In the solar system there is a sun - in the center - many planets, asteroids, Kuiper belt objects and satellites, they are also moons. Although most planets have satellites, and some Kuiper belt objects and even asteroids also have their own satellites, there are no known "satellites of satellites" among them. Either we're out of luck, 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 straightforward. Gravity will be the only working force, and you can place any object in a stable elliptical or circular orbit around it. Under this scenario, it seems, he will be in his position forever. But other factors come into play here:
- the object may have a kind of atmosphere or a diffuse "halo" of particles around;
- the object will not necessarily be stationary, but will rotate - probably quickly - around an axis;
- this object will not necessarily be isolated as you originally thought.
The tidal forces that act on Saturn's moon Enceladus are enough to pull out its ice crust and heat the bowels, so that the subsurface ocean erupts hundreds of kilometers into space
The first factor, atmosphere, only makes sense as a last resort. Typically, an object that orbits a massive and solid world with no atmosphere will only need to avoid the object's surface and it will stick around indefinitely. But if the atmosphere, even incredibly diffuse one, is augmented, 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 just dries up as you go higher and higher. The Earth's atmosphere extends for many hundreds of kilometers; even the International Space Station will go out of orbit and burn if we don't constantly urge it on. 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”.
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Whether it's an artificial satellite or a natural one doesn't really matter; if it orbits a world with a substantial atmosphere, it will de-orbit and fall onto the nearest world. All satellites in low Earth orbit will do so, as will Mars' satellite Phobos
In addition, the object can rotate. This applies to both a large mass and a smaller one rotating around the first. There is a "stable" point where both masses are tidally locked (that is, always facing each other on one side), but any other configuration will create a "torque". This twisting 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 in ideal conditions. But there is one more factor that we need to consider before diving headlong into the problem of the "satellite of satellites".
The Pluto - Charon model shows two main masses revolving around one another. A flyby of the "New Horizons" showed that Pluto or Charon have no internal satellites relative to their mutual orbits
The fact that the object is not isolated is of great importance. It is much easier to keep an object in orbit near a single mass - like a moon near a planet, a small asteroid near a large one, or Charon near Pluto - than to keep an object in orbit near a mass that itself orbits a different mass. This is an important factor and we don't think much about it. But let's look at it for a second from the perspective of our closest to the Sun, the moonless planet Mercury.
Mercury revolves around our Sun relatively quickly, and therefore the gravitational and tidal forces acting on it are very large. If something else revolved around 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, which the Sun bestows on the surface of Mercury, can lead to the expansion of the atmosphere of Mercury. Despite the fact that Mercury is airless, particles on the surface heat up and are thrown into space, creating a faint atmosphere.
3. Finally, there is a third mass that wants to lead to the final tidal blockage: not only between low mass and Mercury, but also between Mercury and the Sun.
Therefore, for any moon of Mercury, there are two extreme locations.
Every planet that orbits a star will be most stable when tidal locked with it: when its orbital and rotational periods coincide. If you add another object to orbit to the planet, its most stable orbit will be mutually tidally locked with the planet and star near L2
If the satellite is too close to Mercury for a number of reasons:
- does not rotate fast enough for its distance;
- Mercury is not rotating fast enough to be tidal locked with the Sun;
- susceptible to solar wind deceleration;
- will be subject to significant friction from the Mercury atmosphere, - it will eventually fall to the surface of Mercury.
When an object collides with a planet, it can lift debris and cause nearby moons to form. This is how the Earth's Moon appeared and the satellites of Mars and Pluto also appeared.
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 spins too fast to be tidally locked with the Sun;
- the solar wind gives additional speed to the satellite;
- interference from other planets push the satellite out;
- the heating of the Sun gives additional kinetic energy to a definitely small satellite.
With that said, keep in mind that many planets have their own moons. Although a three-body system will never be stable, unless you adjust its configuration to ideal criteria, we will be stable for billions of years under the right conditions. Here are some conditions that will make the task easier:
1. Take the planet / asteroid so that the bulk 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 heavily gravitationally and is not accidentally pushed out during other gravitational or mechanical interactions.
3. That the satellite of this planet / asteroid was far enough away from the main body so that tidal forces, friction or other effects do not lead to approach and merger with the parent body.
As you might have guessed, there is a "sweet bullseye" in which the moon can exist near the planet: several times beyond the radius of the planet, but close enough so 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?
Asteroids in the main belt and Trojans near Jupiter may have their own satellites, but they themselves do not consider themselves as such.
The closest we have are Trojan asteroids with their own satellites. But since they are not "satellites" of Jupiter, this is not entirely appropriate. What then?
The short answer: we are unlikely to find something like that, but there is hope. Gas giant worlds are relatively stable and far enough from the Sun. They have many satellites, many of which are tidally locked with their parent world. The largest moons will be the best candidates for satellites. They should be:
- as massive as possible;
- relatively removed from the parent body to minimize the risk of collision;
- not too far away so as not to be pushed out;
- and - this is new - well separated from other moons, rings or satellites that could disrupt the system.
Which moons in our solar system are best suited to acquire their own satellites?
- Jupiter's moon Callisto: the outermost of all Jupiter's large moons. Callisto, which is 1,883,000 kilometers away, also has a radius of 2,410 kilometers. It travels around Jupiter in 16.7 days and has a significant escape velocity of 2.44 km / s.
- Jupiter's moon Ganymede: the largest moon in the solar system (2634 km radius). Ganymede is very far from Jupiter (1,070,000 kilometers), but not enough. It has the fastest escape velocity of all satellites in the solar system (2.74 km / s), but the densely populated system of the giant planet makes it extremely difficult for Jupiter's satellites to acquire satellites.
- Saturn's moon Iapetus: not very large (734 kilometers in radius), but quite distant from Saturn - at 3,561,000 kilometers at an average distance. It is well separated from the rings of Saturn and from other large moons of the planet. The only problem is its small mass and size: the escape velocity is only 573 meters per second.
- Uranus's satellite Titania: With a radius of 788 kilometers, Uranus's largest satellite is 436,000 kilometers from Uranus and completes its orbit in 8.7 days.
- Uranus' satellite Oberon: the second largest (761 kilometers), but the most distant (584,000 kilometers) large moon completes its orbit around Uranus in 13.5 days. Oberon and Titania, however, are dangerously close to each other, so the "moon of the moon" is unlikely to appear between them.
- Neptune's satellite Triton: this captured Kuiper belt object is huge (1355 km in radius), far from Neptune (355,000 km) and massive; the object needs to move at a speed of more than 1.4 km / s in order to leave Triton's field of attraction. Perhaps this is our best candidate for the right to own your own satellite.
Triton, Neptune's largest moon and a captured Kuiper belt object, might be our best bet for a moon with its own moon. But Voyager 2 saw nothing.
With all this, as far as we know, there are no satellites in our solar system with their own satellites. Perhaps we are mistaken and find them at the far end of the Kuiper belt or even in the Oort cloud, where objects are a dime a dozen.
The theory says that such objects can exist. This is possible, but it requires very specific conditions. As for our observations, such have not yet appeared in our solar system. But who knows: the universe is full of surprises. And the better our search capabilities become, the more surprises we will find. No one will be surprised if the next grand mission to Jupiter (or other gas giants) finds a satellite near a satellite. Time will tell.
ILYA KHEL