I Can Satellites in Frozen Orbits of the Moon Study Different Ground Areas?

[Sam Phillips]

TL;DR Summary

Frozen orbits of the moon effect on Satellite ground coverage

Quick question for the people to better understand orbital mechanics.

Due to large mass concentrations on the moon's surface, there are only four orbital inclinations that a satellite can be at to maintain an indefinite orbit: 27°, 50°, 76°, and 86°.

My question is this: If a satellite was in a frozen orbit of the moon, would it then forever be orbiting over the same ground? The reason for the question is that if a company wanted to place a long term satellite into orbit for science, would that satellite then be stuck studying the same section of the ground because it was in a frozen orbit. Or would they be forced to place it in a much higher orbit so that it wasn't forced to be in a frozen orbital inclination?

 

[DaveC426913]

By "frozen" orbits of the Moon, I assume you mean LunaStationary / Selenostationary orbits - where the satellite is stationary above a specific spot on the Moon's surface. That is theoretically at about 88,000km, or almost 1/4 of the way to Earth.

I'm not sure if this is the desired state you're looking for, since you say "would that satellite then be stuck studying the same section of the ground" as if that's not the desired result. In fact, from 88,000km it would be able to see something shy of 50% of the Moon's surface. But there is some question as to whether it would be stable there ,what with Earth's and the Sun's perturbations.

What you could do is put a sat at the Lagrange point L2.

They would be relatively stable there, with just the occasional adjustment.

 

[stefan r]

By "frozen" orbits of the Moon, I assume you mean LunaStationary / Selenostationary orbits - where the satellite is stationary above a specific spot on the Moon's surface. That is theoretically at about 88,000km, or almost 1/4 of the way to Earth.

I'm not sure if this is the desired state you're looking for, since you say "would that satellite then be stuck studying the same section of the ground" as if that's not the desired result. In fact, from 88,000km it would be able to see something shy of 50% of the Moon's surface. But there is some question as to whether it would be stable there ,what with Earth's and the Sun's perturbations.

What you could do is put a sat at the Lagrange point L2.

They would be relatively stable there, with just the occasional adjustment.

View attachment 246049

That is the sun-earth l2 in the picture. Here is earth-luna:

 

[DaveC426913]

Thanks. I was looking for that.

But note, that will have the satellites seeing the same same of the Moon perpetually, something that (I think) is undesirable in the OP's opinion.

 

[Sam Phillips]

The hope is to use very small satellites with weaker sensors to save on cost and launch weight, then make up for that by placing them in an extremely low orbit (hoping for 20km). Apollo missions orbited between 60-70mi from the moon during their time. But from what I am reading, large mass concentrations on the planet cause any satellite below 100mi to suffer rapid orbit decay and crash. Then NASA discovered four orbital inclinations that a satellite could be into avoid the mass concentrations and stay in low orbit indefinitely regardless of the altitude.

Where my research is hitting a snag though is that the orbital inclinations don't tell me if they were chosen to keep the satellites constantly over the same ground so that they never near the mass concentrations, or were they chosen because that path caused the problems to cancel out. This conclusion is compounded by the moons extremely slow spin and tilt of only 1.5°.

What I want is an orbital path like below, but I can't seem to find research to see if it is possible.

 

[DaveC426913]

Where my research is hitting a snag though is that the orbital inclinations don't tell me if they were chosen to keep the satellites constantly over the same ground

This cannot be the reason since it can't happen.

1] Any orbital inclination other than 0 degrees will have the satellite making northward/southward passes.
2] Any orbit less than (or greater than) 88,000km in radius will have the satellite advancing westward/eastward.

*see caveat in sig line

 

[Nik_2213]

IIRC, those 'frozen' orbits are still only meta-stable, subject to longer-term perturbation.

 

[Keith_McClary]

The axis of the orbit will have a fixed direction (pointing at some distant star). The Moon rotates about once a month, so you will get an orbital path similar to your image. The low orbit period is ~2hrs so successive passes will be about 1 degree apart.

 

[Keith_McClary]

Is this what we're talking about?:
A New Paradigm for Lunar Orbits (NASA)

 

[DaveC426913]

This should get you started.

https://xkcd.com/2172/

 

[alantheastronomer]

Is this what we're talking about?:
A New Paradigm for Lunar Orbits (NASA)

This is about Earth's gravity destabilizing high altitude lunar orbits; what we're talking about is lunar mass concentrations destabilizing low altitude lunar orbits, which is covered by Science@NASA's story "Bizarre Lunar Orbits".

 

[alantheastronomer]

What I want is an orbital path like below.

If you did, then orbital drift will inevitably take you over the mascons and your orbit would decay. Same goes for the highly inclined orbits recommended by NASA which DaveC and Keith_McClary pointed out will also drift. These orbits were never intended for extended periods of time; they'd need corrections to help stabilize them, and they were chosen to avoid the mascons. If you were stuck in an orbit over the same patch of the lunar surface, you could still possibly observe more of the Moon depending on your instrument's field of view.

 

[Sam Phillips]

Darn, that means a satellite at 22km would have a limit of 1148.18km of observable distance on the surface. Only 7.7% of the surface area of the moon. Even less when you account for obstructions...

O well, you can't fight physics.

 

[Keith_McClary]

Only 7.7%

I figure 0.3%

 

[DaveC426913]

Darn, that means a satellite at 22km...

Must have missed a post. Where did 22km altitude come from?