Could Earth Support Two Moons in Different Orbits?

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Discussion Overview

The discussion explores the possibility of a stable gravitational system with two moons orbiting Earth, considering both scenarios where the moons are in the same orbit and in different orbits. The conversation touches on theoretical aspects of celestial mechanics and gravitational stability.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the stability of a system with two moons depends on their size and distance from each other, with a focus on the concept of instability in such configurations.
  • One participant notes that having two moons in the same orbit is possible if one moon is significantly smaller than the other, specifically mentioning a size ratio of 25 or more, and describes the concept of trojan moons.
  • Another participant mentions that two moons of similar mass can exchange orbits periodically, referencing the moons Janus and Epimetheus of Saturn.
  • It is proposed that for moons in different orbits, stability increases with larger differences in their orbits and lighter moons.
  • Clarifications are made regarding the arrangement of trojan moons, stating that they cannot be 180 degrees apart, but rather must be 60 degrees ahead or behind the main moon.
  • Participants discuss the implications of having additional moons if considering trojan configurations, leading to some confusion about the total number of moons involved.

Areas of Agreement / Disagreement

Participants express differing views on the stability and arrangement of moons, particularly regarding the conditions under which two moons can coexist in the same orbit or in different orbits. The discussion remains unresolved with multiple competing perspectives on the configurations and stability of such systems.

Contextual Notes

There are limitations in the assumptions made about the masses and distances of the moons, as well as the specific gravitational dynamics involved in such configurations. The discussion does not resolve the complexities of the N-body problem as it relates to this scenario.

petrushkagoogol
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Could we have a stable gravitational system with 2 Moons :

* In the same orbit
* In different orbits ? :wideeyed:
 
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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).
 
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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.
 
Well, two Trojans for a moon would be at least three moons already.
 
mfb said:
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
 
davenn said:
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.
 
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mfb said:
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
 

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