Is there are theoretical limit as to how low a satellite could maintain an orbit around the moon? I know there about mountains so it must orbit above those. There is also a very thin atmosphere but I don’t know if that is a consideration.
I was wondering if a satellite could orbit a few hundred meters above the surface of the moon.
The moon isn’t uniform in its composition, so you end up with some areas of higher mass concentration and some areas of lower mass concentration. These variations in mass concentrations make low lunar orbits unstable.
I don’t know exactly how high you have to go to get a stable orbit, but a few hundred meters ain’t gonna do it.
Areas of higher mass concentration are called “mascons”. Googling that might get you more info.
Lowest possible lunar orbit is determined by various stability and safety factors. These include earth’s gravitational influence, lunar mass concentrations (mascons): https://en.wikipedia.org/wiki/Mass_concentration_(astronomy), and lunar orbital inclination. Some higher orbits are also unstable:
The Apollo spacecraft initially orbited at about 60 nautical miles (111 km) altitude, however some missions used more elliptical orbits. In case the Lunar Module had difficulty upon ascent reaching the required orbit, there were contingency plans for the Command/Service (CSM) module to swoop down to a very low orbit and rendezvous.
I don’t know what the lowest limit was for that ascent contingency, but the unmanned Lunar Prospector probe was in a circular orbit at 20 miles (about 32 km or 105,000 ft) altitude for several months.
During Apollo 15, incomplete mascon surveys resulted in the LM/CSM descending below 15 km (8.1 nautical miles, or 49,000 ft) altitude. It was on a trajectory to reach a perilune of 10 km (32,800 ft or 5.4 nautical miles) but a corrective burn raised this to 13 km.
The highest lunar mountain Mons Huygens is 15,400 ft high (4.7 km) so Apollo 15 would not have hit it, even had the orbital path intersected at the moment of lowest lunar orbital altitude on that mission.
If the gravitational field strength was reasonably constant and there was nothing in the way you could orbit as low as you want. Build a trench and you could orbit sub-surface.
But in reality the previous answers are correct.
The velocity needed would be way the hell in excess of escape velocity.
Just for the heck of it.
The Holes Around Mars by Jerome Bixby
Orbital speed is always less than escape speed.
Yup. Quick calculation so I hope I’m right but, for example, orbital velocity at the moon’s surface would be 1.7 km/s. Escape velocity is 2.4 km/s.
My mistake. Many years ago, I read an article about the science in the story. Obviously, I mis-remembered a part of it.
Aren’t you ignoring the gravitation of Earth and other bodies whose perturbative influence would break stability at some point? (I.e., “gravitational field strength” will not be constant in a 3-body problem.)
Yes, I’m totally ignoring that.
As others have said, the trivial assumption is that the Lunar atmosphere is negligable and any stable orbit that clears the surface features could be maintained indefinitely. The reality is that mascons, perturbations by the Earth, and even potentially like pressure and accrual of suspended dust from ionization on the satellite would all limit the duration that an unadjusted orbit could be sustained. The minimum altitude depends upon specifics of the trajectory, but it is certainly possible to plot selenocentric orbits at inclinations and arguments that avoid the mascons with an altitude of a few tens of kilometers which could easily last thousands or tens of thousands of years without correction.
Stranger
In the long long run, all orbits are unstable. If nothing else, it’ll eventually decay due to gravitational radiation.
In the not-quite-that-long run, you’ll usually get instabilities due to interaction with other objects, masscons, etc.
So really, to answer a question like this, you have to establish how long you want the orbit to last. If 10ish times around is enough, then you probably can get away with something that just barely clears the highest mountain, but if you want 100 years, then you’ll need more leeway.
If you detour to the moon without planning, stay out of low lunar orbit , less than 200 km… Or carry heaps of fuel.
Its a death trap because the mascons, mass concentrations, cause most unplanned, eg not accurately aimed, low lunar orbits to decay quickly. NASA discovered this in the 60’s when they sent a satellite out from the Apollo mission, into very low lunar orbit, and the satellite quickly suffered orbit disturbance . At first it went fine, but then it almost crashed, settled down again, and went haywire again and did crash.
Orbits with inclination of 27°, 50°, 76°, and 86° avoid the effects of these mascons causing trouble… So you could ensure your approach puts you into one of these, or use the fuel to adjust to one. That is, there are some low lunar orbits which are better than others.
My understanding is anything lower than geosync (selenesync?) orbit will eventually result in impact.
No, that’s not true. There is nothing special about geosynchronous orbit other than that the satellite passes over the same point on Earth once a day. A selenostationary orbit is essentially impossible (except for the special case of objects at L1 and L2 libration points) because of the very slow rotation of the moon (rotational period of ~27.3 days).
Stranger
I can see where the misconception would come from, though. For satellites orbiting Earth, there are mostly only two orbital heights we’re interested in: In the first case, you just want a cheap launch and put the satellite in the lowest orbit that’ll stick around long enough, in which case it’ll only stick around about that long. If your satellite will be obsolete in ten years, for instance, you might figure on an orbital height that’ll take 20 years to decay, just to be on the safe side. You could launch into an orbit that would take a thousand years to decay, or one that’ll take ten million years, but why would you? Higher orbits are more expensive.
In the other case, you have some specific reason for wanting to be at some specific height, in which case of course you launch to that height. And by far the most common specific height anyone would want is geosynchronous height. Now, geosynchronous is high enough that it’ll last basically forever, but it’s not the lowest height for which that’s true. You could have a satellite that lasts basically forever at a much lower height, but why would you bother?
earlier thread of mine:
http://boards.straightdope.com/sdmb/showthread.php?t=630888&highlight=moon
There are plenty of orbits in upper LEO and MEO below GSO (geosynchronous or geostationary) that are well suited to a variety of applications, including telecommunications, earth surface and weather surveillance, high latitude highly elliptical (used to achieve long dwell times for television broadcast and satellite internet), et cetera. GSO is actually kind of problematic because it occurs only at a specific altitude and therefore has a limited number of orbital slots in geostationary orbit before spectrum allocations by the International Telecommunications Union are filled,
and geosync orbits have to be planned to not interfere with geostationary or other orbital operations.
LEO is desirable because it is, as you note, easy to get to, transmitters and optical viewers can be smaller and less powerful than those at higher orbits for the same resolution, and because drag from the thermosphere will cause satellite orbits to eventually decay, hopefully meeting the 25 year limit past operational life (though since most of these satellites have no propulsion systems and thermospheric behavior is still not fully characterized, it is often guesswork to see if satellites above the minimum altitude will actually decay). However, LEO is also full of debris from satellite launches, bits of junk that have come off of spacecraft over the years, and detritus from ASAT tests and spacecraft collisions (only one verified so far but it is only a matter of time before others occur). The ISS regularly has to perform collision avoidance maneuvers, and again most satellites don’t even have that capability. So there are good reasons to go above the minimum and below GSO for both mission requirements and reliability. The problem is that there are few launchers that go above LEO and the cost of launch often doubles or more per payload mass to get to MEO.
Stranger
Which is depicted in Iain M Banks last science-fiction novel ‘The Hydrogen Sonata’ where an alien species did exactly that.
Possibly in Asimov’s “Where Do We Go From Here?” which included “The Holes Around Mars” and several other stories, with discussion of the science in each story.