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Can moons have moons? (arxiv.org)
116 points by nopacience 3 months ago | hide | past | web | favorite | 57 comments

Earth's moon has unstable orbits:


Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons) beneath the lunar surface caused by large impacting bodies at some remote time in the past. These anomalies are significant enough to cause a lunar orbit to change significantly over the course of several days.

See also:


Joining an earlier subsatellite PFS-1, released by Apollo 15 astronauts eight months earlier, PFS-2 was to measure charged particles and magnetic fields all around the Moon as the Moon orbited Earth. The orbit of PFS-2 rapidly changed shape and distance from the Moon. In 2-1/2 weeks the satellite was swooping to within a hair-raising 6 miles (10 km) of the lunar surface at closest approach. As the orbit kept changing, PFS-2 backed off again, until it seemed to be a safe 30 miles away. But not for long: inexorably, the subsatellite's orbit carried it back toward the Moon. And on May 29, 1972—only 35 days and 425 orbits after its release—PFS-2 crashed.

Be careful of the orbit chosen for a low-orbiting lunar satellite. "What counts is an orbit's inclination," that is, the tilt of its plane to the Moon's equatorial plane. "There are actually a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27º, 50º, 76º, and 86º"—the last one being nearly over the lunar poles. The orbit of the relatively long-lived Apollo 15 subsatellite PFS-1 had an inclination of 28º, which turned out to be close to the inclination of one of the frozen orbits—but poor PFS-2 was cursed with an inclination of only 11º.

edit: added additional information about frozen orbits.

Our moon, and those of mars, are special cases. They are most likely the result of collisions involving the host planet and some other very large object. They are bigger and closer than any 'natural' moon formed during the formation of the solar system. So it is expected that they are unsymmetrical both in shape and orbit, resulting in them having few stable orbits for sub-moons. Collisions are certainly part of the equation, but there again many of those collisions were associated with debris from the initial impact that created the moon.

A 'natural' moon formed early in the life of a solar system could be much further from its host, with much more symmetrical gravity and many stable orbits ready to host sub-moons. A moon is also in a much better place from which to capture a passing body.

Mars' moons are indeed much too close but our Moon is one of those listed in this paper as being capable of having a moon of its own.

I also got interested in this question a few years back and while browsing for candidates I learned about Neso, the outermost (known) moon of Neptune. https://en.wikipedia.org/wiki/Neso_(moon)

It is on average further away from Neptune than Mercury is from the Sun! It takes more than 26 years to orbit Neptune, it still hasn't completed a full orbit since we have discovered it.

Also, I think there could be "binary-moons", if a binary asteroid gets captured as an irregular moon.

Asteroids with natural satellites exist and have been found.

There could also be very big trojans moons/planets in other star systems.

To the Sun, a moon is like a submoon to the planet, so what would be the difference? With enough distance in between it should be possible. Not probable but possible.

Submoons will probably clump together much easier in the formation phase of any system and a moon will never capture an asteroid before it's planet does.

As the author pointed out, the question remains how stable it is. The almost nonexistent cases (at least in our Solar System) shows how rare it is. Maybe only distance is a factor. Huge Solar Systems with twice the distance between objects will also have more submoons.

There is a limit to how big planets can get before they become stars, and a limit to how small moons can get before electromagnetism becomes dominant over gravity in determining their motion. So there exists a hard (not just probabilistic) maximum number of levels of submoons. Given other constraints like orbital stability and the Roche radius, it would not surprise me if this number was a low single-digit integer.

I'm curious, how does electromagnetism determine the motion of a planet?

All I mean is that if a "moon" were very small, say a few mm or cm, then it can accumulate enough of a charge imbalance for forces from ambient magnetic fields in a stellar system to push it out of it's nominal gravitational orbit. That size limit might seem trivial, but the ratio of an atom to a paper clip is about the same as a paper clip to the Earth's moon, so it cuts off half of the geometric range.

So it's safe to say that, say, a styrofoam pellet wouldn't make a good moon (not even considering lack of durability against cosmic bombardment).


Well the moon has had artificial satellites in the past. I imagine it might not be terribly common, but not impossible either.

>The almost nonexistent cases (at least in our Solar System) shows how rare it is.

It might not exist in our solar system, our solar system isn't a model from which all others operate from. Submoons may be rare, but not because they don't exist in our solar system.

> Submoons may be rare, but not because they don't exist in our solar system

Sure, "submoons are rare because they're rare in our solar system" isn't a proof. (It's a hypothesis.) But the observation is evidence. In the absence of other evidence, it defines the status quo.

> Well the moon has had artificial satellites in the past. I imagine it might not be terribly common, but not impossible either.

There are still artificial satellites around the Moon but the problem as stated in the paper is "long-lived" submoons, on stable orbits, which the Moon can't have.

> which we call submoons

I believe the publicly accepted term for these is “moonmoon”.

Can moonmoons have moonmoonmoons?

Astronomers have adopted Python syntax for further iterations of this. Thus, it is formally referred to as a "moon"*3.

I'm going with moonpies.

"That's no moonmoon..."

Dammit moonmoon, that's not what you're supposed to howl at.

No, a "moonmoon" would be the big moon into whose orbit you park the smaller moon. Reference: https://xkcd.com/2043/

That actually doesn't match the meanings of househouse and boatboat in the chart. The hovertext specifies that you can refer to the contained object by flipping the order, so a moonmoon can also refer to the smaller moon orbiting around a larger one:

> The <x> that is held by <y> is also a <y><x>, so if you go to a food truck, the stuff you buy is truck food.

So, what you're saying is that the earth is a moonmoon of the sun right?

In that case they both would be a moonmoon.

I thought the correct term for anything other than Earths Moon was "natural satelite".

Not at all. The noun 'moon' can refer to any, well, moon (like Deimos or Europa). To distinguish between those and our moon, you can use the proper name 'Moon' (capitalised) or Luna if you are so inclined.

The same goes for the Sun. Our sun is either proper name 'Sun', or if your prefer Luna above, Sol. Any other star with stuff orbiting around it is just another sun (noun).

> Luna

I have also seen Selene or Selena being used from time to time, more often as a adjective (selenic) but also as a noun every now and then.

same goddess. luna is the roman name and selene is the greek name.

> To distinguish between those and our moon, you can use the proper name 'Moon' (capitalised) or Luna if you are so inclined.

No you don't. Not in english at least. The moon or sun is understood in context and with the article "the". "The moon" and "the sun" without qualifications is understood to mean earth's moon or earth's sun. General references like "a sun" or "a moon" connote non-specific and non-identifying moon or sun. If you want to specifically identify other moons or other suns, you qualify it like - the moons of jupiter or sun of X solar system.

As an example, the recent SpaceX announcement.

"The proposed trip—a trip around the moon for two passengers lasting about a week—is almost the same as a scheme hatched by the tourism company Space Adventures, whose president told Quartz last year it had two clients booked for the mission."


Makes me wonder if some languages (Spanish, Catalan, Japanese, ...) will adopt a new word for "soil" that isn't synonymous with "earth" once we start farming on other planets.

In Romance languages, there is a separate Latin term, closely related to the word "soil," which survives in French and Portuguese. Maybe sol/solo will be borrowed back into the Romance languages where it died out.

In Japanese there’s 地球 which means Earth (this planet), and 土 which means soil.

地 can also mean soil, but it’s a different word from 地球 meaning the planet earth. I don’t think it’s commonly used by itself to mean soil either, but I could be wrong.

It means ground, as in the surface on which we live and build things on. 球 means ball or sphere. I wonder if that’s the reason why flat Earthers are rare in China and Japan - the very phrase ‘flat Earth’ is an oxymoron in these languages :)

Right, but the word 地球 can only mean the planet Earth, it’s not synonymous with anything else and there’s no room for confusion.

Soil, or ground (on any planet) is a different word. There’s not much room for confusion.

I was under the impression that 土 could be used for "earth," so I apologize. Thanks for the correction!

I’ve not heard it used that way, but I could be wrong. There’s possibly also a historical usage in this sense.

In Kim Stanley Robinson’s Mars trilogy, they study areology.

I thought there's only one sun regarding of whether you capitalize it or not, but that moon was generic.

“sun” is generic but rarely used as such instead of the proper noun “Sun” outside of interstellar fiction, because “star” is available and denotatively equivalent. (“sun” has a stronger connotation that the object's importance is relative to one or more planets, though.)

They are both generic, but because the context of most texts we write is here on Earth, 'sun' and 'moon' tend to be used as proper names would, but without capitalisation.

That these are essentially generic nouns can show up more clearly in astronomical texts and of course in literature (science fiction).

Of course this is just within the arbitrary distinctions of what constitutes a moon and a planet. A particle of sand in the Sahara desert orbits the earth's core albeit with a lot of collisions with other sand moons.

Technically, aren't planets moons of their star? So if a 'planet' has a moon, wouldn't that be a submoon?

> aren't planets moons of their star?

No. The difference between “star” and “not star,” within the context of a solar system, is well defined. (Brown dwarfs muddy the line in general.)

The difference between “planet” and “not planet” is a little more arbitrary, with the line between “moon” and “non-moon satellite” being more arbitrary still.

The all-encompassing term you’re looking for is “satellite.”

Sub-question. Are there any extant man-made objects in orbit around our moon right now? Around any of the planets?

Currently active: 1 around Venus, 4 around the Moon, 6 around Mars, 1 around Ceres, 1 around Jupiter.


The Lunar Reconnaissance Orbiter is still in operation, as are ARTEMIS P1 and P2.

There are several spacecraft orbiting other planets (both functional and inactive) but none orbiting any of the moons of other planets, if that is what you mean.

I know Mars at least has several ...


So the moon has a moon, but I'd guess that the orbit is not stable over the long term.

TIL: They have detected exo-moons around exo-planets outside our solar system.

May someone tell me where this article was published (magazine, congress, etc), please?

Proper link to abstract on arXiv:


arXiv is an archive of preprints. This is a preprint. It hasn't been published (yet). In some scientific fields, it's customary to share preprints publicly before they are published. Sometimes they aren't published (i.e. by a peer-review journal) at all.

Interesting. Thank you aargh_aargh for the information.

This is an exception to Betteridge's law, isn't it?

Most importantly: can moons have morons?


Your username is well suited to comment this article.


so what is the human behavior denoted by 'moonmoon-ing' someone?

As programmers, the answer is "of course". Of course a function can call a function. But nature isn't a stack. Can submoons have subsubmoons.

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