. This gave grounds to assert that the number of Kuiper Belt objects measuring about 100 meters or more is approximately a quadrillion (10 15). Initially, at earlier stages of the evolution of the Solar System, the mass of Kuiper Belt objects was much greater than now - from 10 to 50 Earth masses. Currently, the total mass of all bodies in 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 modeling shows, almost the entire mass of the primordial disk beyond 70 AU. was lost due to collisions caused by Neptune, which led to the crushing of belt objects into dust, which was swept into interstellar space by the solar wind. All 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 to 100 thousand 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 observed instrumentally until 2003. In March 2004, a team of astronomers announced the discovery of a planet-like object orbiting the Sun at a record distance, making it uniquely cold.
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 along a highly elongated elliptical orbit with a period of 10,500 years. But even during its approach to the Sun, Sedna does not reach the outer limit of the Kuiper Belt, which is located at 55 AU. from the Sun: its orbit lies in the range from 76 (perihelion) to 1000 (aphelion) AU. This allowed the discoverers of Sedna to attribute it to the first observed celestial body from the Oort cloud, permanently located outside the Kuiper belt.
According to their 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.
Currently, the above classification is increasingly expanded and detailed, including new groups. By 2002, their number increased to 24. As an example of a new group, we can mention the M-class of mainly metallic asteroids. However, it should be taken into account that classifying 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 to be studied in detail using ground-based telescopes. Their images can be obtained using radar, but for this they must fly close enough to the Earth. A rather interesting method for determining the size of asteroids is to observe eclipses of stars by asteroids from several points along the path along the straight line star - asteroid - point on the Earth's surface. The method consists of calculating the points of intersection of the star-asteroid direction with the Earth using the known trajectory of the asteroid, and telescopes are installed along this path at some distances from it, determined by the estimated size of the asteroid, tracking the star. At some point, the asteroid obscures the star, it disappears for the observer, and then reappears. Based on 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 be obtained. There is now an organized community of amateur astronomers who are successfully carrying out coordinated measurements.
Flights of spacecraft to asteroids open up incomparably more opportunities for their study. The asteroid (951 Gaspra) was first photographed by the Galileo spacecraft in 1991 on its way to Jupiter, then in 1993 it photographed the asteroid 243 Ida and its satellite Dactyl. But this was done, so to speak, incidentally.
The first vehicle specifically designed for asteroid research was the NEAR Shoemaker, which photographed the asteroid 253 Matilda and then entered orbit around 433 Eros and landed on its surface in 2001. It must be said that the landing was not initially planned, but after successful exploration 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 devices for landing and its control system did not provide for such operations, following commands from the Earth, it was possible to land the device, and its systems continued to function on the surface. In addition, the Matilda flyby made it possible not only to obtain a series of images, but also to determine the mass of the asteroid from the disturbance of the apparatus’s trajectory.
As a side task (while performing the main one), the Deep Space probe explored asteroid 9969 Braille in 1999 and the Stardust probe explored asteroid 5535 Annafranc.
With the help of the Japanese apparatus Hayabusa (translated as “hawk”) in June 2010, it was possible to return to Earth soil samples from the surface of asteroid 25 143 Itokawa, which belongs to the near-Earth asteroids (Apollos) of the spectral class S (silicon). The photograph of the asteroid shows rugged terrain with many boulders and cobblestones, of which more than 1,000 are over 5 meters in diameter, and some are up to 50 meters in size. We will return to this feature of Itokawa next.
The Rosetta spacecraft, launched by the European Space Agency in 2004 to the comet Churyumov-Gerasimenko, safely landed the Philae module on its core on November 12, 2014. Along the way, the device made a flyby of asteroids 2867 Steins in 2008 and 21 Lutetia in 2010. The device received 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 name to the landing module. Texts are carved on the stone in two languages: ancient Egyptian and ancient Greek, which provided the key to unlocking the secrets of the civilization of the ancient Egyptians - deciphering hieroglyphs. By choosing historical names, the project developers emphasized the goal of the mission - to reveal the secrets of the origin and evolution of the Solar system.
The mission is interesting because at the time the Philae module landed 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 while in the center of events.
The ongoing Dawn mission, carried out under the NASA program, is very interesting. The device was launched in 2007, reached the asteroid Vesta in July 2011, then was transferred to the orbit of its satellite and conducted research there until September 2012. Currently, the device is on its way to the largest asteroid - Ceres. It is powered by a low-thrust electric rocket ion engine. Its efficiency, determined by the flow rate 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 a satellite of one asteroid to the orbit of a 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, their physical characteristics are very different. If Vesta is a “dry” asteroid, then on Ceres, according to ground-based observations, water, seasonal polar caps of water ice, and even a very thin layer of atmosphere have been discovered.
The Chinese have also contributed to asteroid research 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 photo 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 interesting feature is its very uneven density. One part of the asteroid’s volume has a density of 1.95 g/cm 3 , the other - 2.25 g/cm 3 . In this regard, it has been suggested that Tautatis was formed as a result of the connection of two asteroids.
As for asteroid mission projects in the near future, a place to start is the Japanese Aerospace Agency, which plans to continue its research program with the launch of the Hayabusa-2 spacecraft in 2015 in order to return soil samples from asteroid 1999 JU3 to Earth in 2020. The asteroid belongs to spectral class C, is in an orbit intersecting the orbit of the Earth, and its aphelion almost reaches the orbit of Mars.
A year later, that is, in 2016, the NASA OSIRIS-Rex project starts, the goal 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 expert on the asteroid problem, Donald Yeomans: “If we do not fly to asteroids, then they fly to us " This emphasized the importance of another aspect of asteroid research - the asteroid hazard and solving problems related to the possibility of asteroid collisions 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 famous American philanthropist who founded a foundation to support scientific research in the United States in 1954. In the project, the initial condition was that the task of exploring an asteroid was solved with human participation, in other words, the mission to the asteroid should be manned. But in this case, the duration of the entire flight with return to Earth will inevitably be at least several months. And what is most unpleasant for a manned expedition is that 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 the asteroid to Earth using unmanned vehicles. But not to the surface, as naturally happened with the Chelyabinsk asteroid, but to an orbit similar to the Moon, 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 the event of an emergency, astronauts will be able to return to Earth within a week. NASA has already selected the near-Earth asteroid 2011 MD, a member of the Amurs, as the main candidate for the role of the asteroid moved in this way. Its diameter is from 7 to 15 meters, its density is 1 g/cm 3, that is, it can look like a loose pile of crushed stone weighing about 500 tons. Its orbit is very close to the Earth's orbit, inclined to the ecliptic by 2.5 degrees, and its 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 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 an asteroid and transfer it to a new orbit, will be equipped with low-thrust electric rocket engines running on xenon. Operations to change the asteroid's orbit also include a gravity 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 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 destruction of Phaeton did not confirm this hypothesis in all its varieties, ranging from the rupture of the planet 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. Currently, the generally accepted theory suggests that the solar system arose from a primordial gas and dust accumulation. From the cluster a disk was formed, 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 remarkably similar to photographs of planetary systems at certain phases of their development.
At the initial stage of planet formation, so-called planetesimals arose - “embryos” of planets, onto which dust then adhered due to gravitational influence. As an example of such an initial phase of planet formation, they point to the asteroid Lutetia. This rather large asteroid, reaching 130 kilometers in diameter, consists of a solid part and an adhering thick (up to a kilometer) layer of dust, as well as boulders scattered across 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. The substance was heated and melted, 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 resulting 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, some of the resulting celestial bodies were thrown to the outskirts of the system, and some were thrown onto the Sun. This process took place 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 forming 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 in shape 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 up during collisions, as well as objects that did not have time to become parts of larger bodies and, thus, represent examples of more ancient formations. As mentioned above, the asteroid Lutetia is such an example. This was confirmed by studies of the asteroid carried out by the Rosetta spacecraft, including photography 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 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 distant region of the solar system. It is assumed that the influence of the giant planets extends to the even more distant Oort cloud, which, however, formed closer to the Sun than it is now. In the early phases of the evolution of the approach to 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 exposed to the influence of the stars of our Galaxy - the Milky Way, which leads to their chaotic transition on a trajectory of return to a close region of circumsolar space. We observe these planetesimals as long-period comets. As an example, we can point out the brightest comet of the 20th century - Comet Hale-Bopp, discovered on July 23, 1995 and reached perihelion in 1997. Its period of revolution around the Sun is 2534 years, and aphelion is at a distance of 185 AU. from the sun.
Asteroid-comet danger
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 danger for the Earth. But such a reference is not entirely correct, since the overwhelming majority of these craters were formed during the “heavy bombardment period.” Nevertheless, on the surface of the Earth, with the help of modern technologies, including analysis of satellite imagery, it is possible to detect traces of collisions with asteroids that date back to much later periods in 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 by an asteroid impact 65 million years ago, equivalent to the explosion energy of 100 teratons (10 12 tons) of TNT. It is now believed that the extinction of the dinosaurs was a consequence of this catastrophic event, which caused tsunamis, earthquakes, volcanic eruptions and climate change due to the formation of a dust layer in the atmosphere that obscured 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 that attracted the most attention was near Chelyabinsk, when on February 15, 2013, an asteroid entered the atmosphere in this area, the size of which was estimated at approximately 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 over a fairly large area, the roof of the Chelyabinsk zinc plant collapsed, and about 1,500 people were injured by glass fragments. It is believed that the disaster did not happen due to an element of luck: the trajectory of the meteorite’s fall was gentle, otherwise the consequences would have been much more severe. The explosion energy is equivalent to 0.5 megatons of TNT, which corresponds to 30 bombs dropped on Hiroshima. The Chelyabinsk asteroid became the most thoroughly described 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 occurs around the world approximately once every 100 years. As for the Tunguska event, when trees were burned and felled over an area with a diameter of 50 kilometers as a result of an explosion at an altitude of 18 kilometers with an energy of 10-15 megatons of TNT, such disasters happen approximately once every 300 years. However, there are cases where smaller bodies colliding with the Earth more often than those mentioned have 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 consisted almost entirely of iron and turned out to be the largest iron meteorite 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 onto the earth's surface) was uninhabited, but in 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 likelihood of a particular person dying 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 Table 1 shows the probabilities of an asteroid fall, correlated with the mortality rate from other events.
It is unknown when the next asteroid impact will occur, comparable or more severe in its consequences to the Chelyabinsk event. It may fall in 20 years, or in several centuries, but it may fall tomorrow. Receiving early warning of an event like Chelyabinsk is not only desirable - it is necessary to effectively deflect potentially dangerous objects larger than, say, 50 meters. As for collisions of smaller asteroids with the Earth, these events occur more often than we think: approximately once every two weeks. This is illustrated by the following map of impacts of asteroids measuring a meter or more over the past twenty years, prepared by NASA.
.Methods for deflecting potentially dangerous near-Earth objects
The discovery in 2004 of the asteroid Apophis, the probability of a collision with the Earth in 2036 was then considered quite high, led to a significant increase in interest in the problem of asteroid-comet protection. 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 sharply increased, so that by now more of them have been discovered than was known before the start of work on the program. Various methods have also been proposed for deflecting asteroids from their impact trajectories with the Earth, including quite exotic ones. For example, covering the surfaces of dangerous asteroids with paint, which will change their reflective characteristics, leading to the required deviation 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 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 guided spacecraft. For the most powerful Russian carrier, Proton-M, it cannot exceed 5–6 tons.
Let's 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 deviation, but this “long time” will take many decades. It is currently impossible to predict the trajectory of an asteroid so far with acceptable accuracy, especially considering that there is uncertainty in knowing the parameters of the impact dynamics and, therefore, in assessing the expected change in the asteroid’s velocity vector. Thus, in order to deflect a dangerous asteroid from colliding with the Earth, it is necessary to find an opportunity to direct a more massive projectile at it. As such, we can propose another asteroid with a mass significantly greater than the mass of the spacecraft, say 1500 tons. But to control the movement of such an asteroid, too much fuel will 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 itself does not require any fuel consumption.
By gravitational maneuver we mean the flyby of a space object (in our case, an asteroid projectile) of a fairly massive body - the Earth, Venus, other planets of the solar system, as well as their satellites. The meaning of the maneuver is to choose the parameters of the trajectory relative to the body being flown (height, initial position and velocity vector), which will allow, due to its gravitational influence, to change the orbit of the 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 the controlled object using a rocket engine, we receive this impulse due to the gravity 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 device. And for the gravity maneuver method, fuel is needed only to bring the vehicle onto 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 rocketry F.A. Zander. Currently, this technique is widely used in space flight practice. It is enough to mention once again, for example, the European spacecraft Rosetta: during the implementation of the mission, over ten years it performed three gravity maneuvers near the Earth and one near Mars. One can recall the Soviet spacecraft Vega-1 and Vega-2, which for the first time flew around Halley's comet - on the way to it they performed gravity 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 exhausted by these examples.
The use of a gravitational maneuver to guide relatively small near-Earth asteroids towards dangerous celestial objects in order to deviate them from their collision trajectory with the Earth was proposed by employees of the Space Research Institute of the Russian Academy of Sciences at an international conference on the problem of asteroid danger, organized in Malta in 2009. And the next year a journal publication appeared outlining this concept and its rationale.
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 was established approximately ten years before its expected collision with the Earth. Accordingly, a scenario was constructed for the asteroid to deviate from the trajectory passing through it. First of all, from the list of near-Earth asteroids whose orbits are known, one was selected, which will be transferred into the vicinity of the Earth into an orbit suitable for performing a gravitational maneuver, ensuring that the asteroid hits Apophis no later than 2035. As a selection criterion, we took the magnitude of the velocity impulse that must be imparted to the asteroid in order to transfer it to such a trajectory. The maximum permissible impulse was considered to be 20 m/s. Next, a numerical analysis of possible operations to point the asteroid to Apophis was carried out in accordance with the following flight scenario.
After the head unit of the Proton-M launch vehicle is launched into low Earth orbit using the Briz-M upper stage, the spacecraft is transferred to a flight path to the projectile asteroid with subsequent landing on its surface. The device is fixed on the surface and moves together with the asteroid to the point at which it turns on the engine, imparting to the asteroid an impulse that transfers it to the calculated trajectory of the gravitational maneuver - orbiting the Earth. During the movement, the necessary measurements are taken to determine the movement parameters of both the target asteroid and the projectile asteroid. Based on the measurement results, the projectile trajectory is calculated and its correction is made. With the help of the propulsion system of the device, the asteroid is given speed impulses that correct errors in the parameters of the trajectory of movement towards the target. The same operations are performed on the vehicle’s flight path to the projectile asteroid. The key parameter in developing and optimizing the scenario is the velocity impulse that needs to be imparted to the projectile asteroid. For candidates for this role, the dates of the impulse message, the arrival of the asteroid to the Earth and the collision with a dangerous object are determined. These parameters are selected in such a way that the magnitude of the impulse imparted to the projectile asteroid is minimal. During the research process, the entire list of asteroids whose orbital parameters are currently known was analyzed as candidates - about 11,000 of them.
As a result of the calculations, five asteroids were found, the characteristics of which, including sizes, are given in Table. 2. It was hit by asteroids whose dimensions significantly exceed the values corresponding to the maximum permissible mass: 1500–2000 tons. In this regard, two remarks need to be made. First: the analysis used a far from complete list of near-Earth asteroids (11,000), while, according to modern estimates, there are at least 100,000 of them. Second: the real possibility of using not an entire asteroid as a projectile, but, for example, located on its surface there are boulders, the mass of which falls within the designated limits (one can recall the Itokawa asteroid). Note that this is precisely the approach that is assessed as realistic in the American project to deliver a small asteroid into lunar orbit. From the table 2 it can be seen that the smallest velocity impulse - only 2.38 m/s - is required if asteroid 2006 XV4 is used as a projectile. True, it itself is too big and exceeds the estimated limit of 1,500 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 speed of 3200 m/s, spending 1.2 tons of fuel. As calculations have shown, a device with a total mass of more than 4.5 tons can be landed on the surface of this asteroid, so fuel delivery 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 on the required speed pulses given in the table refer to the ideal case, when the required change in the speed vector is implemented absolutely accurately. 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 to date, correction may require a total of up to 30 m/s, which exceeds the nominal values of the speed change to solve the problem of intercepting a dangerous object.
In our case, when the controlled object has a mass three orders of magnitude greater, 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 ten times for the same corrective impulse. In addition, to increase the accuracy of guidance, it is proposed to use a navigation system that includes a small device equipped with a transceiver, which is placed in advance on the surface of a dangerous asteroid, and two subsatellites accompanying the main device. Transceivers are used to measure the distance between devices and their relative speeds. Such a system makes it possible to ensure that an asteroid projectile hits a target with a deviation of within 50 meters, provided that in the last phase of approach to the target a small chemical engine with a thrust of several tens of kilograms is used, producing a speed impulse within 2 m/s.
Among the questions that arise when discussing the feasibility of the concept of using small asteroids to deflect dangerous objects, the most important question is the risk of a collision with the Earth of an asteroid transferred to the trajectory of a gravitational maneuver around it. In table 2 shows the distances of asteroids from the center of the Earth at perigee when performing a gravity maneuver. For four they exceed 15,000 kilometers, and for asteroid 1994 the GV is 7427.54 kilometers (the average radius of the Earth is 6371 kilometers). The distances look safe, but it is still impossible to guarantee the absence of any risk if the size of the asteroid is such that it can reach the Earth’s surface without burning up in the atmosphere. A diameter of 8–10 meters is considered the maximum permissible size, provided that the asteroid is not iron. A radical way to solve the problem is to use Mars or Venus for 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 conducting research with direct human participation. As such, an orbit close to the lunar one was proposed. As another option for changing the asteroid orbit, the IKI RAS considered methods for controlling the movement of asteroids using gravitational maneuvers near the Earth, similar to those that were developed for pointing small asteroids towards dangerous near-Earth objects.
The goal of such maneuvers is to transfer asteroids into orbits that are resonant with the orbital motion of the Earth, in particular with a 1:1 ratio of asteroid and Earth periods. Among the near-Earth asteroids, there are thirteen that can be transferred to resonant orbits in the specified ratio and at the lower permissible limit of the perigee radius - 6700 kilometers. To do this, it is enough for any of them to provide a speed impulse not exceeding 20 m/s. Their list is presented in table. 3, which shows the magnitude of the velocity impulses that transfer 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’s, that is, one year. The maximum and minimum velocities of the asteroid in its heliocentric motion achievable by the maneuver 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, asteroid 2012 VE77 will be able to be sent into an orbit with an aphelion at the distance of the orbit of Saturn, and the rest - beyond the orbit of Mars.
The advantage of resonant asteroids is that they return to the Earth's vicinity every year. This makes it possible to send a spacecraft to land on an asteroid at least every year 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 an orbit similar to the Moon, as planned in the Keck project, since it requires a noticeable fuel consumption to return. For unmanned missions, this may be decisive, but for manned flights, when it is necessary to ensure the fastest possible return of the device to Earth in an emergency (within a week or even less), the advantage may be on the side of the ARM project.
On the other hand, the annual return of resonant asteroids to Earth allows for periodic gravity maneuvers, each time changing their orbit to optimize research conditions. At the same time, the orbit must remain resonant, which is easy to achieve by performing multiple gravitational maneuvers. Using this approach, it is possible to transfer the asteroid to an orbit identical to the Earth’s, but slightly inclined to its plane (towards the ecliptic). Then the asteroid will approach the Earth twice a year. The family of orbits resulting from a sequence of gravitational 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 around planets, including the construction of resonant orbits, then the idea arises of using the Moon. The fact is that a gravitational maneuver near a planet in its pure form does not allow capturing an object into a satellite’s orbit, since when it flies around the planet, the energy of its relative motion does not change. If at the same time it circles the natural satellite of the planet (the Moon), then its energy can be reduced. The problem is that the decrease must be sufficient to transfer to the satellite's orbit, that is, the initial speed relative to the planet must 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 the maneuver can be repeated in a year. Thus, it is possible to capture an asteroid into the orbit of the Earth’s satellite by using gravitational maneuvers near the Earth while maintaining the resonance condition and a coordinated flyby of the Moon.
It is obvious that individual examples confirming the possibility of implementing the concept of controlling the movement of asteroids using gravitational maneuvers do not guarantee a solution to the problem of asteroid-comet danger for any celestial object threatening a collision with the Earth. It may happen that in a particular case there is no suitable asteroid that can be aimed at it. But, as the latest calculation results, carried out taking into account the most “recent” cataloged asteroids, show, with the maximum permissible velocity impulse required to transfer an asteroid into the vicinity of the planet 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 whose movement can be controlled by means of modern rocket and space technology. The list is growing rapidly, with an average of two to five asteroids being discovered per day. Thus, during 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 coming when this becomes possible.
Glossary for the article
Bode's law(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 its 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 true for most planets with an accuracy of 3%, but it seems to have no physical meaning. There is, however, an assumption that at the stage of formation of the Solar system, as a result of gravitational disturbances, 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 not always satisfied: the orbits of Neptune and Pluto, for example, are much closer to the Sun than it predicts (see table).
(L-points, or libration points, from lat. Libration- swinging) - points in a 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, performing oscillations of small amplitude, provided that only gravitational forces act on it.
The Lagrange points lie in the orbital plane of both bodies and are designated by indices from 1 to 5. The first three - collinear - lie on the straight line connecting the centers of the 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 located in orbit on both sides of the line connecting bodies of large mass, at angles of 60 ° from the line connecting them (for example, the Sun and Earth).
Point L 1 of the Earth-Moon system is a convenient place for placing a manned orbital station, allowing astronauts to reach the Moon with minimal fuel consumption, 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 the construction of 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, etc.
At point L 3, on the other side of the Sun, science fiction writers have repeatedly placed a certain planet - Counter-Earth, which either arrived from afar, or was created simultaneously with the Earth. Modern observations have not found 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 on 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 semimajor axis of the ellipse; e - eccentricity - a value that is constant for any given curve. The eccentricity of the ellipse is less than 1 (for a parabola e = 1, for a hyperbola e > 1); when e > 0, the shape of the ellipse approaches a circle; when e > 1, the ellipse becomes increasingly elongated and compressed, ultimately degenerating into a segment - 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 (the major axis of the orbit).
sunny wind- a constant flow of plasma from 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) is a large circle of the celestial sphere along which the visible annual movement of the Sun occurs. In reality, since the Earth moves around the Sun, the ecliptic is the 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 what has been known since ancient times: solar and lunar eclipses occur when the Moon is near the point where its orbit intersects the ecliptic.
An asteroid is a relatively small, rocky cosmic body similar to a planet in the solar system. Many asteroids orbit the Sun, and the largest cluster of them is located between the orbits of Mars and Jupiter and is called the asteroid belt. The largest known asteroid, Ceres, is also located here. Its dimensions are 970x940 km, i.e. almost round in shape. But there are also those whose sizes are comparable to dust particles. Asteroids, like comets, are remnants of the substance from which our solar system was formed billions of years ago.
Scientists suggest that more than half a million asteroids with a diameter greater than 1.5 kilometers can be found in our galaxy. Recent research has shown that meteorites and asteroids have similar compositions, so asteroids may well be the bodies from which meteorites are formed.
Asteroid exploration
The study of asteroids dates back to 1781, after William Herschel discovered the planet Uranus to the world. At the end of the 18th century, F. Xaver gathered a group of famous astronomers who searched for the planet. According to calculations, Xavera should have been located between the orbits of Mars and Jupiter. At first the search did not produce any results, but in 1801, the first asteroid was discovered - Ceres. But its discoverer was the Italian astronomer Piazzi, who was not even part of Xaver’s group. Over the next few years, three more asteroids were discovered: Pallas, Vesta and Juno, and then the search stopped. Only 30 years later, Karl Louis Henke, who showed interest in studying the starry sky, resumed their search. Since this period, astronomers have discovered at least one asteroid per year.
Characteristics of asteroids
Asteroids are classified according to the spectrum of reflected sunlight: 75% of them are very dark carbonaceous class C asteroids, 15% are grayish-siliceous class S asteroids, and the remaining 10% include metallic class M and several other rare species.
The irregular shape of asteroids is also confirmed by the fact that their brightness decreases quite quickly with increasing phase angle. Due to their large distance from the Earth and their small size, it is quite problematic to obtain more accurate data about asteroids. The force of gravity on an asteroid is so small that it is not able to give them the spherical shape that is characteristic of all planets. This gravity allows broken asteroids to exist as separate blocks that are held close to each other without touching. Therefore, only large asteroids that avoided collisions with medium-sized bodies can retain the spherical shape acquired during the formation of planets.