Greg Autry notes that Low Lunar Orbits although they seem safe (from 20-30m up!) are usually not. I mean, even leaving aside that "30m up" has to clear all the craters and other ridges.
The Moon's mass, being so much lighter than Earth's, is poorly distributed around the Moon's surface. Autry finds four really stable low lunar orbits, at 27°, 50°, 76°, and 86° inclinations from the lunar equator
. Those stand to get crowded since, at LLO, the "surface" of that globe is so much less than our LEO over here. Especially 86° which is near-enough polar, going over the water-reservoirs.
"Eager Space" offers a chart on various orbits. It turns out that LLO needs stationkeeping, at least at "polar" (=86°); so he goes ahead and includes TLL2 halo too, which also needs stationkeeping but (I gather) less of it. Also LLO, so close to more-reflective parts of the Moon, has trouble radiating heat away.
Before we worry about cluttering LLO we should figure how to get to the moon in the first place, LLO not being a "feasible" orbit from Earth, at least directly. If we're just using the Moon as a gravity-anchor, en route to (say) STL1 and Venus, then TLL2 halo is better; this is the one Hop David really likes.
Gateway is on the list to the extent it's doing Near [Halo] Rectilinear Orbit (NRO here) with comparable stationkeeping cost as TLL2. This one can inject to LLO-polar. Phil Metzger likes NRO inasmuch as it works for "Mars" as well (meaning Deimos to some of us).
Rounding out the rest: "Frozen Lunar Orbit", which I guess is a high orbit; Prograde Circular Orbit; Elliptical Lunar Orbit; Distant Retrograde Orbit. I don't really... know what ELO is for. Kicking off for off-ecliptic bodies like near-Earth asteroids? DRO might work as a shuttle between TLL2 and TLL1 (the gravitational halfway to Earth); I guess retrograde on the figure-eight plan.
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