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Let’s say I wanted to build a space habitat in orbit around the moon. Better still, let’s say I want to build a ring of habitats occupying the same orbital path, encircling Luna, to oversee the mining/exploration of this body.

Given that Luna is tidally locked, and most orbital paths around it decay over time, what is most stable orbit these objects could occupy?

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    $\begingroup$ Seems an odd idea to build orbital habitats and an office to oversee the mining operation when the ground is there waiting to be used, some ground presence is presumably needed anyway and potential building materials are on the ground. You might also want to check out Worldbuilding and Space Exploration $\endgroup$ Feb 18, 2023 at 9:37

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Low lunar orbits

For low lunar orbits it's a challenge because the Moon as a lot of mass concentrations or mascons near the surface, and these will continue to perturb a low orbit and ultimately bring it down to impact the surface after months or at best years.

There are certain sets of orbital parameters, basically certain combinations of inclination and semi-major axes... these are called frozen orbits because while perturbations affect them they tend to remain somewhere near the same orbital elements rather than veer into surface impact.

Lunar frozen orbits

Through a study of many lunar orbiting satellites, scientists have discovered that most low lunar orbits (LLO) are unstable.3 Four frozen lunar orbits have been identified at 27°, 50°, 76°, and 86° inclination. NASA expounded on this in 2006:

Lunar mascons make most low lunar orbits unstable ... As a satellite passes 50 or 60 miles overhead, the mascons pull it forward, back, left, right, or down, the exact direction and magnitude of the tugging depends on the satellite's trajectory. Absent any periodic boosts from onboard rockets to correct the orbit, most satellites released into low lunar orbits (under about 60 miles or 100 km) will eventually crash into the Moon. ... [There are] a number of 'frozen orbits' where a spacecraft can stay in a low lunar orbit indefinitely. They occur at four inclinations: 27°, 50°, 76°, and 86°"—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 28°, which turned out to be close to the inclination of one of the frozen orbits—but less fortunate PFS-2 had an orbital inclination of only 11°.4

  • 3https://arc.aiaa.org/doi/abs/10.2514/2.5064?journalCode=jgcd
  • 4Bell, Trudy E. (November 6, 2006). Phillips, Tony (ed.). "Bizarre Lunar Orbits". Science@NASA.

Another problem with low lunar orbits is that any particular spot on the surface is only visible for a few minutes every pass, and you may pass over a given site for about one day a month - the moon will slowly rotate around it's axis each month while the orbit will mostly remain fixed.

See also:

Higher lunar orbits

For earth, we get 24/7 access to a big chunk of one hemisphere by going to geostationary orbits (or at least geosynchronous). However luna-stationary orbits don't really exist because the moon rotates only once a month, and by the time you would get high enough to orbit that slowly you're interacting with the gravity of the Earth and Sun too much. Not an option for the Moon.

However there is the distant_retrograde_orbit meaning far from the Moon but moving the wrong opposite direction - counterclockwise if looking down from above (north of) the Earth-Moon system. At least two spacecraft have used this:

Halo and other 3-body Lagrange point-associated orbits

There are a whole host of 3-body orbits associated with the Earth-Moon Lagrange points, primarily L1, L2, that will rotate together with the Moon and stay on one side of it.

The most famous right now (but not necessarily the most suitable for you) is the near-rectilinear halo orbit planned for the Lunar Gateway.

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    $\begingroup$ Are there medium orbits beyond low-lunar orbit but well below rectilinear and lagrange that might be stable? Like in the 500 - 2000 km range? $\endgroup$
    – antlersoft
    Feb 18, 2023 at 1:46
  • $\begingroup$ @antlersoft I don't know - I guess what is and isn't "stable" is a question - that altitude is much less susceptible to gravitational perturbations from the lunar mascons but more susceptible to gravitational perturbations from the Earth and Moon. Anyway, thanks for your comment - I added a section for the distant retrograde lunar orbit/ $\endgroup$
    – uhoh
    Feb 18, 2023 at 3:16

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