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Is there any feasible way for a satellite to have a long lasting higher spin orbit resonance than 1:1 (tidal locking) BESIDES having an eccentric orbit, having its own satellite, or being severely irregularly shaped (highly non-spherical)? Also, does a rocheworld count as an irregularly shaped body such that it would likely have a higher spin orbit resonance or none at all?

The reason I'm asking this question is so that my moon(s) for world building don't have to be tidally locked, and therefore can have a more varied skybox as well as a more complex "day" and "night" cycle. I don't want the organisms inhabiting any given side of the moon to be so limited on what they see in the sky. Also, tidally locked worlds are naturally more dark on one side than any other, causing survivabilty issues among other things.

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  • $\begingroup$ I'm not sure what you're asking. You want like a semi-synchronous orbit? $\endgroup$
    – SIGSTACKFAULT
    Commented Oct 17, 2017 at 1:24
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    $\begingroup$ I'm not sure Worldbuilding is the best place for this question. Perhaps Astronomy.stackexchange? $\endgroup$
    – GrinningX
    Commented Oct 17, 2017 at 1:47
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    $\begingroup$ I've asked various forms of ideas related to this question with increasing complexity going back in time. The reason I made it so simple, and honestly pretty much devoid of world building stuff in the post is because stuff keeps happening so that my question won't get answered, and it seems like the more information I put, the more I get shot down. All I want is an answer so I can move on with my life, and I can't seem to get anything right. $\endgroup$
    – LanceLercher
    Commented Oct 17, 2017 at 2:07
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    $\begingroup$ @LanceLercher - don't worry about whether it's possible. Let it be plausible, and make it work the way you want it. This is the heart of sci-fi - to take something plausible, and make it work for your story. Rather than getting worried about it, instead remember you are the master of the world you're creating. If you want fast-spinning satellites, then make them. Go for it without worrying whether it would work in the real world. $\endgroup$
    – Tim
    Commented Oct 17, 2017 at 2:20
  • $\begingroup$ @Blacksilver, a spin orbit resonance is a ratio of how many times a body rotates vs how many times it orbits its primary in the same amount of time. Most moons have a spin orbit resonance of 1:1, therefore they are what's called "tidal locked", and the same side of the moon always faces the same side of the planet it orbits. This also happens to some planets without any large opposing mass like moons orbiting them. In certain situations however, these conditions can be averted for a higher spin orbit resonance. Mercury has a resonance of 3:2 because of its eccentric orbit. Does that help? $\endgroup$
    – LanceLercher
    Commented Oct 17, 2017 at 2:20

2 Answers 2

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Is there any feasible way for a satellite to have a long lasting higher spin orbit resonance than 1:1 (tidal locking) BESIDES having an eccentric orbit, having its own satellite, or being irregularly shaped?

Depends on what you mean by "being irregularly shaped". What counts as "irregular"?

Spin-orbit resonances resulting from tidal interactions generally cause irregular shapes, in the broad sense of stretching the satelite into a not-perfectly-spherical-ellipsoid, if there wasn't some pre-existing irregularity already. Where it not so, there would be nothing for tidal torques to act on. So, wherever you have a higher-numbered resonance, it will generally be the result of both high eccentricity, and irregularity.

Unless the body is completely rigid, so no spin energy can be lost to tidal deformation, you need the eccentricity to make a higher resonance stable. And if the body is completely rigid (or close to it), then you don't have a resonance--you have a coincidence.

The amount of stretching you need, however, isn't all that high. Mercury, after all, still looks pretty much like a sphere from a distance.

Also, does a rocheworld count as an irregularly shaped body such that it would likely have a higher spin orbit resonance or none at all?

Most definitely! That's an extreme case of being longer in one axis than in another, so there's plenty of gravitational quadrupole for tidal torques to grab on to.

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  • $\begingroup$ I sincerely apologize if I'm not understanding you correctly. I just recently learned about horseshoe orbits and spin orbit resonance, so my astronomical terminology isn't nearly up to snuff as it otherwise could be. On your answer to the second question, are you saying that it could have a stable higher resonance like Mercury? If so, would it necessarily have to have an eccentric orbit too, or no? Also, thank you for your answer! I've gained a little more information as to how this stuff works, and I apologize as I didn't mean for you to focus on irregular shape as much. $\endgroup$
    – LanceLercher
    Commented Oct 17, 2017 at 2:56
  • $\begingroup$ "gravitational dipole" is so wrong term here. For now we only know one charge. It is a monopole, only irregularly shaped. Gravitational dipole, and opposite gravitational charge, is one of the wet dreams of some writers and scientists. $\endgroup$
    – Mołot
    Commented Oct 17, 2017 at 6:46
  • $\begingroup$ @Mołot Quite right. I had a brain fart there, and have edited accordingly. Still maybe not the best way of explaining it, but "irregularly shaped" sources do have a legitimate multiple expansion (scienceworld.wolfram.com/physics/… ) -- just one with a vanishing dipole term, indeed because there is no negative gravitational charge. $\endgroup$
    – Logan R. Kearsley
    Commented Oct 17, 2017 at 14:20
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    $\begingroup$ @LanceLercher Yes, a Rocheworld could easily have a higher stable resonance (well, sort of--unless the lobes are perfectly rigid, they will bleed energy into tidal deformation rather quickly), but it also require an eccentric orbit. $\endgroup$
    – Logan R. Kearsley
    Commented Oct 17, 2017 at 14:23
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Let it be what you wish it to be

I think the best thing to do here is state that your populace is sufficiently technologically advanced to do whatever they damn well please.

Rather than worry about setting up natural spin, have it that your society creates their own spin, artificially.

After all, they are so advanced that they can move asteroids, or build large artificial satellites sufficient to house a population.

It therefore follows that they can make those things spin as fast as they wish. Screw you, physics.

You'd then have some places that like to speed up or slow down their homes. You'd have places full of hippies demanding that everyone should only use a "natural" rotation.

Given the level of tech. available, they can make those things spin as fast as they wish. Even the orbits could be whatever they feel like having.

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  • $\begingroup$ I apologize if I misworded anything. My world is actually set up with people who don't have the means for space travel, let alone moon or planet Manipulation. There are beings nearby that do this to a degree, but I'm trying to lower my reliance on them. You're idea does work well for those with high technological advances, and therefore I do hope that those who see this later, find high use for your idea. Thank you again for the input, and I'm sorry if I worded things badly. $\endgroup$
    – LanceLercher
    Commented Oct 17, 2017 at 3:03

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