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Because the Moon has no atmosphere a spacecraft can theoretically cruise at a very low orbit. What is the lowest possible lunar orbit and which spacecraft achieved it? Was it the Lunar Module of Apollo (began its landing sequence with a Descent Orbit Insertion /DOI/ burn to lower their periapsis to about 15 km)?

Is such low orbit rather circular or elliptical? Are there any quirks to a very low lunar orbit (perturbation effects due to gravitational anomalies etc.)?

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    $\begingroup$ Highly related: space.stackexchange.com/q/20583/6944 space.stackexchange.com/q/19944/6944 2nd link states "For orbits at 100 km and below, stationkeeping maneuvers are required to maintain altitude control. " which may be your answer. $\endgroup$ – Organic Marble Feb 17 at 18:01
  • $\begingroup$ In my opinion, the question should be asking for the spacecraft with the lowest altitude for an orbit. Not asking the lowest possible orbit and if any spacecraft achieved it. Because the former is guaranteed an answer, while the latter is not. $\endgroup$ – Star Man Feb 18 at 3:22
  • $\begingroup$ @StarMan it's totally up to the OP what question to ask as long as it's clear and not off-topic. I can't imagine any reason to suggest that only questions that are guaranteed an answers should be asked. Different but related discussion: [What's best to do about questions that might not be answerable for a few months?]([space.meta.stackexchange.com/q/1407/12102) $\endgroup$ – uhoh Feb 18 at 4:31
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The lowest orbit achieved would probably be PFS-2, a small satellite deployed from Apollo 16's service module. It was intended to go into a 55x76-mile orbit (88.5x122 km), but due to variations in the Moon's gravity field, it made passes of six miles (9.6 km) or less before crashing into the Moon's surface.

There are very few stable low orbits around the Moon due to the extremely lumpy gravity field. Most Lunar orbiters stay far enough up to not be affected. The closest deliberate trajectory is probably Chang'e 2 with a periapsis of 15 km (9.3 miles); other low flyers are the Lunar Reconnaissance Orbiter at 12 miles (20 km), and LADEE (20km x 60km, 12mi x 37 mi).

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"Lowest possible lunar orbit..."

As pointed out in comments and in answers to the linked questions

very close orbits around any body with an imperfect, lumpy gravity field will evolve over time and can eventually intersect the body's surface.

"Lumpy gravity" first became a thing when the term mascon or mass concentration was coined to explain the perturbers that lead to a low lunar orbit evolving to intersect the surface (i.e. something crashed into the Moon earlier than expected).

From Apollo_16; Lunar subsatellite PFS-2:

The Apollo 16 Particles and Fields Subsatellite (PFS-2) was a small satellite released into lunar orbit from the service module. Its principal objective was to measure charged particles and magnetic fields all around the Moon as the Moon orbited Earth, similar to its sister spacecraft, PFS-1, released eight months earlier by Apollo 15. "The low orbits of both subsatellites were to be similar ellipses, ranging from 55 to 76 miles (89 to 122 kilometres) above the lunar surface.

"The orbit of PFS-2 rapidly changed shape and distance from the Moon. In 2-1/2 weeks the satellite was swooping to within a hair-raising 6 miles (9.7 km) of the lunar surface at closest approach. As the orbit kept changing, PFS-2 backed off again, until it seemed to be a safe 30 miles away. But not for long: inexorably, the subsatellite's orbit carried it back toward the Moon. And on May 29, 1972—only 35 days and 425 orbits after its release"—PFS-2 crashed into the Lunar surface.

enter image description here source "This figure shows the topography (top) and corresponding gravity (bottom) signal of Mare Smythii at the Moon. It nicely illustrates the term 'mascon'."

So topographically this looks like a crater, i.e. missing mass, but gravitationally it looks like a mass excess or mass concentration. Every time a spacecraft passes near a mascon its orbit will be perturbed in some complicated way.

For very low orbits it will almost certainly be necessary to have some available propulsion to perform regular station keeping maneuvers (small propulsive impulses or continuous forces) to counter the effects of the many mascons.

As pointed out in several answers, the lower you are, the larger the effects. You can think of a mascon as a separate, small gravitational object in addition to the Moon's average gravitational field, and the $a \sim Gm/r^2$ law would then apply to the acceleration it produces as well where $r$ is now the distance between the spacecraft and the subsurface mascon.

So the lower you want to go, likely the stronger the propulsion system you will need for station keeping.

There may be some altitude at which passive propulsion like a solar or electric/magnetic sail reflecting sunlight or solar wind charged particles might work for a very low mass craft, but below it you'd need a supply of propellant and some kind of thruster to accelerate it.

And that means you'd eventually run out of propellant and then crash.

The second effect is of course the Moon's surface is not a perfect sphere. Wikipedia's Topography of the Moon tells us that in addition to the few km roughness of the surface:

that the elevations are on average about 1.9 km higher on the far side than the near side.

and that relates to the questions

This image is a flatbed scan from the book Recreations in Astronomy by H. D. Warren D. D., published in 1879. 1879, source


However since the Moon's lumpy gravity is extremely well mapped and understood, and we have computers, there may be certain specific frozen orbits even at very low altitudes that minimize the amount of station keeping necessary, or at some altitude even allow for fairly long duration orbiting without any.

This answer should be read completely and all its sources consulted. Briefly paraphrased:

LRO was inserted into a polar frozen orbit for commissioning, which required no stationkeeping. (31.5 km x 199 km polar orbit with periapsis over the South Pole, "frozen" = line of apsides and eccentricity remain fixed.

Then moved into 50 km circular polar orbit (+/- 20 km) for science mission requiring a stationkeeping maneuver once a month, with a budget of about 150 m/s for the year.

moved again into a polar frozen orbit after mission, where no stationkeeping was required.

"...and has any spacecraft achieved it?"

I think that other answerers can address this with more historical thoroughty1 than I, so I'll defer to their expertise, but it sounds like the LRO frozen orbit is likely the best representative example of what's been achieved to date.

That doesn't mean at all that something closer and cleverer isn't possible!

But you'll need

  • a detailed lunar gravity map
  • a detailed lunar topography map
  • a computer, some time, and some interest

Lunar Gravity Model 2011

above: Lunar Gravity Model 2011 source, below: 20x magnified lunar topography source, click images for full size.

20x magnified lunar topography


1Could “thoroughty” have been a word if thoroughness hadn't been there?

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    $\begingroup$ The usefulness of an answer is not proportional to the amount of info in it. $\endgroup$ – Innovine Feb 18 at 7:05
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    $\begingroup$ @Innovine that's incorrect or at least indeterminate; there's no way for you to know how useful an answer will be to all future readers. Personally I love long, well-linked answers! They help readers find additional information that may be useful to them and may answer additional questions that arise to them while reading the current answer. Questions are not isolated, floating islands or planets in a vast void, the more that bits of information in the site connect to other bits, the better! $\endgroup$ – uhoh Feb 18 at 7:38
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    $\begingroup$ personally I upvote answers to the questions posted, not essays around the same subject which might or might not be relevant. $\endgroup$ – Innovine Feb 19 at 11:28
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    $\begingroup$ I upvoted this because of the quotes about what exactly happened to PFS-2, and the info and false-colour images of mascons. Some of the later stuff I only skimmed; I feel like I have an accurate enough big-picture of the challenges involved, and especially the key point that the lower you go, the more perturbation you get from a near-surface mass concentration (which makes perfect sense given inverse-square gravity). Having more links for more depth than I personally was interested in didn't detract from the answer for me. It has sufficient headings and bolding for good skimming. $\endgroup$ – Peter Cordes Feb 19 at 12:05
  • $\begingroup$ @PeterCordes thank you for the helpful feedback! :-) There will be more detailed information about PFS-1 and 2's trajectories available. If you are interested you can ask a new question about it and I am sure others will be able to provide even more details. $\endgroup$ – uhoh Feb 19 at 12:23
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Maybe, depending on your definition of "orbit".

The lowest possible orbit around the Moon is a highly elliptical orbit where the orbiting body just barely avoids grazing the surface at its lowest point. This would mean that the Apollo landers, during their descent to the Lunar surface, would have been in such an orbit, since the landing process would involving burning rockets to adjust their orbit into one that allowed them to land on the Lunar surface.

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  • $\begingroup$ None of the Apollo landers was in a descent orbit lower than that of PFS-2. The lowest was Apollo 17, at a periapsis of 7.1 miles/11.5 km. $\endgroup$ – Mark Feb 19 at 21:21
  • $\begingroup$ @Mark The periapsis of their orbit during landing was at or under the Lunar surface, because they landed on the surface; the rocket burns during the landing process lower the periapsis of the orbit down to those points in order to land. $\endgroup$ – nick012000 Feb 19 at 22:31
  • $\begingroup$ In general, it's not considered an orbit if you're thrusting or if your trajectory intersects the surface. $\endgroup$ – Mark Feb 20 at 1:34
  • $\begingroup$ @Mark If you're thrusting, your orbit is changing over time, but I'd say you still have one. If you're playing a game like Kerbal Space Program, when you fire your engines, you can watch the orbit of your rocket change in real time. $\endgroup$ – nick012000 Feb 20 at 7:23

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