TO THE QUESTION OF THE ORIGIN OF SATELLITES OF THE PLANETS AND ASTEROIDS.
In general, an interesting and informative article by N. Garkavy and Doctor of Physical and Mathematical Sciences V. Prokofieva-Mikhailovskaya "Double asteroids and the loneliness of the Moon" in the journal "Science and Life", 2015, No. 11, pp. 44-52) is not free from contradictions. Let's consider some of them.
“The moon was formed .. at a distance of 3-4 planetary radii (about 19,000 kilometers - A. M.) .. due to many .. weak collisions that threw part of the matter from the earth’s mantle into the protolunar disk .. and only then moved away to a distance at 60 radii of the Earth (384,400 kilometers - A.M.) ... The moon is still moving away from the Earth at a speed of 4 centimeters per year. (p. 52).
Neglecting the time required for the formation of the Moon according to this theory (at least several million years) and the fact that the initial removal rate of the Moon has increased to the current 4 centimeters per year, assuming it to be constant, we obtain the maximum possible removal during the existence of the Earth (about 4.6 billion years) Moons at 184,000 kilometers (4,600,000,000 years x 0.00004 km). That is, at the time of its origin, the Moon was at a distance of 200,400 km from the Earth. \u003d 384 400 -184 000, which is 31-32 earth radii, and not 3-4 as the authors of the article believe. To remove the Moon by 56 Earth radii (358,400 kilometers) after its formation under the above conditions, it would take about 9 billion years, which is almost twice the generally recognized lifetime of the Earth.
These facts give rise to doubts about the realism of the multi-impact model of the formation of the Moon promoted by the authors, because the radius of the geostationary orbit, where the centrifugal force is balanced by the Earth's gravity, is only 35,786 kilometers.
I dare to offer a model that does not contradict the currently known facts of the almost simultaneous formation of the Earth and the Moon from one protoplanetary cloud with two accretion centers at a distance of about 200,000 kilometers from each other. If there is only one accretion center in a protoplanetary cloud, a planet is formed without a satellite. For example, Venus or Mercury. There may well be several accretion centers in protoplanetary clouds. Then, the planets formed from them will have, respectively, several satellites: Jupiter, for example, has four of them, and Pluto has five.
N. Garkavy and V. Prokofieva-Mikhailovskaya see and note the shortcomings of the mega-impact model for the formation of asteroid satellites: “.. the most important drawback of the mega-impact theory (the formation of satellites due to collisions of comparable masses / from 10 to 45% / A.M. cosmic bodies) in that it does not explain the appearance of many thousands of satellites around asteroids with weak gravity, unable to keep fragments of a strong impact near the central body. In addition, such a number of collisions of bodies comparable in mass is simply statistically unbelievable.” (p. 51).
But the multi-impact model, of which they are supporters, sins the same: “..the probability of the presence of a satellite steadily increases with an increase in the speed of rotation of the asteroid; it (probability - A.M.) is large for small and large asteroids and minimal for medium-sized asteroids” (p. 47). However, if satellites of asteroids are formed from the rocks of their surface layer, knocked out as a result of bombardment by micrometeorites, then at the same rotation speed, the possibility of holding bombardment fragments in their gravitational field is certainly higher for medium-sized asteroids than for small asteroids, and, therefore, should be greater. and the probability of occurrence of satellites; if the asteroid and its satellites are formed simultaneously from a single protoasteroid cloud, then the absence of a satellite or satellites for a particular medium-sized asteroid means that the protoasteroid cloud has only one accretion center.
The assertion that the multi-impact (multi-impact) model of the formation of asteroid satellites explains the mass loss of the asteroid belt is also highly controversial, because the mechanism of satellite formation described in the article illustrates only the redistribution of matter between asteroids and their satellites inside the asteroid belt. The authors themselves write that: “Satellites of asteroids are self-organizing structures that grow, feeding on dust flying away from asteroids. ... the emergence of numerous satellites of asteroids (to which this lost mass went) ”.
The model I proposed for the simultaneous formation of planets and their satellites from single protoplanetary clouds with several accretion centers, and asteroids and their satellites from single protoasteroid clouds also with several accretion centers, claims to be the main (most common) as the most consistent with the currently known facts , but does not exclude the fundamental possibility of the formation of satellites around planets and asteroids in some cases according to the multi-impact and mega-impact models.
11/16/2015 Alexander Malchukov.

Reviews

It is interesting to write about asteroids and satellites.
I'm more interested in their mineral composition. Many have a crystalline structure and are similar to terrestrial basalts, gabbro, diorites, but there are no granites in them. I saw sections of iron-nickel meteorites. They have a widmanstatt texture - almost perpendicularly intersecting strokes. This is a sign of a very long slow solidification of the initial melt (millions of years).
The conclusion to everything is that asteroids, meteorites are fragments of planets with an initial internal molten composition and with a long period of solidification and crystallization of minerals and rocks inside them. This conclusion is not new, the presence of Phaethon between Mars and Jupiter is assumed. Asteroid belts could have been picked up by the Sun from outer space as well.
How do you think - how could crystalline structures form in asteroids and meteorites?

After the big bang, if there was one, all matter was in a molten state and slowly (maybe millions of years) cooled down. Then the legend of Phaeton becomes redundant.

Here you have a big mistake. After the Big Bang, there was no matter yet - only radiation in the form of energy quanta. Then, as it cools, the stage of formation of elementary particles from quanta began - electrons-positrons, protons-antiprotons, and then the stage of formation of atoms of matter - hydrogen and helium.
This allegedly took 1 billion years (according to Shklovsky and Ginzburg). And other atoms were formed much later - in the depths of stars and their subsequent explosion. It took several billion years.
So the substance in the Cosmos was nowhere in the melt - there the temperature is below zero - 150 degrees. The melt of mineral matter could only be in the bowels of planets with a diameter of at least 2000 km. There is a book - Small planets.

And what exploded if there was no substance? And what did all these quarks, cracklings, positrons, electrons come from? And the temperature in the space covered by the explosion was -273 degrees?

It was not the substance that exploded, but the "Singular Point of the Physical Vacuum" that lost stability - such is the hypothesis. The human mind cannot understand this.

That's it, when the "geniuses" do not know what to say, they invent "singular points", secretly laughing at the simpletons amazed by their genius.

"The Unified Theory of Matter by V.Ya. Bril".
In my opinion, this is a masterpiece of another nonsense of a person poorly educated in the natural sciences, trying to create "his own theory." This is evidenced by a hodgepodge of scientific terms with religion and esotericism: “kinetic (quantum) theory of gravity”, “unified theory of matter”, “fundamental strings”, “elementary particles”, soul, spirit, aura, “information field”, “world mind ”, “field form of life”. To save from such a dish, I propose a remedy from REAL science:

A BRIEF DETERMINER OF SCIENTIFIC QUARRATIVITY.
Book counters, periodicals, TV programs, Internet sites and forums are full of anti-scientific rubbish. Sincerely sympathizing with the victims of pseudoscience and charlatanism, let's try to make a short guide to "brechology", like the guides to dangerous animals and poisonous mushrooms.
FIRST ORDER FEATURES
If the publication contains the words: aura, biofield, chakra, bioenergetics, panacea, energy-informational, resonant-wave, psychic energy, thought-form, telegony, wave genetics, wave genome, supersensible, astral, then you can be sure that you are dealing with charlatan writing.
The list could go on, but it doesn't make much sense. The terminology of the charlatan brethren is expanding all the time, so orientation by "signal words" is not always sufficient for a correct assessment of the text.
SIGNS OF THE SECOND ORDER
This is information about the identity of the author. As a rule, the main specialty of the authors of pseudoscientific works is far from the areas of knowledge to which their opuses are devoted. I deliberately use the term "opus" (from the Latin opus - business), so as not to specify whether this is a book, an article or a TV show.
The scientific regalia of the author are of great interest for analysis. The more of them and the more carefully they are listed, the more carefully one should treat the text. For real scientists, vanity is considered bad manners.

"Honorary membership" in various academies is especially alarming because of the significant differences between a member and an honorary member.
Without a doubt, many truly outstanding people have received many awards. But, alas, their works are understandable only to the same professionals, and they almost never condescend to popular publications.
In the works of professionals, there is not only no self-praise, but also a general mention of the value of this work.

Expressions like: "Our research completely changes the idea of ​​so-and-so"; "It has a special value"; "Everything that was before us is of no value" - coupled with promises of fundamental changes in science, immediate huge effect at negligible costs, with the humiliation of predecessors and competitors - are reliable symptoms of charlatanism.
An author's definition of his work as revolutionary is a very serious reason to doubt both the author's competence and the value of his creation.
SIGNS OF THE THIRD ORDER.
These signs are found, in fact, in the content of creation. Some of the points that pertain to this section have already been mentioned above. The authors of fantasmic and charlatan writings are by no means interested in quickly identifying their anti-science. Some have achieved remarkable success in mimicry and are remarkably adept at disguising the pseudoscientific nature of their creations in the midst of perfectly reasonable reasoning. Limiting the scope of medicine and biology, let me remind you that in biological systems and in living organisms, all known physical laws operate just as rigorously as in non-living ones. Specific biological laws are no less powerful and are also not violated. Therefore, if the author seriously talks about paranormal abilities - seeing through the wall, reading letters in closed envelopes, levitation, telekinesis, reviving the dead, operations without a knife (with the extraction of giblets, but without a wound and scar),

The use of pseudoscientific terminology is designed not so much for the mind of the reader, but for the hypnotizing effect of incomprehensible words that serve as a conductor of author's ideas in the brain of readers / listeners. The reader is simply not left time to comprehend the verbal flow. He only has time to grasp individual pieces written in normal language. They also contain the thoughts that, according to the author's intention, the consumer of the product of his philosophizing should assimilate. In theory, one should read thoughtfully, slowly ... But where there, we are accustomed (and forcedly accustomed) to speed reading. So we swallow without chewing. This method of absorbing spiritual food is more dangerous for the brain than hastily absorbing bodily food for the stomach.
So, an increased concentration of foreign language terms where it is quite possible to get by with words mother tongue, an abundance of complex grammatical structures

SIGNAL FOR THE READER: "Look, do not get in trouble!" Charlatan opuses are characterized by the absence of doubts and intolerance to objections. An undoubted sign of quackery is the lack of response to criticism on the merits and the transition to the personality of the opponent.
Pseudoscientific "inventions" are characterized by universality and universality. The charlatan does not stoop to solve narrow problems. If he made a revolution in science, then global. If he treats oncological diseases with an aspen stick (by God, there is such a patent!).
If he invented a miraculous diet, then it suits everyone, heals completely and without the right to appeal. If he describes a miraculous drug, then it has no contraindications and you can give it to anyone.

When an author lacks factual or logical (often both) arguments, he resorts to referring to authorities. At the same time, the late authorities are often credited with statements and views that were absolutely alien to them during their lifetime. A well-known case: the dead have no shame. In such cases, familiarity with the biography of the greats allows you to quite reliably determine the forgery and treat the author's creation accordingly.

If the "revolutionary doctrine" offered to the consumer has no scientific background, this is a very, very reliable sign of Brechology. Science develops progressively, the basis for new knowledge is always the old, proven. If the author has no predecessors, and his "science" jumped out into the light of day, like the devil out of a snuffbox, it will be completely natural to treat it as evil spirits. Similarly, I propose to treat all kinds of "insights", "influences" and other God's gifts. Any esotericism, hysteria and mysticism by their very presence in a "scientific" opus unequivocally determine its belonging to Brechology.

Another sign of the third order, I would call "Occam's unshaven". Occam's razor was the principle formulated back in the 14th century by the Franciscan monk William of Occam, which states: Entia non sunt multiplicanda sine necessitate - "Entities should not be multiplied without necessity." In other words, one should not invent a complex explanation where a simple one suffices. Einstein slightly changed the wording: "Everything should be simplified as long as possible, but no more." In pseudoscientific opuses, this principle is not respected.
An example of a violation of Occam's principle can serve as reasoning about bermuda triangle. In an area with extremely intensive navigation, with very unstable air currents and sea currents, ships and aircraft disappear from time to time. Brechologists explain these catastrophes by the action of otherworldly forces. Accidents due to natural causes (loss of communication with the aircraft due to power failures; falling into the sea due to navigation errors and excessive fuel consumption; the death of the ship under the impact of an abnormally high solitary wave) are rejected in favor of beautiful and unsubstantiated fabrications.
A simple recommendation: use your common sense to distinguish between science and bullshit.

If the lotteries have not yet gone bankrupt, the prophets are worthless. If there are still sick people, all miraculous drugs are garbage. If someone offers a miracle - he is a charlatan.
Source according to the Handbook: JOURNAL "SCIENCE AND LIFE" 2005.

God, how many bukoff and sloff!
I am absolutely not going to comment on Bril's theory from a scientific point of view, but there are no "auras" and other esotericism there at all, everything is scientific from a person who has been engaged in science all his life.

For some reason, you like bukffy and sloff Bril, but you don’t like real science? What would it be from?
They read Bril poorly - there are words there: soul, spirit, aura, “information field”, “world mind”, “field form of life”.
And you start talking about something you don't know. This is not good. Read again - have you read it for a long time?

I read it more than once, but for a long time. In any case, there the physical picture of the world is not presented through esotericism, and the hypothesis of "elementary strings" was discussed by physicists quite seriously thirty or forty years ago.
If there are words about "soul", "aura", etc., they do not define the main content of the text. I repeat, I do not have sufficient knowledge to discuss Brill's hypotheses from a scientific point of view, but one should definitely not drag esotericism here by the ears.

Modern scientific theories go through a stage of hypothesis with a long and repeated experimental verification by the scientific community. Only after practical confirmation do they move to the rank of theory. But even after that, they continue to be under experimental verification and elimination of inconsistencies.
And then immediately a theory based on postulates - that is, axioms from the head. The author of this "theory" at the end writes that it is not science that can test it, but only a higher mind. That is, he believes that his theory is above the human mind. Such fashionable "theories" are now filled with the Internet. Their collection is given on the site scorche.ru and there is also a critical analysis of specialists.

Since I regularly come across the fact that I am credited with what I allegedly believe, I also try not to guess what the author believed in relation to others, especially when there is a reference to the "higher mind". With all the achievements that humanity has achieved, it seems to me that it sometimes suffers from a certain self-confidence.
I don’t want to blame anyone, but experts are sometimes in the grip of their knowledge and experience and are not always receptive to alternative views, because then they will have to admit their own delusions. Especially applies to the so-called. humanities. In principle, there is nothing new in this, it has always been so. Of course, as long as this or that theory is not supported by experimental material, it is not of particular interest. I repeat again that I am not speaking here in defense of Bril, but the same Einstein theory did not immediately receive experimental confirmation, and even then the opinion about it is still ambiguous, but more than a century has passed.
Over the past few decades, the LHC has been built to test some assumptions about the structure of matter, but although the discovery of the Higgs boson was announced, it was somehow vague, and the collider itself almost burned down, they have been repairing it for several years. But how many people are at work.

Here you have a more objective view of reality. It is difficult to be objective, especially without knowledge of the basics of the natural sciences. Humanitarians and journalists tend to believe in miracles. Even Mikhail Veller believes in Chumak's "wonderful abilities" - he invited him to his program. Weller says - "I know physics at the level of Peryshkin's school textbook", and he himself undertook to create an "energy-information theory". Do they have some kind of itching with these "creators" of the current ones?
The Higgs boson fit into the hypothesis quite confidently, even Higgs himself was pleased. Two competing groups of scientists (collaborations), using different search methods, came to a common opinion - the boson exists.
The power of the collider is gradually growing and new discoveries are possible ahead. The collider is better than fiction. But they will still appear - this is how the human mind works, the unknown weighs on him and he fills this void with fantasy - at best, with a hypothesis. Again a lot of sloff I wrote?

Here you can see the distrust of the sciences. Naturally, everyone has the right to doubt the discoveries and laws of science. One can doubt even Newton's laws. But our everyday doubts such as a conversation - "Science say? Something is unbelievable" cannot be compared with the doubts of a specialist. They are as different as heaven from earth.
Do you remember Chekhov's short story "Letter to a learned neighbor"? There, an inquisitive neighbor doubted whether there were spots on the Sun and proved their apparent absence as follows: "It cannot be, because it can never be."
The Higgs boson is not a theoretical fiction, but it was highlighted in the course of experiments as the "missing link" in the system of elementary particles. Higgs roughly described its characteristics based on the behavior of other particles. This is very similar to the discovery of Pluto - the "missing planet" of the solar system and it was discovered by predicted characteristics, that is, calculated.
Interpretation scientific facts- this is again not an everyday matter, but a purely professional matter. The world community will never miss a hack, as it repeatedly checks any new facts. If there is an ambiguous interpretation, then he speaks openly about it and collects new experimental data.
Science for some 300 years has led mankind from a torch and a candle to electrification, telegraph, telephone, radio, electronics, computer, information revolution, space exploration. And all the same, there are critics of science and its homegrown debunkers - especially among believers and esotericists, who at the same time very willingly use the benefits of science and technology.
Such a contradictory human nature. Psychology Mystery?

To talk about distrust of science in relation to me is not entirely correct. I draw the conclusion to something else: it is impossible to fall into euphoria from the received scientific data and build far-reaching forecasts. Firstly, it has repeatedly happened that the experimental data were not given a completely correct or complete explanation, and secondly, one should not forget that each subsequent theory must include the previous one as a special case.
If we talk specifically about Newton's laws, then we can, for example, pay attention to the following nuance.
There is a "gravitational constant" (~6.67x...) in the Law of universal gravitation. At one time, many years of experiments were carried out in order to accurately calculate its value, but in the end we can only talk about a probabilistic characteristic. I fully admit that Newton's formula in the usual sense is valid only for relatively small masses, as mentioned by Bril (not the fact that it is so!).
By the way, it is interesting that for the interaction of electric charges the formula looks almost the same, only instead of the "gravitational constant" - "dielectric" (as applied to a specific medium).

In the Higgs boson, I am greatly confused by its declared mass, which is many times greater than even the mass of a proton. I'm surprised it wasn't opened earlier. In general, experiments on accelerators remind me of an attempt to find out, for example, how a house works, smashing it to smithereens and then building a picture from the wreckage.
Finally, there are many testimonies (especially regarding history) that do not fit into the usual ideas, but they try not to remember them so as not to confuse the minds.

(PS I am always annoyed by a long exchange of opinions in the margins of other people's reviews. While maintaining further interest in the dialogue, if you don't mind, I suggest continuing it on my pages or, even more conveniently, via regular e-mail.)

The daily audience of the Proza.ru portal is about 100 thousand visitors, who in total view more than half a million pages according to the traffic counter, which is located to the right of this text. Each column contains two numbers: the number of views and the number of visitors.

Objects and , they are moons. While most planets have moons, and some Kuiper belt objects and even asteroids have moons of their own, there are no known "moon moons" among them. Either we were unlucky, or the fundamental and extremely important rules of astrophysics complicate their formation and existence.

When all you need to keep in mind is one massive object in space, things seem pretty simple. will be the only workforce and you will be able to place any object on a stable elliptical or circular around it. According to this scenario, it seems that he will be in his position forever. But here other factors come into play:

• the object may have some sort of or diffuse "halo" of particles around;
• the object will not necessarily be stationary, but will rotate - probably rapidly - about an axis;
• this object will not necessarily be isolated as you originally thought

The tidal forces that act on the satellite are enough to pull out its ice crust and heat the interior, so that the subsurface ocean erupts hundreds of kilometers into space.

The first factor, atmosphere, only makes sense as a last resort. Usually an object that orbits a massive and solid world with no atmosphere will just need to avoid the surface of that object and stay close indefinitely. But if you add an atmosphere, even an incredibly diffuse one, any body in orbit will have to deal with the atoms and particles surrounding the central mass.

Even though we usually think that our atmosphere has an "end" and that space begins at a certain altitude, the reality is that the atmosphere simply dwindles as you go higher and higher. The atmosphere extends for many hundreds of kilometers; it will even go out of orbit and burn up if we do not constantly adjust it. By the standards of the solar system, a body in orbit must be at a certain distance from any mass in order to remain "safe".

In addition, the object can rotate. This applies to both a large mass and a smaller one rotating around the first one. There is a "stable" point where both masses are tidally locked (i.e., always facing each other on the same side), but any other configuration would produce a "torque". This twist will either spiral both masses inward (if the rotation is slow) or outward (if the rotation is fast). On other worlds, most satellites are not born under ideal conditions. But there is one more factor we need to consider before diving headlong into the “satellite of satellites” problem.

Mercury revolves around our Sun relatively quickly, and therefore the gravitational and tidal forces acting on it are very large. If something else were orbiting Mercury, there would be many more additional factors.

1. "Wind" from the Sun (a stream of outgoing particles) would crash into Mercury and an object near it, knocking them out of orbit.
2. The heat that the Sun bestows on the surface of Mercury can cause the expansion of Mercury's atmosphere. Despite the fact that Mercury is airless, the particles on the surface are heated and thrown into space, creating an atmosphere, albeit weak, but.
3. Finally, there is a third mass that wants to bring about a final tidal lock: not only between a small mass and Mercury, but also between Mercury and the Sun.

Therefore, for any satellite of Mercury, there are two limiting locations.

Every planet that orbits a star will be most stable when tidally locked with it: when its orbital and rotational periods match. If one more object is added to an orbit to a planet, its most stable orbit will be mutually tidally locked with the planet and the star near the point

If the satellite is too close to Mercury for a number of reasons:

• does not rotate fast enough for its distance;
• Mercury doesn't spin fast enough to be tidally locked with the Sun;
• susceptible to slowdown from ;
• will be subject to the significant friction of the Mercurial atmosphere,

it will eventually fall to the surface of Mercury.

When an object collides with a planet, it can kick up debris and cause nearby moons to form. This is how the earth's moon appeared and so did the satellites of Pluto.

Conversely, it risks being ejected from Mercury's orbit if the satellite is too far away and other considerations apply:

• the satellite is rotating too fast for its distance;
• Mercury rotates too fast to be tidally locked with the Sun;
• the solar wind gives additional speed to the satellite;
• interference from other planets pushes the satellite out;
• the heating of the Sun imparts additional kinetic energy to a distinctly small satellite.

With all that said, do not forget that many planets have their own satellites. Although a three-body system will never be stable, unless you tweak its configuration to ideal criteria, we will be stable for billions of years under the right conditions. Here are a few conditions that will make the task easier:

1. Take a planet/asteroid so that the main mass of the system is significantly removed from the Sun, so that the solar wind, flashes of light and tidal forces of the Sun are insignificant.
2. So that the satellite of this planet / asteroid is close enough to the main body so that it does not dangle much gravitationally and is not accidentally pushed out in the process of other gravitational or mechanical interactions.
3. To ensure that the satellite of this planet/asteroid is far enough from the main body that tidal forces, friction or other effects do not lead to convergence and merging with the parent body.

As you may have guessed, there is a "sweet apple" in which the moon can exist near the planet: several times further away from the radius of the planet, but close enough that the orbital period is not too long and still significantly shorter than the orbital period of the planet relative to the star. So, if you take all this together, where are the satellites of the satellites in our solar system?

There are currently 34 known satellites of the planets, but their number is constantly increasing as a result of new discoveries. The number of discovered asteroids, which has already exceeded 2000, is growing even faster. At the same time, our knowledge of the nature of these bodies is enriched. Both satellites of Mars - Phobos and Deimos, Jupiter's satellite Ganymede are photographed from spacecraft. The time is not far off when the researchers will have in their hands photographs of a number of satellites of Jupiter and Saturn, obtained at close range.

By size, planetary satellites and asteroids can be divided into three groups. The first group should include four Galilean satellites of Jupiter (Po, Europa, Ganymede, Callisto), Saturn's satellite Titan, Neptune's satellite Triton, and also our Moon.

These bodies are 3-5 thousand km in diameter and, in terms of their physical properties, closely adjoin the terrestrial planets, especially such as Mercury, Mars and Pluto. The three largest satellites are larger than the planet Mercury (and, even more so, Pluto). Data about them are summarized in Table. 6.

Table 6

The second group includes the remaining satellites of Saturn and Neptune, the satellites of Uranus and Pluto (16 in total), as well as several of the largest asteroids.

Table 7

Contrary to popular belief, not four, but 26 asteroids have a diameter of 200 km or more. Information about 15 satellites of the second group and about 15 largest asteroids is given in Table. 7.

Rice. 36. Comparative sizes of satellites of planets and some asteroids.

Finally, the third group includes the small satellites of Mars and Jupiter and all other asteroids, that is, bodies with a diameter of less than 200 km (Fig. 36).

Of greatest interest are, of course, large satellites. The best studied among them is our Moon, but we do not dwell here in detail on the description of its nature, since another book in this series will be devoted to the Moon.

By it's nature, internal structure and surface structure, the Moon is very similar to Mercury, but the average density of the Moon is 3.33 g/cm3 versus 5.45 g/cm3 for Mercury. As we said in $13, this is due to the enrichment of Mercury's substance with iron and ferrous compounds.

In the "seas" of the Moon, heavy igneous rocks predominate - basalts (average density 3.3 g / cm3), in the continents - lighter anorthosites (density 2.8 g / cm3).

In the four Galilean satellites of Jupiter, we observe a transition from "moon-like" (Io, Europa) to "ice-like" (Ganymede, Callisto). The low average densities of the last two satellites, in spite of their relatively large size and mass, clearly indicate that they are mostly composed of ice.

In 1971, the American theoretical scientist J. Lewis built models of these satellites and came to the conclusion that they must have a dense solid core, an extensive almost isothermal mantle made of an aqueous solution of ammonium, and a thin ice crust. Such a model with a liquid mantle may surprise readers, but it is based on the assumption that “water (the most common hydrogen compound in nature) makes up about 55% by mass of“ ice-like ”satellites, ammonia and methane - about 15%, the remaining 30% (account for There should be ice on the surface of the satellites (their temperatures are in the range from 120 to l60 ° K), but as you go to deeper layers, the temperature will rise and the ice will melt, turn into liquid water enriched with ammonium and other This is how the "ice-like" Ganymede, Callisto and Titan are built.On the contrary, Io, Europa and, apparently, Triton in many ways resemble the Moon.

However, it must be noted that the dimensions of Triton are determined very inaccurately, so there is no certainty in the values ​​\u200b\u200bof its density and albedo. Triton is the only one large satellites planets in the solar system, having a reverse motion.

In addition, its orbit is inclined to the plane of the equator of Neptune by 20 °, while the orbits of other large satellites lie almost in the plane of the equator of their planet (this is the case in the systems of Jupiter, Saturn and Uranus). These features of Triton's orbit, as well as the small size and nature of Pluto's orbit, forced R. Littleton back in 1936 to put forward a hypothesis that Pluto is a former satellite of Neptune that experienced a close approach to another massive satellite, Triton, as a result of which Pluto was ejected from the system Neptune and became an independent planet, and Triton moved into a reverse orbit with a high inclination. This hypothesis continues to be discussed today.

In 1976, two groups of American astronomers, headed by W. Fink and D. Morrison, independently conducted studies of the infrared spectrum and reflectivity of the four "medium" satellites of Saturn: Rhea, Iapetus, Tethys and Dione. The results indicated that their surfaces were at least partially covered with ordinary water ice. This explains the high albedo values ​​of these satellites.

On the other hand, Titan is the darkest of all large satellites, excluding the Moon. The low albedo of Titan is all the more incomprehensible because back in 1944, J. Coyper discovered an atmosphere containing methane in it, and this discovery was then repeatedly confirmed. According to the American spectroscopy astronomer L. Trafton, the total amount of gas in the vertical column of Titan's atmosphere is 1.6 km-atm, i.e., 25 times more than in the atmosphere of Mars. The pressure of the atmosphere near the surface of Titan is estimated at 0.1 atm. Apparently, methane is the main gas of Titan's atmosphere, although the presence of pure hydrogen is also assumed.

According to some details of the spectrum of Titan in the "window" near the wavelength of 4.9 microns, T. Owen and his colleagues concluded that the surface of Titan is also covered with ice. On the other hand, the study of the absorption bands of methane shows that the atmosphere of the satellite cannot be purely gaseous: it must contain clouds, and very dark clouds that absorb the incident sunlight, otherwise it will be difficult to explain the low albedo of Titan.

What substance they may consist of is not yet clear.

Titan is supposed to have an ionosphere containing at least. Moreover, Titan is able to intercept and neutral atoms and thereby make up for the loss of its atmosphere.

Titan, like a number of other satellites of Saturn, faces its planet with the same side, like the Moon to the Earth. This is not surprising: Saturn's mass is 95 times Earth's, and although Titan is more than three times farther from Saturn than the Moon is from Earth, tidal acceleration on Titan's surface is five times stronger than on the surface of the Moon. It is even stronger in the satellites of Rhea, Dione, Tethys and Enceladus, which are closer to Saturn (18, 35, 66 and 90 times more compared to the Moon). Photometric observations have confirmed that all these satellites also face Saturn on the same side. Undoubtedly, the closer Mimas and Janus are also oriented in the same way (there are no observational data for them yet). Turned one side to Saturn and the more distant Iapetus. It has long been noticed that its front (in the direction of movement) hemisphere is five times darker than the back (their albedo is 0.07 and 0.35, respectively). Therefore, in western elongations, Iapetus is two magnitudes brighter than in eastern ones (see § 7). But the tidal acceleration created by Saturn on Iapetus is 18 times weaker than that created by the Earth on the Moon. But over the 4.5 billion years of the existence of the solar system, it slowed down the rotation of Iapetus and forced it to turn to Saturn on one side.

Let us now turn to the Galilean satellites of Jupiter. The processing of 20-year observations of their surfaces at the Pic-du-Midi observatory allowed the French astronomer O. Dollfus and the American astronomer B. Murray to make the final conclusion that their rotation is synchronous, like that of the satellites of Saturn: their periods of rotation are equal to the periods of revolution around Jupiter, and they all face the planet on one side.

If we calculate tidal accelerations for them, it turns out that Io has 250 times more than the Moon, Europa - 53 times, Ganymede - 22 times, Callisto - four times. It is obvious that the closest satellite to Jupiter, Amalthea, also faces it on one side: despite its small size (it is 20 times smaller than Io), the tidal acceleration from Jupiter is 150 times greater than that experienced by the Moon.

Rice. 37. Photograph of Ganymede taken by Pioneer 11 at the end of 1974. Noticeably bright spot.

Spectral observations of Europa and Ganymede have shown that there is ice on their surfaces.

The variation of the albedo of both satellites along the spectrum also agrees with this conclusion. Europe has white spots near the poles, similar to polar caps. In Ganymede, the white spots are more chaotic (Fig. 37). There are suggestions that there may be ammonia frost on Ganymede. The rest of the surface of Ganymede, as shown by direct images of Pioneer 11 and radar, is very rough, stronger than that of Mercury. Most likely, outer layer The surface of Ganymede is an icy matrix interspersed with stone and iron. The density of the surface layer, according to D. Morrison and D. Cruikshank, 0.15 g/cm. The density of the outer layers of Io and Callisto is the same. Deposits of ammonium chloride are possible on the surface of Callisto.

No traces of an atmosphere of methane and ammonia were found in all four of Jupiter's Galilean satellites: according to the data of W. Fink's group, the amount of both gases in no case exceeds 0.5 cm-atm.

Jupiter's most interesting moon is undoubtedly Io. We have already spoken in § 16 about the influence of Io on Jupiter's magnetic field and its radiation of decameter radio waves. But No gave scientists a few more surprises.

In early 1974, the American astronomer R. Brown, using an echektrograph mounted on a 1.5-meter reflector at the Mount Hopkins observatory, obtained a series of spectrograms of Io, which clearly revealed the emission lines of the yellow sodium doublet. Their intensity showed that the optical depth of the sodium layer exceeded unity. None of the atmospheres of the planets of the solar system was observed to emit sodium. It was not found among the neighbors of Io: Europa, Gadamed and Callisto.

The glow of sodium in Io's atmosphere has attracted the attention of theorists. American scientists M. McElroy, L. Trafton, and others proposed the following explanation. Sodium atoms are "knocked out" from the surface of the satellite by impacts of high-energy particles from Jupiter's radiation belts. Other satellites are further from the planet and these particles do not reach them.

It soon became clear that the sodium glow is concentrated not only in the immediate vicinity of Io, but is stretched along the satellite's orbit and has the shape of a torus.

In addition, an ionosphere was discovered near Io with a maximum concentration of electrons (the highest concentration in the Earth's ionosphere), which means that sodium atoms are ionized. The main mechanism of ionization is the impact of electrons from Jupiter's radiation belts. Sodium ions are transported over long distances (mostly forward in Io's orbit), where they turn into neutral atoms, which create the glow.

Where do the sodium atoms on the surface of Io come from? What does it consist of? F. Faneuil, D. Matson and T. Johnson from the Jet Propulsion Laboratory (USA) conducted a series of experiments on sample bombardment rocks protons. For common salt (NaCl), an emission intensity was obtained comparable in magnitude to that observed in Io. The albedo of the satellite, according to the French astronomer O. Dollfuss, is very high: 0.83 in the equatorial zone (like that of a snow cover) and 0.46 in the polar regions. On this basis, the Faneuil group hypothesized that the surface of Io is covered with deposits of evaporation products of aqueous solutions saturated with salts coming from the warm or hot bowels of the satellite.

According to the theory of F. Faneuil and his collaborators, salt deposits should be rich not only in sodium, but also in sulfur. However, the presence of sulfur on the surface of Io has not been directly confirmed.

The search for other emission lines in the spectrum of Io began. In 1975, L. Trafton succeeded in using the 2.7-meter telescope of the McDonald Observatory (USA) to detect the glow of resonance potassium lines at wavelengths of 7665 and 7699 A at 20,000 km from the satellite. The intensity of these lines weakens with distance from And about.

Observations of emission lines in the spectrum of Io were also carried out by N. B. Ibragimov and A. A. Atai using a 2-meter reflector of the Shemakha Astrophysical Observatory of the Academy of Sciences of the Azerbaijan SSR. In addition to the already known doublet of sodium 5890-5896 A, they found many weak bands of iron, magnesium and calcium in the spectral range of 5900-5170 A in the spectrograms with large dispersion.

In March 1979, the American spacecraft Voyager 1 passed near Jupiter and Io. An analysis of images of Io taken from a relatively close distance showed that there are at least six active volcanoes on this satellite, spewing gases and dust to a height of about 500 km. Thus, the cloud of metal vapors around Io is possibly associated not with the carving of metal particles from the surface of Io by impacts of elementary particles, but with powerful volcanic eruptions from the surface of the satellite. What is the reason for the differences in the structure of the surfaces of these four satellites of Jupiter, future research will show.

Jupiter has two more groups of "irregular" or anomalous satellites. One of them, which includes VI Himalia, VII Elara, X Lysitea and the recently discovered XIII Leda, is located at distances of 11-12 million km from Jupiter. These satellites have direct motion, but their orbits have significant eccentricities (0.15-0.21) and inclinations to the plane of the planet's equator (25-29°). Another group includes VIII Pasiphe, IX Sinop, XI Karme and XII Ananke, which are located at distances of 21-24 million km from Jupiter and have a reverse movement. The eccentricities of these satellites are even greater (0.17-0.38), the inclinations are from 147 to 163°. Most likely, these satellites, whose radii range from 85 km (Himalia) to 5-8 km (Leda), are asteroids captured by Jupiter.

Some idea of ​​the external appearance of these satellites can be given by photographs of Mars' satellites Phobos and Deimos (Fig. 38) obtained from American spacecraft. These satellites are irregular shape boulders measuring 27x21x15 km (Phobos) and 15x12x8 km (Deimos), dotted with meteorite craters of various sizes from 10 km to very small ones. Devoid of atmospheres, these satellites have preserved for us the history of cosmic bombardment not only of themselves but also of their planet.

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Rice. 38. Photograph of Phobos (above) and Deimos obtained by Mariner 9.

The density of craters per unit surface makes Phobos and Deimos almost the same age as Mars. Both satellites are also facing Mars on the same side. Calculations have shown that it took tens of millions of years for Deimos to establish such a rotation, and only hundreds of thousands of years for Phobos - an insignificant time period in terms of cosmogonic scales.

In the photographs of Phobos taken in 1976-1977. from the Vikint-Orbiter spacecraft, long parallel furrows about 500 m wide are clearly visible (Fig. 39). They cross the most ancient craters, but young craters, in turn, are superimposed on furrows.

The location of the furrows turned out to be symmetrical with respect to the 10-km-long Stickney crater. This gave rise to the American astronomers J. Veverke, T. Duxbury and P. Thomas to put forward a hypothesis that the furrows are associated with deep faults formed during the impact of a giant meteorite that gave rise to the Stickney crater.

Back in 1945, the American astronomer B. Sharpless discovered a secular acceleration in the motion of Phobos. For 30 years, there has been a great discussion on this issue, both about the reality of the acceleration itself and about its possible explanations. The most complete processing of all observations of Phobos over 100 years led the Leningrad astronomer V. A. Shor and his colleagues to the conclusion that the effect is real. Phobos is gradually approaching Mars and in about 20-25 million years will fall on its surface. Thus, the explanation of the origin of the furrows according to Soter and Harris has some grounds. As for the secular acceleration of Phobos itself, back in 1959 Corr. N. N. Pariyskiy of the Academy of Sciences of the USSR showed that the cause of it is the tidal deceleration of the satellite: the tidal humps created by it in the crust of Mars slow down the movement of the satellite, it goes into a lower orbit and therefore its movement is accelerated.

The more distant Deimos does not experience such strong tidal drag, its orbit is more or less stable, and no furrows have been found on its surface.

The surface of the moons of Mars is very dark, their albedo is 0.05, like that of the lunar seas. Direct photographs, photoelectric and polarimetric observations indicate that the outer layer of the surface of both satellites is finely divided dust, the layer of which is about 1 mm thick. Its composition is apparently basaltic with a significant admixture of carbonates.

Infrared observations indicate an extremely low thermal conductivity of the outer cover, which confirms the hypothesis of a dust layer.

Let us now turn to the nature of asteroids. We will not consider the structure of the asteroid ring here, referring readers to A. N. Simonenko's brochure "The Asteroid Belt" (M.: Znanie, 1977) and to the articles listed in the bibliography at the end of the book. Consider the physical characteristics of these bodies.

Rice. 40. Reflectivity of asteroids as a function of wavelength (according to K. Chapman and T. McCord).

The reflectivity (albedo) and color of a number of large asteroids are eloquently indicated by the graphs constructed by K. Chapman and T. McCord (Fig. 40). A number of interesting conclusions follow from them. Thus, the asteroid Vesta reflects light almost 10 times more than Bamberg; Ceres and Pallas are almost gray (their reflectivity does not change with wavelength), and Juno is noticeably reddish (albedo in red rays increases). Vesta has a deep absorption band in the region of 0.9 μm, which was previously discovered in the spectrum of Mars by T. McCord and J. Adams. It is characteristic of the group of ferrosilicates (for example, olivines) and some iron oxides.

The low albedo of Ceres is comparable to that of the Moon and Mercury. But Nemause and especially Bamberg have an almost black surface, approaching the darkest meteorites, carbonaceous chondrites, on this basis.

Systematic measurements of the albedo and size of 187 asteroids were carried out over last years two groups of American astronomers led by D. Morrison and O. Hansen.

At the same time, they used two new methods: polarization, proposed in 4970 by J. Veverka from Cornell University and based on the known dependence of the nature of the change in polarization with phase on the albedo value, and radiometric, developed by D. Allen (University of Minnesota) and based on comparison of radiation fluxes at wavelengths of 10 and 20 μm. Both methods gave good agreement with each other.

It turned out that all the studied asteroids can be divided according to their reflectivity into three groups: dark (class C), similar in this respect to carbonaceous chondrites, light (class S), resembling ordinary silicates, and very light (class U) with an unclear mineralogical composition. Their distribution according to albedo clearly reveals two main groups: C and S (Fig. 41). Few asteroids belong to the U class, in which the albedo exceeds 0.2; in fig. 41 they form the right “wing” of a group of class S asteroids. Among them are (4) Vesta, (44) Nisa, (64) Angelina, (113) Amalthea (not to be confused with the nearest satellite of Jupiter - there are namesakes in the solar system too), (182) Elsa, (349) Dembowska and (434) Hungary.

Rice. 41. Distribution of asteroids according to their albedo (according to D. Morrison).

Among the darkest are (313) Chaldea (albedo 0.014), (95) Aretuza (albedo 0.019), (537) Pauli (albedo 0.021), (65) Cybele (albedo 0.022) and a number of others. 26 out of 187 asteroids (14%) have an albedo less than 0.03, i.e. less than that of Bamberga. By the way, Jupiter's anomalous jokers Himalia and Elara also have an albedo of 0.03, which confirms the assumption of their asteroidal nature and subsequent capture by Jupiter.

It is curious that class C asteroids have orbits located farther from the Sun than class S, and among the minor planets with large orbital semiaxes 3 AU. e. and more they make up 95% of all asteroids. In the inner part of the asteroid ring, the proportions of classes C and S are approximately equal.

G class asteroids are almost gray, class S are reddish.

Some asteroids are similar in their reflective and polarization properties to iron-nickel meteorites. These include (16) Psyche, (21) Lutetia and (89) Julia. Their albedo is close to 0.09.

A comparison of the orbits of asteroids and meteorites shows that these are bodies with common origin. As a rule, the orbits of meteorites have aphelion in the region of the asteroid belt. If we add here the similarity of their optical characteristics noted above, it becomes clear that the nature of these two groups of bodies is common. As is known, some asteroids of the Apollo group passed near the Earth, in particular, Hermes in 1937 passed only 580 thousand km from the Earth. In principle, the fall of such bodies to the Earth is not only possible, but has also taken place more than once in the past, as evidenced by numerous meteorite craters on Earth up to 100 km or more in diameter,

Satellites are celestial bodies that orbit around a certain object in outer space under the influence of gravity. There are natural and artificial satellites.

Our space portal site invites you to get acquainted with the secrets of the Cosmos, unthinkable paradoxes, exciting mysteries of the worldview, providing in this section facts about satellites, photos and videos, hypotheses, theories, discoveries.

There is an opinion among astronomers that a satellite should be considered an object that rotates around a central body (asteroid, planet, dwarf planet) so that the barycenter of the system, including this object and the central body, is located inside the central body. In the event that the barycenter is outside the central body, then this object cannot be considered a satellite, since it is a component of the system that includes two or more planets (asteroids, dwarf planets). But the International Astronomical Union has not yet given a precise definition of the satellite, arguing that this will be done in the near future. For example, the IAU continues to consider Pluto's satellite Charon.

In addition to all of the above, there are other ways to define the concept of "satellite", which you will learn about below.

Satellites to satellites

It is generally accepted that satellites can also have their own satellites, but the torrential forces of the main object in most cases would make this system extremely unstable. Scientists have assumed the presence of satellites in Iapetus, Rhea and the Moon, but to date, natural satellites have not been identified in the satellites.

Interesting facts about satellites

Among all the planets of the solar system, Neptune and Uranus never had their own artificial satellite. Satellites of the planets are small cosmic bodies of the solar system that revolve around the planets through their attraction. To date, 34 satellites are known. Venus and Mercury, the planets closest to the Sun, do not have natural satellites. The Moon is the only satellite of the Earth.

The moons of Mars - Deimos and Phobos - are known for their small distance to the planet and relatively fast movement. The Phobos satellite sets twice during the Martian day and rises twice. Deimos moves more slowly: more than 2.5 days pass from the beginning of its sunrise to sunset. Both satellites of Mars move almost exactly in the plane of its equator. Thanks to spacecraft, it was found that Deimos and Phobos in their orbital motion have an irregular shape and remain turned to the planet with only one side. The dimensions of Deimos are about 15 km, and the dimensions of Phobos are about 27 km. The moons of Mars are composed of dark minerals and are covered with numerous craters. One of them has a diameter of 5.3 km. Probably, the craters were born by meteorite bombardment, and the origin of the parallel furrows is still unknown.

The mass density of Phobos is approximately 2 g/cm 3 . The angular velocity of Phobos' movement is very high, it is able to overtake the axial rotation of the planet and, unlike other luminaries, sets in the east and rises in the west.

The most numerous is the system of satellites of Jupiter. Among the thirteen satellites orbiting Jupiter, four were discovered by Galileo - these are Europa, Io, Callisto and Ganymede. Two of them are comparable in size to the Moon, and the third and fourth are larger than Mercury, although they are significantly inferior in weight to it. Unlike other satellites, the Galilean satellites have been studied in more detail. In good atmospheric conditions, it is possible to distinguish satellite data disks and notice certain details on the surface.

According to the results of observations of changes in the color and brightness of the Galilean satellites, it has been established that each of them has a synchronous axial rotation with the orbital one, therefore they face Jupiter with only one side. The Voyager spacecraft took pictures of the surface of Io, on which active volcanoes are clearly visible. Bright clouds of eruption products rise above them, which are ejected to a great height. It has also been observed that there are reddish spots on the surface. Scientists suggest that these are salts evaporated from the bowels of the earth. An unusual feature of this satellite is the cloud of gases surrounding it. The Pioneer 10 spacecraft provided data that led to the discovery of the ionosphere and rarefied atmosphere of this satellite.

Among the number of Galilean satellites, it is worth highlighting Ganymede. It is the largest among all the satellites of the planets in the solar system. Its dimensions are more than 5 thousand km. From Pioneer-10, images of its surface were obtained. Spots and a bright polar cap are clearly visible in the image. Based on the results of infrared observations, it is believed that the surface of Ganymede, just like that of another satellite, Callisto, is covered with frost or water ice. Ganymede has traces of an atmosphere.

All 4 satellites are objects of 5-6th magnitude, they can be seen with any binoculars or telescope. The remaining satellites are much weaker. The closest satellite to the planet is Amalthea, it is only 2.6 radius of the planet.

The remaining eight satellites are at great distances from Jupiter. Four of them revolve around the planet in the opposite direction. In 1975, astronomers discovered an object that is the fourteenth satellite of Jupiter. To date, its orbit is unknown.

In addition to the rings, which consist of a swarm of numerous small bodies, ten satellites have been discovered in the system of the planet Saturn. These are Enceladus, Mimas, Dione, Tethys, Titan, Rhea, Iapetus, Hyperion, Janus, Phoebe. The closest to the planet is Janus. It moves very close to the planet, it was possible to identify it only during the eclipse of the rings of Saturn, which created a bright halo in the field of view of the telescope.

Titan is the largest moon of Saturn. In terms of mass and size, it is one of the largest satellites in the solar system. Its diameter is about the same as that of Ganymede. It is surrounded by an atmosphere that consists of hydrogen and methane. Opaque clouds are constantly moving in it. Only Phoebe of all satellites rotates in the forward direction.

The satellites of Uranus - Ariel, Oberon, Miranda, Titania, Umbriel - rotate in orbits whose planes almost coincide with each other. In general, the entire system is distinguished by an original inclination - its plane is almost perpendicular to the average plane of all orbits. In addition to satellites, a huge number of small particles move around Uranus, which form peculiar rings that are not similar to the known rings of Saturn.

The planet Neptune has only two moons. The first was discovered in 1846, two weeks after the discovery of the planet itself, and is called Triton. It is larger than the Moon in mass and size. Differs in the reverse direction of orbital motion. The second - Nereid - is small, characterized by a strongly elongated orbit. Direct direction of orbital motion.

Astrologers discovered a moon of Pluto in 1978. This discovery of scientists has great importance, because it makes it possible to calculate the mass of Pluto as accurately as possible from the data on the period of revolution of the satellite, and in connection with the discussion that Pluto is the “lost” satellite of Neptune.

One of the key issues of modern cosmology is the origin of satellite systems, which in the future can reveal many secrets of the Cosmos.

Captured satellites

Astronomers aren't entirely sure how moons form, but there are many working theories. Most of the smaller moons are thought to be captured asteroids. After the formation of the solar system, millions of space boulders roamed the skies. Most of them were formed from materials left over from the formation of the solar system. Perhaps others are the remnants of planets that have been shattered into pieces by massive cosmic collisions. The greater the number of small satellites, the correspondingly more difficult it is to explain their occurrence. Many of these may have originated in a region of the solar system such as the Kuiper Belt. This zone is located on the upper edge of the solar system and is filled with a thousand small planet-like objects. Many astronomers believe that the planet Pluto and its moon may actually be Kuiper Belt objects and should not be classified as planets.

The fate of the satellites

Phobos - the doomed satellite of the planet Mars

Looking at the moon at night, it's hard to imagine that she would be gone. However, in the future, the Moon may indeed not exist. It turns out that the satellites are not permanent. By making measurements with laser beams, scientists have found that the Moon is moving away from our planet at a rate of about 2 inches per year. The conclusion follows from this: millions of years ago it was much closer than it is now. That is, when dinosaurs still walked on Earth, the Moon was several times closer than it is today. Many astronomers believe that one day the Moon may break free of the Earth's gravitational field and go into space.

Neptune and Triton

Other satellites also faced similar fates. For example, Phobos is actually, on the contrary, approaching the planet. And someday he will end his life, plunging into the atmosphere of Mars in fiery agony. Many other satellites can collapse under the influence of the tidal forces of the planets around which they constantly revolve.

Many of the rings surrounding the planets are composed of particles of stone and fire. They may have formed when the moon was destroyed by the planet's gravity. These particles form thin rings over time, and you can see them today. The rest of the satellites next to the rings help keep them from falling. The satellite's gravitational force keeps the particles from rolling back to the planet after being pulled out of orbit. Among scholars, they are called shepherd satellites, as they help keep the rings in line, like a shepherd grazing sheep. If there were no moons, Saturn's rings would have long since disappeared.

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