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Say we decide at some point that we'd like to keep the Moon where it is, stop its recession. What would be some efficient ways to achieve this (in terms of rate of money or energy spent)? The motivation is that we'd like to keep experiencing angular eclipses where the moon is large enough to hide the photosphere of the Sun but not large enough to hide the corona.

I'm looking for answers grounded in current science.

One obvious method that comes to mind is to blast it with some material or light tangential to its orbit with Earth, increasing the orbital velocity it loses.

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    $\begingroup$ Please keep in mind for future posts that providing your own answer and asking for more is prohibited by the help center. The rationale is simple: you have an answer... what's the point of asking again? From our perspective, if you explain why the answer you've already considered doesn't meet your needs, that elevates it to a condition of the question. Is there a reason why your answer is insufficient? $\endgroup$
    – JBH
    Aug 6 at 20:19
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    $\begingroup$ BTW: +1 for giving us an explanation of what you meant by "efficient." Not everyone realizes that's necessary. Thanks. $\endgroup$
    – JBH
    Aug 6 at 20:21
  • $\begingroup$ Thanks. My answer is lacking quantification. Also, it might not be the best way to do this and missing some considerations. I think of this as a design document for doing something like that if we need to. A design document should have multiple possible approaches. Understand that this might be slightly divergent from the needs of the website. On that note, I think world building does often have questions that require speculation to answer and benefit from multiple answers as opposed to one really good one. $\endgroup$
    – Rohit Pandey
    Aug 6 at 21:56
  • $\begingroup$ There was a question on the Astronomy SE in the HNQ recently and I read there that the moon recedes at a rate of around 1cm per year. It is actually quite amazing we can measure this at all seeing that the moon is around 400.000 km away from us. $\endgroup$
    – quarague
    Aug 7 at 13:47

2 Answers 2

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Repeated gravity assists.

Get a big spaceship, or a lot, or a big asteroid or moon, and repeatedly use energy from sunlight to slingshot it past the moon.

Using an ion drive with an efficiency of 90% and a futuristic solar panel with an efficiency of around 50% the government can simply force the many spaceships going around the solar system to repeatedly do slingshot maneuvers around the moon and pay for their energy costs and only lose 55% of the energy.

The spaceships can then continue their missions away from the earth or towards the earth. This doesn't damage the moon at all, and simply costs a bit of time and energy for spaceships.

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    $\begingroup$ I've gotta ask... the OP states that they're looking for "answers grounded in current science." I'm having trouble believing "a big spaceship" or "a lot [of spaceships]" etc. meet that expectation. In a sense, the OP has limited themselves to the answer, "we can't do this today and meet your efficiency expectation." But I'm curious to see if you can get this all into his/her conditions, because the rest of it is plausible. $\endgroup$
    – JBH
    Aug 6 at 20:24
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    $\begingroup$ Without numbers this is not an answer. $\endgroup$
    – AlexP
    Aug 6 at 21:16
  • $\begingroup$ We can build spaceships today, so per current science this is perfectly feasible, it would just take a lot of resources. I gave numbers, this is a very efficient method if you have enough traffic. $\endgroup$
    – Nepene Nep
    Aug 7 at 0:05
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    $\begingroup$ The space ships we can build today cannot do what you're suggesting.... $\endgroup$
    – JBH
    Aug 7 at 0:38
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    $\begingroup$ The spaceships we can build today can certainly do that, and if we had the political will we could build far more. They already routinely use gravitational slingshots to go faster. Doing it in reverse is weird, but very possible. $\endgroup$
    – Nepene Nep
    Aug 7 at 9:38
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Casually assuming that civilizations with these concerns have lots of space infrastructure and plan millions of years ahead.

Reducing tides

The recession is mainly caused by the the tides in the earths oceans. This also has an other (probably undesirable) effect, the lengthening of the earths day. So reducing this would kill two birds with one stone.

On the wiki page on Tidal Acceleration a paper is referenced that suggest that the current rate of recession is abnormally high and that it used to be 1/3th to half the current rate. Williams, George E. (2000). "Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit" It is suggested that this is caused by resonances in the oceanic currents that accompany the tidal wave.

The civilization might have some other earth reforming projects going and needs to tightly control oceanic currents for climate control and preserving bio diversity. So reducing the average of the tides might be incorporated in these plans.

Counter force

The effect of the tides can be understood a a virtual force acting upon the moon in the direction of its orbital travel. Causing it to spiral outwards.

Calculating the virtual force the tides exert on the Moon is quite difficult, because articles on the topic only give the angular decrease in orbital speed which works together with the change in orbital distance to absorb the energy of this force. This makes it difficult to calculate a single force from those numbers. What we want is an actual average force in Newtons.

However On the Wiki a number is given of 0.12 Terra Watt of energy that is added to the Moons orbit. Dividing that by the Moons orbital speed of 1022 m/s gives around 118 Mega Newton. (or 118 kilo ton of thrust) This is not that much, about 3 and a half Saturn V rockets, but it has to be given continuously until the end of time.

Chemical rockets

Doing this with chemical rockets would spray valuable propellants in a orbit around the Sun, which would be blown out of the solar system by the solar winds. Not efficient.

Particle beam

Doing this with a high efficient propulsion device like powerful ion drive, a nuclear torch or something else in that category, would effectively be a lunar death ray that spreads around high energy particles like a lighthouse. With a rotation period of one month. Not nice for space vessel approaching Earth.

The effect might be lessened by using a divergent beam. At some considerable distance the radiation of the beam will be less that the natural solar radiation. Still spacecrafts have to avoid flying directly though the beam.

Solar sails

An other method for creating this force would be a cloud of lunar satellites with solar sails. The satellites orbit the Moon and any force on them is eventually propagated to the Moon. Operating them in a clever way can change the orbit of the Moon.

According to the wiki on Solar Sails a 800m by 800m meter solar sail maxes out on 5 Newton. To create a force of 118MN would require a square of 3900km on the sides. This is slightly larger than the Moon and not counting for operating losses. This cloud of satellites would defeat the initial purpose of preserving Earths view on the Moon.

Mass driver

The Moon is an important source of building material. Ejecting this material in one direction with a mass driver will influence the orbit of the Moon. To eject this material into an orbit around the Sun it has to be ejected with at least the orbital velocity of the Moon which is 1022 m/s. Perfect for building a ring of habitats in an orbit around the Sun close to Earths.

To produce a force of 118 MN and assuming a mass driver speed of 2000 m/s, 59 Ton of material needs to be launched every second. That is almost 2 giga Ton per year. Which has negligible effect on the mass of the Moon of 7.3 * 10^22 Kg. But it will mean that the Moons surface is mined by 1 meter every 100,000 years. Precaution needs to be taken to keep the Moons appearance aesthetic.

Conclusion

Several techniques can be combined and that is the most likely solution, also depending on other uses. Reducing the tides a bit seems most favorable, because of its side benefit. But it will not solve the problem completely. To solve the reduced problem, the mass driver approach is the solution with the least side effects. And it might be done anyway, because of the material needs near Earths orbit around the Sun.

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