# Satellites and their orbits

Could we have a stable gravitational system with 2 Moons :

* In the same orbit
* In different orbits ?

## Answers and Replies

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mfb
Mentor
In the same orbit: Possible if one moon is much smaller than the other (factor 25 or more). The smaller moon can be 60 degrees ahead or behind of the other moon. Those objects are called trojans. There is also a weird configuration where two moons of similar mass exchange their (very similar) orbits periodically, the Saturn moons Janus and Epimetheus do that.

In a different orbit: yes, if the orbits are different enough. Lighter moons and larger differences lead to more stable systems. A more massive planet far away from the sun helps as well (that's the reason Jupiter and Saturn can have to many moons).

petrushkagoogol
tony873004
Science Advisor
Gold Member
The factor of 25 or more refers to the ratio of the total mass of the moons to the mass of the planet. Two trojans can have the same mass.

mfb
Mentor
Well, two Trojans for a moon would be at least three moons already.

davenn
Science Advisor
Gold Member
2019 Award
Well, two Trojans for a moon would be at least three moons already.
can you clarify please ?

are you saying you cannot have 2 Trojans 180 deg from each other ?
that there must be 3 reasonably evenly spaced ?

D

mfb
Mentor
are you saying you cannot have 2 Trojans 180 deg from each other ?
Trojans are 60 degree ahead or behind of the main body. There is nothing 180 degrees apart.

You can have a moon (#1), and a smaller moon (moon #2) as trojan ahead or behind. If you want a second trojan, you are at moon number 3. Which is perfectly fine, but the question was about two moons.

davenn and petrushkagoogol
davenn
Science Advisor
Gold Member
2019 Award
Trojans are 60 degree ahead or behind of the main body. There is nothing 180 degrees apart.

You can have a moon (#1), and a smaller moon (moon #2) as trojan ahead or behind. If you want a second trojan, you are at moon number 3. Which is perfectly fine, but the question was about two moons.

ahhhh OK ... wasn't totally sure of the arrangement

thanky