Can a LUN be your moon?

In the Solar system is the Sun — in the center — many planets, asteroids, Kuiper belt objects and the moons, they are moons. Although most of the planets have satellites, and some Kuiper belt objects and even asteroids too, have their own satellites, known “satellites of satellites” is not among them. If we are not lucky, or basic and very important rules of astrophysics complicate their education and existence.

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

the object can be in a certain kind of atmosphere or diffuse “halo” of particles around;
the object is not necessarily stationary, but will rotate — probably fast — axis;
this object will not necessarily be as isolated as you initially thought.

Tidal forces acting on Saturn’s moon Enceladus, enough to stretch its icy crust and heat the subsurface so that the subsurface ocean is erupting hundreds of miles into space

The first factor, the atmosphere, makes sense only in the most extreme case. Usually an object that revolves around a massive and solid the world without an atmosphere, would be enough to avoid the surface of this object, and it will stick around indefinitely. But if you increase the atmosphere, even incredibly diffuse, any body in orbit will have to deal with atoms and particles, surrounding the Central mass.

Despite the fact that we usually believe that our atmosphere is “the end” and at a certain height begins space, the reality is that the atmosphere is just exhausted when you climb higher and higher. The Earth’s atmosphere extends for hundreds of kilometers; even the international space station will descend from orbit and burn up if we don’t it to constantly adjust. By the standards of the Solar system body in orbit must be at a certain distance from any mass to stay “safe”.

Whether it’s an artificial satellite or a natural, not a big deal; if it will be in orbit of the world with a significant atmosphere, he will descend from an orbit and will fall on the next world. All satellites in low earth orbit will do so as a satellite of Mars, Phobos

In addition, the object can rotate. This applies to both a large mass and a smaller orbiting around the first. There is a “stable” point in which both masses are tidal locked (always facing each other on one side), but any other configuration will be “torque”. This torsion or twist, both masses will spiral inwards (if rotation is slow) or outward (if the rotation is fast). In other worlds most of the satellites are not born in ideal conditions. But there is another factor that we need to consider before you head dive into the problem of “satellites of satellites”.

The model of the system Pluto — Charon shows two masses, rotating around one another. Flyby New horizons showed that Pluto or Charon no inner satellites concerning their mutual orbits

The fact that the object is not isolated, is important. Much easier to keep an object in orbit near a single mass — like the moon near the planet, a small asteroid near the big or Charon near Pluto than to keep the object in orbit near the mass, which itself rotates around another mass. This is an important factor, and we think little about it. But let’s for a moment consider it from the perspective of our closest to the Sun, the moonless planet mercury.

Mercury revolves around the Sun relatively quickly, and therefore the gravitational and tidal forces acting on it are very high. If something else was spinning around mercury, it would be much more additional factors.

“Wind” from the Sun (flux of outgoing particles) hit would be in the mercury and the object near it, knocking them out of orbit.
The heat which the Sun gives to the surface of mercury, can lead to the extension of the atmosphere of mercury. Despite the fact that mercury vacuum, and the particles on the surface are heated and thrown into space, creating a kind of weak, but the atmosphere.
Finally, there is a third mass, which wants to lead to a final tidal locking: not only between low weight and mercury and between mercury and the Sun.

Therefore, for any satellite of mercury there are two ultimate location.

Each planet that orbits a star, will be the most stable when tidal locked with it: when its orbital and rotational periods are the same. If you add another object in orbit to the planet, its the most stable orbit will be mutually tidal locked with the planet and star near the point L2

If the satellite is too close to mercury for several reasons:

rotates fast enough for its distance;
Mercury rotates fast enough to be tidal locked with the Sun;
susceptible to slow solar wind;
will be subject to significant friction Mercurian atmosphere

ultimately it falls on the surface of mercury.

When the object collides with the planet, it can raise the wreckage and lead to the formation of moons nearby. So came the earth, the Moon and also appeared the moons of Mars and Pluto

And on the contrary, he runs the risk of being ejected from mercury’s orbit, if the satellite is too far and would be applicable other considerations:

the satellite is rotating too fast for its distance;
Mercury spins too fast to be tidal locked with the Sun;
the solar wind imparts an additional velocity to the satellite;
interference from other planets push the satellite;
the heat of the Sun imparts additional kinetic energy is definitely small satellite.

With all that said, we should not forget that many planets have their satellites. Although the system of the three bodies will never be stable unless you adjust its configuration under ideal criteria, we will be stable for billions of years under the right circumstances. Here are a few terms that will simplify the task:

To take the planet/asteroid, so that the bulk of the system was far removed from the Sun to the solar wind, flashes of light and tidal forces of the Sun were unimportant.
For the satellite of this planet/asteroid was close enough to the main body, so not much hanging gravity and has not been accidentally pushed in the process of gravitational or other mechanical interactions.
For the satellite of this planet/asteroid was far enough away from the main body to tidal forces, friction or other effects did not lead to rapprochement and merger with the parent body.

As you may have guessed, there is a “sweet Apple” in which a moon may exist near the planet: several times further than the radius of the planet, but close enough to the orbital period was not too long and still much shorter than the orbital period of the planet relative to the stars. So, if you take all this together, where is satellites satellites in our Solar system?

The asteroids in the main belt and the Trojans near the Jupiter can have its own satellites, but they don’t consider themselves as such.

The closest thing we have is the Trojan asteroids with their own moons. But since they are not “satellites” of Jupiter, is not quite right. What then?

The short answer is: hardly we will find something similar, but there is hope. Gas giant worlds are relatively stable and sufficiently removed from the Sun. They have many satellites, many of which are tidal locked with its parent world. The largest moon will be the best candidates for placing satellites. They should be:

maximum massive;
relatively removed from the parent body to minimize the risk of collision;
not too far, not to be ejected;
and is new — well separated from the other moons, rings, and satellites, which could interfere with the system.

What moon in our Solar system better suited to acquire their own satellites?

Jupiter’s moon Callisto: the outermost of the large moons of Jupiter. Callisto, which is located at a distance of 1 883 000 km, also has a radius of 2410 km. Around Jupiter it is held for 16.7 per day and has a large 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 km), but not enough. It has the highest escape velocity of all satellites in the Solar system (2.74 km/s), but densely populated system giant planets extremely complicates the process of acquiring satellites, the satellites of Jupiter.
Saturn yapet: not particularly large (734 kilometers in radius), but is very far from Saturn — 3 561 000 km average distance. It is well separated from the rings of Saturn and from all other major moons of the planet. The only problem is his low weight and size: the escape velocity is only 573 meters per second.
Satellite of Uranus Titania: radius of 788 kilometers, the largest satellite of Uranus is 436 000 km from Uranus and completes an orbit in 8.7 days.
Moon of Uranus Oberon: the second largest (761 km), but the remote (584,000 kilometer) big moon completes orbit around Uranus for 13.5 days. Oberon and Titania, however, dangerously close to each other, so the “moon moon” between them is unlikely to emerge.
The satellite of Neptune Triton: this captured object from the Kuiper belt a huge (1355 km in radius), far from Neptune (355 000 km) and massive; the object needs to move at a speed greater than 1.4 km/s to leave the field of attraction of the Triton. This might be our best candidate for ownership of own companion.
Triton, the largest moon of Neptune and the captured object from the Kuiper belt, may be our best bet at the moon with its own moon. But “Voyager 2” saw nothing.

With all this, as far as we know, in our Solar system have satellites with their own satellites. Perhaps we are wrong and you will find them at the far end of the Kuiper belt or even Oort cloud, where objects are a dime a dozen.

Theory says that such objects can exist. It is possible, but requires very specific conditions. As far as our observations in our Solar system, such has not yet occurred. But who knows: the universe is full of surprises. And the better will become our search capabilities, the more surprises we will find. Nobody will be surprised if the next big mission to Jupiter (or other gas giants) will find a satellite near satellite. Only time will tell.

Can a LUN be your moon?
Ilya Hel