
NASA finds Neptune moons locked in 'dance of avoidance' - dnetesn
https://phys.org/news/2019-11-nasa-neptune-moons.html
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jonplackett
Is this some kind of mathematical thing based on how orbits work, or is it
more like natural selection, in that any moons that don't avoid each other
just aren't there anymore.

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FreeFull
It is a mathematical thing based on how orbits work. Orbits that are very
close to a resonance but not quite there will tend towards being even closer,
so it's a stable system. If this wasn't the case, you wouldn't ever expect to
observe anything like this, due to disturbances to the orbits (from other
bodies tugging gravitationally on the objects) building up.

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mirekrusin
Resonance is natural selection.

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lmilcin
No, it is not.

It would be if we saw many colliding orbits and only those that had resonance
surviving in long run.

But resonances are perfect ratios and no random orbits will ever have perfect
ratios of orbit times.

Instead what happens is that collisions seem to be extremely rare occurrence.
Once two moons find themselves on close orbits they will tend to exchange
energy until they find some kind of resonance that will keep true for a very
long time until disturbed.

~~~
patcon
It might be more apt a metaphor than it seems...?

Resonant orbits might not be _natural_ selection, but they might participate
in a form of thermodynamic selection: two simple forms and structures
selecting for and reinforcing one another.

They are both subsets of the same thermodynamic processes of
structures/arrangements sticking arouns when they mutually support one another
creating persistence in a chaotic system

My understanding is that some folks believe there are similar mechanisms
underlying the emergence of both -- a generalized form of selection between
mutually reinforcing structures that encode elements of prediction of the
chaotic environment they reside in, be that the periodicity of orbits (that
"know* a stable path that will reasonably persist) or life itself (that have
similar baked in prediction abilities we perceive subjectively as
consciousness).

Disclaimer: biochemist, technologist and armchair follower of complexity
science

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gus_massa
The idea is that the gravity of a moon affect the trajectory of the other moon
and vice versa, until they are in resonance. It's a bad analogy, but it's
closer to Lamarckism than Darwinism, but it's still a bad analogy. There are
nothing similar to random mutations.

~~~
mirekrusin
By natural selection I meant that if you have two (or more) bodies on the same
orbit they'll crash or they'll find stable orbit (resonance). Of course you
see resonating ones because ones that wouldn't would collide millions of years
ago.

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chmod775
Reminds me of the plot of "Pushing Ice", where a weird moon turns out to be
way more than just a piece of rock and ice.

[https://www.goodreads.com/book/show/89186.Pushing_Ice](https://www.goodreads.com/book/show/89186.Pushing_Ice)

Saturn's moon, Janus, is in a co-orbital configuration with Epimetheus, and...
just look at these "orbits":
[https://upload.wikimedia.org/wikipedia/commons/1/1c/Animatio...](https://upload.wikimedia.org/wikipedia/commons/1/1c/Animation_of_Epimetheus_orbit_-
_Rotating_reference_frame.gif) (Janus in green)

The author is an astronomer turned writer, and the stuff he comes up with (or
uses in his stories) is always incredibly interesting.

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205guy
Those are horseshoe orbits in a rotating reference frame (your link does not
provide the caption). Janus and Epimetheus do orbit around Saturn elliptically
(except when they switch positions) when seen from an inertial frame.

[https://en.wikipedia.org/wiki/Epimetheus_(moon)](https://en.wikipedia.org/wiki/Epimetheus_\(moon\))
[https://en.wikipedia.org/wiki/Horseshoe_orbit](https://en.wikipedia.org/wiki/Horseshoe_orbit)

~~~
chmod775
I didn't give it much thought what that gif would look like in a vacuum. I
should've probably linked to this instead:
[https://upload.wikimedia.org/wikipedia/commons/f/ff/Epimethe...](https://upload.wikimedia.org/wikipedia/commons/f/ff/Epimetheus-
Janus_Orbit.png)

The orbits probably wouldn't look very fancy if you were observing from Saturn
(besides the speeding up and slowing down), but when seen from either of the
moons, that's roughly what things would seem like to you.

~~~
chmod775
> The orbits probably wouldn't look very fancy if you were observing from
> Saturn (besides the speeding up and slowing down)

Unless you were moving around the thing at precisely the right velocity I
suppose. But it being a gas giant I suppose the point is kind of moot. It's
not like you or a manmade device can stand touch down anywhere while still
observing the sky.

~~~
205guy
"moving around the thing at precisely the right velocity" would be the
rotating frame of reference that gives you the horseshoe orbits seen in
chmod775's 2 links.

My point is that for people like myself, referential frames are not intuitive,
and temporarily misleading. If they are unlabeled, one might even confuse them
for the inertial frame, and then if one were uncritical enough, assume
erroneously that these moons "bounce" back and forth like billiard balls
([https://www.teachersource.com/product/newtons-kinetic-
yoyo-s...](https://www.teachersource.com/product/newtons-kinetic-yoyo-set-
of-12/physics-laws)).

It took a bit of searching, but here is an animation showing how the two moons
look in the inertial frame of reference. The interesting bit is how they
interact gravitationally so that the faster one never overtakes the slower
one, rather it causes the faster one to slow down and the slower one to speed
up and pull ahead.

[https://youtu.be/r9PSimuA9a8?t=45](https://youtu.be/r9PSimuA9a8?t=45)

Of course, this is what a knowledgeable reader grasps from the rotating frame
horseshoe diagram, but I needed the inertial frame animation to get it.

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peter303
There are now several dozen multiplanet exo-solar systems that exhibit various
kinds of resonances. They are a real time laboratory for testing dynamics
mathematics.

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devicetray0
> They are a real time laboratory for testing dynamics mathematics.

I wouldn't say "testing", but perhaps "observing"?

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novelAdTech
This is a reversal of cause and effect.

We're looking at billion-year-old hydrostatic, spheroid satellites that
currently orbit in a system that stabilized a long, long time ago.

They are ancient survivors of a chaotic distillation that predates all of life
on earth, a dynamic system that stands as a persistent, unchanging
choreography, undisturbed in a quiet part of space for eons.

None of the bodies are engaging willpower, in order to avoid one another.

What scientists are observing just happens to be a closed circuit scale-model
racetrack, and the little toy race cars couldn't jump out of their well-worn
grooves if they tried.

But hey, "science" headlines need clicks too...

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hanniabu
It's amazing that we're still discovering stuff like this with large objects
in our own solar system. It's humbling and makes me realize how much we still
don't know.

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username90
It is nice and all, but these two moons are pretty small, just 30 and 40 km
radius. For comparison our moon has a radius of 1700 km.

~~~
ryan_j_naughton
This is bc our moon is abnormally large. We're almost a binary planet with our
moon bc of it's size in comparison to earth.

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whamlastxmas
I never knew to be thankful that we have a big ol moon in the sky

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macintux
There’s been speculation, as I recall, that we have the moon to thank for
intelligent life.

Some references:

[https://www.scientificamerican.com/article/moon-life-
tides/](https://www.scientificamerican.com/article/moon-life-tides/)

[https://www.npr.org/2011/11/18/142512088/is-a-moon-
necessary...](https://www.npr.org/2011/11/18/142512088/is-a-moon-necessary-
for-a-planet-to-support-life)

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jvanderbot
It always amazes me how they dress up the most mundane results (not that all
dressed-up results from NASA are mundane!).

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pmarreck
Could survivorship bias be playing a role here?

In other words, the moons that did not stabilize in a fashion similar to this
either crashed into each other or their planet already a long time ago?

~~~
cwlrs
Well yeah - Masses on a colliding path would have merged over the billions of
years. These masses were not, so remain like this

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pbosko
Could this cause a natural occurrence of resonance cascade?

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tolerate
Is hackers news really censoring and deleting comments ?

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dang
No. Why did you think that we might be?

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foobarbecue
The three body problem is so fun!

