Astronomers calculated how Saturn captures bodies from the surrounding space and "calculated" dozens of such exotic satellites at once, making the most accurate prediction in the history of astronomy. Let's try to figure out how they did it.
In 1995, scientists from the post-Soviet space built the Saturn-2 model. She was able to predict new celestial bodies, a whole vast area where Saturn's moons, created from material captured from space, rotate. By October 2019, 41 such satellites have already been discovered, and at the moment 82 moons have been discovered near Saturn. Perhaps, we are talking about the most accurate prediction in the history of astronomy.
Astronomy makes fun of mathematical modeling. They say, they have questionable accuracy, and often their results are incompatible with the observed reality.
What is the only theory of the formation of the Moon due to a collision with another planet: no one has yet been able to explain why the earth's water remained on the planet (judging by the ratio of deuterium and hydrogen, it is not cometary-asteroid, but local), although it should have boil off immediately after the mega-impact. But in October 2019, it's time to admit that sometimes models perform better than we expect.
How to calculate the invisible
In October 2019, Scott Sheppard's group from the Carnegie Institution (USA) announced the discovery of 20 new moons of Saturn this year alone. In the past, by the way, they found 12 more. All these discoveries were made with the 8, 2-meter Japanese telescope "Subaru". As a result, Saturn has 82 satellites - the most in the solar system and three more than Jupiter.
However, there are things more amazing than the discovery of 32 new satellites on one planet in two years. Almost all newly discovered bodies are in line with a prediction made a long time ago. In 1995, researchers from the Simeiz Observatory (Crimea), Nikolai Gorkavy and Tatiana Taydakova published an article in Astronomy Letters, where they made an unexpected discovery. It turned out that some of the bodies flying past Saturn must collide with the particles of the circumplanetary disk, lose speed and be captured by the gravity of a huge planet. Moreover, after capture, these bodies will rotate "against the grain": in astronomy, this is called retrograde or reverse rotation.
As they were able to show, the reason for the formation of many "anti-wool satellites" is pure celestial mechanics. If a body flying past the planet falls into a circumplanetary disk, which rotates in the same direction as this body is flying, then the velocities of this body and gas and dust particles from the circumplanetary disk do not differ very much. If the body flies against the direction of rotation of the gas and dust of the near-planetary disk, then the speed will be very different. A simple analogy: a collision of cars traveling in the same lane has a low energy, but if they collide in an opposite lane, the energy of this event is much higher. Losing a lot of energy on oncoming collisions, a celestial body flying past quickly loses its speed. It falls so hard that the planet's gravity can keep the "stranger".
Often times it is blown into a lot of debris. But even such debris is effectively captured by the planet's gravity, and then gradually merge with each other, forming the next "satellites rotating against the grain."
In 1995, when this article was published, astronomers knew about 17 "forward" and only one "backward" satellite of Saturn - Phoebe.She was considered a strange exception, only by a strange coincidence captured by the gravity of a gas giant from the surrounding space. After all, no one knew about braking in the circumplanetary disk. But even then it was clear that "in place" a satellite with the direction of rotation opposite to the planet could not arise.
Let us explain why - using a simple analogy. If you pick up a ball on a string and start spinning in one place, then the direction of rotation for you and the ball will be the same. For normal satellites of the planet, the direction of rotation also coincides with the direction of rotation of their planets. This is because satellites and planets are formed from the same material of common origin - from a protoplanetary disk, and in this disk the planet itself and the debris rushing around it arise, which is why they rotate in one direction.
Saturn II and Zone IV
Having tried to calculate in which zones near Saturn bodies with reverse rotation can accumulate, Gorkavy and Taidukova found that there is a so-called zone IV - extending strongly beyond the orbit of Phoebe, at a distance of 18 to 31 million kilometers from Saturn. The density of satellites with a normal, "straight" orbit, rotating in the same direction as Saturn, in zone IV turned out to be negative. This meant not only that, according to the model, the appearance of "forward" satellites was unlikely, but also that "reverse" satellites were accumulating there.
The last paragraph of their 1995 work ended like this: “Based on our model of Saturn-2, we assume that near Saturn there may be a group of backward rotation satellites that has not yet been discovered …” As we can see, this is a very rare and ambitious statement: so far, calculations at the tip of a feather in the solar system have succeeded in discovering exactly one body - and that was Neptune. The discovery was so significant that it sparked a heated debate with an attempt to steal scientific priority in the discovery, and the echoes of this scandal are still raging in 2019, more than a century and a half after the discovery.
The last paragraph of their 1995 work ended like this: "Based on our model of Saturn-2, we assume that near Saturn there may be a group of satellites of reverse rotation that has not yet been discovered, farthest from the planet …"
The question naturally arises: to what extent was the prediction about zone IV justified, where satellites with reverse rotation should dominate? Was it possible for the first time since the 19th century to predict celestial bodies in the solar system?
In October 2019, Nikolai Gorkavy reacted to the announcement of the discovery of 20 new satellites of Saturn, indicating on a chart from 1995 work where the orbits of those that are open for 2019 lie. It turned out that in a somewhat compressed ("Saturn-2" was calculated in the first half of the 90s, with a different level of computer technology), zone IV contains 41 satellites with reverse rotation and only two with forward rotation. In 1995, no satellites from Zone IV were discovered yet. In 2019, "the group of satellites of reverse rotation that has not yet been discovered, the farthest from the planet," was discovered - and it contains half of Saturn's satellites: 41 out of 82.
The model "Saturn-2" from 1995 was able to predict not one, but dozens of celestial bodies in the solar system at once
The model "Saturn-2" from 1995 was able to predict not one, but dozens of celestial bodies in the solar system "at the tip of a feather", and today this is the only achievement of this kind known for the XX-XXI centuries.
What does this mean for the future of astronomy
It should be understood: by this mechanism of "oncoming collisions", asteroids are decelerated and captured by planets, becoming reverse satellites, not only at Saturn. Jupiter has a similar group of moons (it was discovered earlier because Jupiter is closer). Moreover, in the system of Neptune's satellites, the most massive is Triton, just a satellite of reverse rotation, in appearance not similar to an ordinary satellite, but to a body from the Kuiper Belt, a large debris ring on the outskirts of the solar system, from where Triton's material came from.
All this means that models similar to Saturn-2, but at a new technical level, can predict the location of satellites not yet discovered in other distant planets of the solar system, as well as in the distant future and satellites of exoplanets. The importance of such a tool can hardly be overestimated. Who knows, perhaps similar mechanisms could be responsible for the formation of larger bodies - perhaps entire planets?