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Imagine the equivalent of a sun-synchronous Earth orbit, just substituting the Moon for Earth and Earth for Sun. Such an orbit would have to have the following properties:

  • It would be a polar or near-polar orbit around the Moon
  • Its orbital plane would precess with the same rate as the Moon orbits around Earth so the angle between the Earth-Moon line and the orbital plane would be constant.

For example, if the orbital plane was (nearly) perpendicular to the Earth-Moon line, a satellite in this hypothetical orbit would be permanently visible from the Earth.

Can such an orbit exist?

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  • $\begingroup$ It's basically what's done with the near rectilinear halo orbit proposed for the lunar gateway. I don't have hard facts (that's why I don't post an anwer) but I'd hazard a guess that they would have taken a geosynchronous polar orbit instead of the metastable halo orbit if it had been an option. As for the reason: the moon is tidally locked to earth, tidal forces can't introduce precession into the orbit of the spacecraft... $\endgroup$
    – TrySCE2AUX
    Aug 19, 2022 at 13:13
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    $\begingroup$ @TrySCE2AUX Yeah, I also suspect as much, but my knowledge of orbital mechanics is limited to the nice clean two-body/patched conics realm. I don't know much about all those scary perturbations (but I'm sure others here do). $\endgroup$
    – TooTea
    Aug 19, 2022 at 13:16
  • $\begingroup$ Would you be ok with en.wikipedia.org/wiki/Sun-synchronous_orbit#Orbital_precession (the part about Orbital Precession) as an answer? Since the moon is tidally locked to earth it can't get enough precession into the satellites orbit... $\endgroup$
    – TrySCE2AUX
    Aug 19, 2022 at 13:21
  • $\begingroup$ @TrySCE2AUX I'm really not competent enough to judge that. From my basic understanding of SSOs, the precession is at least partly due to the oblateness of the earth, not just due to tidal forces. The Moon certainly isn't a perfect homogeneous sphere either, so there could be a similar effect (but maybe it isn't; if you can support your suggestion with some physics, feel free to post an answer). $\endgroup$
    – TooTea
    Aug 19, 2022 at 13:33
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    $\begingroup$ Actually, turns out this is a duplicate question and the answer is a resounding No. If a lunar sun-synchronous orbit isn't possible (as the answer to linked duplicate explains) because the perturbation is too weak, a geosynchronous variant of the same is even less possible (it would have to precess a full circle in a month vs a year for SSO). $\endgroup$
    – TooTea
    Aug 19, 2022 at 21:10

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