NASA finds Neptune moons locked in 'dance of avoidance'

https://phys.org/news/2019-11-nasa-neptune-moons.html

by dnetesn • 7 years ago

178 48 7 years ago

hanniabu• 7 years ago

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.

username90• 7 years ago

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• 7 years ago

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.

foobarbecue• 7 years ago

The three body problem is so fun!

jonplackett• 7 years ago

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.

FreeFull• 7 years ago

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.

mirekrusin4• 7 years ago

Resonance is natural selection.

kkylin3• 7 years ago

Presumably any moons that did not end up in a nice stable resonant orbit would have had a higher chance of being ejected or colliding with something.

hcrisp• 7 years ago

So could two earth-sized planets be in resonant orbits around a sun that would otherwise intersect? If so, what does this mean for the definition of a planet as something which "clears the neighborhood"?

thevsb_dnebbexj• 7 years ago

I think it’s more along the lines of moons that don’t avoid each other merge.

phkahler• 7 years ago

You may find this informative: https://en.m.wikipedia.org/wiki/Attractor

kkylin3• 7 years ago

Not quite -- orbital mechanics are Hamiltonian, which means they conserve energy, phase space volume, and often a bunch of other things. Dynamical systems that conserve phase space volume cannot have "strange" attractors (attractors with zero volume) like the ones described in the article.

chongli• 7 years ago

Resonance plays an incredibly large role in orbital mechanics [1]. All of the orbits of the moons of Jovian planets are in small fraction ratios of one another.

[1] https://en.wikipedia.org/wiki/Orbital_resonance

gorgoiler• 7 years ago

Great animation. Everything orbits everything.

We are just trained as children to see the small things independently orbiting the big thing, but in reality all bodies orbit each other.

What would this animation look like with one of the moons at the center with Jupiter in a fixed position?

peter303• 7 years ago

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.

devicetray0• 7 years ago

They are a real time laboratory for testing dynamics mathematics.

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

pmarreck• 7 years ago

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• 7 years ago

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

pbosko• 7 years ago

Could this cause a natural occurrence of resonance cascade?

novelAdTech• 7 years ago

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...

tolerate• 7 years ago

Is hackers news really censoring and deleting comments ?

dang• 7 years ago

No. Why did you think that we might be?

jvanderbot• 7 years ago

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

chmod7753• 7 years ago

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

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... (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.

205guy2• 7 years ago

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/Horseshoe_orbit

chmod7753• 7 years ago

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...

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.

NASA finds Neptune moons locked in 'dance of avoidance'

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