My understanding is that earth orbits mostly decay because of the object in orbit striking the extremely tenuous atmosphere at that height which slows it down over time. Would an object put in orbit of the moon, say a space station, stay in orbit basically forever since the atmosphere is already basically nil compared to earth? Or would some interaction between the earth/moon system make that orbit unstable?
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Quite the opposite, actually. Most lunar orbits are unstable. The moon isn’t perfectly uniform. Variations in density causes orbits to become more eccentric over time until the periapsis becomes low enough to hit a mountain somewhere.
A high enough orbit might avoid that but then it would be perturbed by interaction with the Earth and eventually crash or get ejected from the system.
Scott Manley has an excellent video explaining this:
https://youtu.be/EadClM4Y45A?si=7WXHLqkBxDSchs8–
That’s a very good guess. Based on what causes orbits to decay around the Earth it certainly looks like orbits around the moon would be stable long term.
However the strength of the moon’s gravity isn’t uniform. There are pockets of more dense material inside the moon near the surface that mean the gravity is slightly higher in some areas than others. This is also true on Earth but I think it’s more extreme for the moon. This means if you’re in a close orbit around the moon you’re being tugged slightly differently by the changing gravity fields in different parts of your orbit and it can nudge you off course. It doesn’t take a lot of disruption to move you into an unstable orbit where your closest approach to the moon is very close and you end up hitting the surface.
The Apollo 10 mission was a dress-rehearsal of the Apollo 11 landing mission, intended to test out the hardware for the lunar lander and practice all the procedures. But they were also double-checking some of the analysis on lunar gravity measurements taken from the uncrewed Surveyor spacecraft. They didn’t want the Apollo 11 descent to go off course because of inconsistent gravity strength. As it happened the lander went off course anyway but not because of gravity issues.
This effect is more extreme with very close orbits (The kind of orbits that would be impossible with Earth because of the atmosphere). And just like on Earth there are plenty of stable orbits further out.
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>that earth orbits mostly decay because of the object in orbit striking the extremely tenuous atmosphere
That’s the case in LOW Earth orbit, which is a feature not a bug. It’s very easy to install a small thruster capable of giving the nudges required to maintain orbit over the satellite’s service life and it means that space junk will naturally clean itself even if you lose control of the satellite and can’t manually deorbit.
There is still loss all the way out at geostationary orbits, but we’re talking centuries to millenia of stability.
For that matter the Earth’s orbit around the Sun is decaying, albeit so slowly that the Sun will go red giant and destroy the planet first.
>Or would some interaction between the earth/moon system make that orbit unstable?
This is poking at the three body problem. Well, technically the four body problem. The answer is mostly we don’t know. There is no analytic solution to the three body problem. We can calculate out the path of bodies numerically to some arbitrary point in the future, but there is no formula representing its indefinite trajectory.
There is a class of orbit called Distant Retrograde Orbits which are very stable around the Moon. But as the name implies, they’re pretty far away, so only suitable for certain types of missions.
It would be stable for hundreds of years for most elliptical orbits since oblateness affects the rotation about the polar axis but wouldn’t cause it to lose energy. But it would be most stable at L4 and L5 lagrange points