
Can moons have moons? - nopacience
https://arxiv.org/abs/1810.03304
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fxj
Earth's moon has unstable orbits:

[https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effec...](https://en.wikipedia.org/wiki/Lunar_orbit#Perturbation_effects)

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:

[https://science.nasa.gov/science-news/science-at-
nasa/2006/0...](https://science.nasa.gov/science-news/science-at-
nasa/2006/06nov_loworbit/)

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.

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sandworm101
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.

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svachalek
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.

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jobigoud
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)](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.

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Medox
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.

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Brockenstein
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.

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JumpCrisscross
> _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_.

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saagarjha
> which we call submoons

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

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

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daveFNbuck
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.

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

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Freak_NL
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).

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

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dragonwriter
“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.)

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SubiculumCode
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.

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

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JumpCrisscross
> _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.”

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aj7
Sub-question. Are there any extant man-made objects in orbit around our moon
right now? Around any of the planets?

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

[https://en.wikipedia.org/wiki/List_of_extraterrestrial_orbit...](https://en.wikipedia.org/wiki/List_of_extraterrestrial_orbiters)

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asmithmd1
TIL: They have detected exo-moons around exo-planets outside our solar system.

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

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aargh_aargh
Proper link to abstract on arXiv:

[https://arxiv.org/abs/1810.03304](https://arxiv.org/abs/1810.03304)

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.

~~~
gustavooliveira
Interesting. Thank you aargh_aargh for the information.

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

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pbduring
Most importantly: can moons have morons?

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moon_of_moon
Yes.

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S4M
Your username is well suited to comment this article.

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cleanyourroom
DOUBLEMOON

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

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ape4
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.

