Could 2 moons that orbit same terrestrial planet never see each other if they orbit the planet at same time?
Moons have different mass and gravity.
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Sign up to join this communityCould 2 moons that orbit same terrestrial planet never see each other if they orbit the planet at same time?
Moons have different mass and gravity.
In theory if the two moons were in the exact same orbit on opposite sides of the planet then yes. Having the moons closer to the planet and smaller also makes that easier. For example geostationary satellites over opposite sides of earth will never have direct line of sight to each other.
In practice though that would be a very unstable arrangement (even if there were no other moons to disrupt things) and would also be very unlikely to form naturally.
So it would be very unlikely to form naturally and if it did form it would be unstable ... so realistically the answer is "no" but if you can explain away the improbabilities somehow then "yes".
The moons having different masses doesn't change their behavior in this case. If they are in the same orbit they are in the same orbit.
Yes, this is possible.
A large moon and a smaller moon can share the same orbit if one is 60 degrees ahead of the other. In such an orbit, the smaller moon would be at one of the stable Lagrangian points L4 and L5. If the orbital radius is less than
$$\frac{1}{\cos (30^{\circ})} = \frac{2}{\sqrt{3}}R_P \approx 1.155 R_P$$
(where $R_P$ is the radius of the planet), then the planet will block the line of sight between the two moons. That is, each moon will be beyond the horizon as seen from the other moon.
Of course, such orbits would be very close to the planet. Would the moons break apart due to tidal forces? The answer to that is given by the Roche limit, which for a rigid satellite is
$$ d = R_P \left( 2\frac{\rho_P}{\rho_m} \right)^{1/3} $$
where $\rho_P$ and $\rho_m$ are the densities of the planet and the moon respectively. If the moons orbit outside this radius, they will survive. If they are inside the radius, they will break apart. For our scenario, we need the Roche limit to be less than $1.155 R_P$, so the density of the moons must be at least 30% larger than the density of the planet. More precisely, the density ratio must be at least
$$2\cdot\left(\frac{\sqrt{3}}{2}\right)^3 \approx 1.299$$
Orbits are elliptical, normally quite eccentric - our moon's almost circular orbit is unusual. For two moons not to see each other, both their orbits would have to be extremely circular and almost exactly in the same plane.
The system would be unstable. If one moon lead the other by a tiny fraction it would be accelerated by the lagging moon and the lagging moon would be dragged by the leading one. This would rapidly cause the system to collapse.
However, it is not impossible.
Yes. Tim B got half the answer--you need moons in matching orbits 180 degrees apart. In general he's right, it's an unstable situation.
However, lets introduce a third, much larger moon. Your two moons that never see each other are both in resonance orbits with the big moon. Now the relationship between the moons is stable.