# Can two moons, moving at different velocities, periodically overtake each other and share an atmosphere?

Idea being, can two moons orbiting a large planet move at v_1 and v_1 + delta, but the distance between the two moons is small enough that when their positions along an axis periodically overlap they temporarily, say less than 24 hours, share an atmosphere allowing for material transfer?

Note, not two planets but rather moons in the gravity well of a larger planet such as a gas giant. Wondering if this is a somewhat reasonable work around for Roche limit.

• This is almost certainly a question that requires a hard-science tag for the math involved, but a simple answer is "definitely not", because you're describing a three body problem. Ordinarily, moons are so far apart that the effect of the central body's gravity overwhelms that of other moons, but in this setup, that cannot be true, and the perturbation exerted on the orbits of the two proximate moons would likely result in their deorbit or ejection from the system. Apr 5, 2022 at 0:08
• When the two satellites are so close as to share their atmospheres, what is it that prevents them from collapsing into a single molten satellite? (That accursed gravitational force of Newton's works between the two satellites just like it works between each of them and their primary.) Apr 5, 2022 at 0:26
• @AlexP angular velocity of the pair. That's not to say that binary moons would be stable (too close and you get torn by tidal effects, too far and they'll escape their parent body or crash into it), but if they're orbiting fast enough, then a single body with that angular momentum would be unstable. Apr 5, 2022 at 2:03

YES, MOONS CAN - AND DO - EXCHANGE ORBITS

In fact, this is the case for the two small Saturnian moons Janus and Epimetheus. Their orbits are only about 50 km apart; a fraction of their individual diameters of about 120 and 180 kilometers. As the one in lower orbit approaches the other, their mutual gravities make them swich places, so the moon that was outermost becomes innermost.

NO, THEY COULD NOT SHARE AN ATMOSPHERE

In spite of the closeness of the orbits of the two moons, they never come anywhere near enough to share an atmosphere - even if they had one. In fact, they never get closer than about 15,000 km to each other. Even the Earth's atmosphere is only about 100 km thick, far less than this. The lower-orbiting, faster-moving moon never actually catches up to the outer, slower-moving moon; instead, they exchange angular momentum, making the forward moon move into a lower, faster orbit, while the rear moon moves into a higher, slower orbit (see image below). It is kind of like relay runners passing batons to each other.

DIFFERENT ORBITAL CONFIGURATIONS ARE VERY UNLIKELY TO ALLOW REGULARLY SHARING AN ATMOSPHERE

We can imagine very different orbital configurations from the one above, such as the moons having perpendicular orbits or orbits of very different eccentricity. No matter the configuration, if the moons were to move close enough to share an atmosphere, the friction with the atmosphere will slow their relative speeds down and change their orbits. This will likely cause the moons to crash into each other after a number of passages.

More importantly, there is the Roche limit to consider. When two celestial bodies come too close, the tidal forces between them will make one or both disintegrate - their closer parts are accelerated faster than the more distant parts. Our Moon could not get closer than about 9,500 km to the Earth without breaking up. The smaller the two moons in your question are, the lower is the Roche limit - but so, also is the thickness of the atmosphere their gravities could hold. For two moons of roughly equal density, the Roche limit is at least 1.26 times the radius of the smaller moon (and higher if their sizes are more different), meaning that the closest the two moons can get is more than one-quarter the radius of the smaller moon. Saturn's moon Titan has a very thick atmosphere (much thicker than the Earth's) of about 600 km. The Roche limit for two such moons is 670 km, so they could actually get close enough to share an atmosphere, however briefly (their passage will be at a very high relative velocity)- but the tidal forces of their passage will strip their atmospheres off, losing a lot to space. I don't imagine it would take very many passages before their atmospheres thin too much to be shared.

To sum up: In very extreme cases, with two very similarly sized moons with very thick atmospheres, they could just barely pass each other close enough to share atmospheres without breaking up from tidal stresses - though these stresses would be very hard on the moons, causing major earthquakes and massive tidal waves if they have oceans. However, friction and loss of atmosphere would make it impossible for such passages to be regular events.

• +1. The question is tagged hard science, this is a hard science answer
– Qami
Apr 5, 2022 at 13:59
• Thank you for the response! I think this gives me a solid foundation to work off. Perhaps rather than having a perpetual system of this nature, it could make more sense that an impact from a sufficiently dense object could slightly alter the orbit of one of these near-adjacent moons and from then start a clock before the moons either destroy each other or lose atmosphere... Might need some hand waving also hah. Apr 6, 2022 at 5:43
• @chonchon: If you accept this as a right answer, please mark it as such. :-) Apr 6, 2022 at 10:03
• I imagine that such a system would allow for one pass. Friction and tidal stresses would so substantially modify their shared orbit that the next pass would either be too far apart or terminal. It would effectively be a second impact like the one that potentially set the system in motion. Apr 6, 2022 at 18:18

In addition to the arguments already presented, suppose you have a pair of moons in such an arrangement. You have an extreme deflection of their orbits upon every such close encounter. This deflection will be incredibly sensitive to the exact encounter conditions--and thus vulnerable to being disrupted by otherwise minor things. It's extremely unstable.

• The question is tagged hard science, this is not a hard science answer
– L.Dutch
Apr 5, 2022 at 3:36

I assume you just want any conditions where two planets can exchange atmospheres, while having solid ground.

One option that could help is disk-planet. Planet that spins so quickly that it changes its shape from a sphere to a disk. Then again, highly elliptical orbit to make the contact short term. This also helps that it allows the surfaces to be at much lower speed relative to each other during the fly by. This also allows to extend the atmosphere to the point of it flying away freely.

In other words - if you cant bring anything from outside to the Roche limit - bring it from the inside closer to the Roche limit, so that the counterpart on the outside doesnt have to cross the Roche limit.

So the system is like this: heavy disk planet of x10 Earth, spun to a level close to destruction, planet x1 Earth just to keep the atmosphere with normal rotation speed to make sure it survives the fly by.

This way big planet wont be disturbed by the small planet. And small planet can come close to the Roche limit intact because it is not spinning.

Such a configuration is likely impossible naturally.

Math for making a planet more like a disk from rotation is here:

What would a planet spinning fast enough to allow geostationary orbit near the surface look like?

Once planet's large radius of the atmosphere touches the Roche limit, you can have a close fly by with atmospheric contact with a smaller body, without destruction.

• The question is tagged hard science, this is not a hard science answer
– L.Dutch
Apr 5, 2022 at 3:35