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Related Question : Can we 'beam' energy from the moon?

I was wondering if instead of 'beam'ing energy from the moon, could it be possible to extend a power cable from the earth to the moon?

If the power cable was fixed at a stationary point on earth, it would be at one of the poles, I guess, otherwise the cable would wrap around the earth as the moon revolves. What other reasons could prevent this from becoming a reality, or what type of improvement in technology would be required to make this achievable?

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I asked this question primarily because it was stated in the above question that 'beam'ing energy from the moon to earth would have a pretty bad efficiency. Now, as several people mentioned in the comments, I get it that a cable would be no better as it too would lose power during transmission. However, for the sake of this question, let's assume that the cable material is a superconductor, and has the adequate properties (tensile strength, etc.) to withstand the rigors of satellite-tethering!

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    $\begingroup$ Just something I'd like to point out, the difference between the furthest and closest the moon gets from the earth is about 42,000 km, which is more than 10% of its average distance. $\endgroup$
    – overactor
    Commented Dec 18, 2014 at 7:32
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    $\begingroup$ Make the cable "curly" to solve that issue ;) $\endgroup$
    – Ghanima
    Commented Dec 18, 2014 at 7:34
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    $\begingroup$ We're trying to go 'wireless' everywhere and you want to tether the moon! ;) $\endgroup$
    – bowlturner
    Commented Dec 18, 2014 at 13:45
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    $\begingroup$ A really big issue, in addition to the ones already presented, is the power drop-off over long transmission lines. $\endgroup$
    – Adam Miller
    Commented Dec 18, 2014 at 18:37
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    $\begingroup$ While you're making a cable to the moon, you may as well put Earth's end on a track that you build around the equator, and let it drive around following the Moon. $\endgroup$
    – AlbeyAmakiir
    Commented Dec 18, 2014 at 22:00

8 Answers 8

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The first and major issue is already presented in the question. Since the moon is not in geostationary orbit around the earth, that is: it doesn't orbit the earth at the same speed that the earth rotates (aka lunar month is different from earth day), thus the moon appears over different places of earth. Any cable attached to earth and the moon would wrap around earth over time. Attaching the cable in the polar region of earth is not going to help much without some kind of an "untangler" in space because the plane of the moon's trajectory (its orbital inclination) is not polar with respect to earth.

A second major issue is the material to fabricate such a cable: mainly its (tensile) strength to support its own weight. Refer to the concept of a space elevator to find out more about that issue. A material with a large tensile strength vs. density ratio would be required.

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    $\begingroup$ You could have the cable on a cart on rails at 5.16 degrees (the inclination of the moon's orbit) from the north pole. That cart would race around the pole at about 150 km/h. $\endgroup$
    – overactor
    Commented Dec 18, 2014 at 7:41
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    $\begingroup$ Would be a funny thing, that's for sure. But a cable attached near the polar region of earth would experience an additional load from not leaving perpendicular to earth. There are however many other issues (you mentioned) changing distance earth-moon, as well as libration of the moon. $\endgroup$
    – Ghanima
    Commented Dec 18, 2014 at 7:49
  • $\begingroup$ plus, the cable would begin to twist, causing it to break over time as well $\endgroup$
    – Flotolk
    Commented Apr 16, 2015 at 13:24
  • $\begingroup$ Twisting isn't a problem: see every electrical motor ever (no, really). The real problem is that the moon moves around the earth in a non-stationary way. Even if you built a track at 5.16 degrees off from the plane of the equator...that track would need to be mounted on another track. There's a reason total solar eclipses happen in different places. $\endgroup$
    – Draco18s no longer trusts SE
    Commented Jul 5, 2017 at 18:12
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It's not really feasible, and even if it was it's not what people would do. There are lots of good reasons not to place the generators on the moon (for example the obvious generation to do in space is solar, but the moon has nights and days).

On the other hand though if we were running a Space Elevator out to just past geosynchronous orbit (so that centripetal force keeps it in place) then it would be quite easy to have massive solar panels on the station at the end of the space elevator and run the power from those back down the cable. That has most of the advantages of the moon and few of the disadvantages.

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    $\begingroup$ I second the part about a space elevator. I think that is the obvious alternative. There would be a need for a space elevator anyways, why not also use it for power generation? There could even be multiple space elevators. If you really want to produce energy on the moon have it transported to the nearest space elevator, with multiple existing. $\endgroup$
    – dsollen
    Commented Dec 18, 2014 at 13:47
  • $\begingroup$ The space elevator rotates with the earth and its magnetic field. The distortion of the field due to solar wind would need to change the number of flux lines that cross the cable. That means the cable can't be too long (we want the extended tail to be cut off) or too short (the field lines are not compressed near the earth). $\endgroup$
    – JDługosz
    Commented Apr 16, 2015 at 13:08
  • $\begingroup$ @JDługosz I don't understand what you mean? $\endgroup$
    – Tim B
    Commented Apr 16, 2015 at 13:11
  • $\begingroup$ Look at a picture of the earth's magnetic field. Imagine the wire moving around that. If it cuts the same number of flux lines, even if they are rearranged, it won't generate because nothing is changing. $\endgroup$
    – JDługosz
    Commented Apr 16, 2015 at 13:19
  • $\begingroup$ I was suggesting solar power generation, not magnetic flux power generation...flux lines are not relevant... $\endgroup$
    – Tim B
    Commented Apr 16, 2015 at 13:21
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You need space elevator. And not only one. If my calculations are correct, you need four of them, possibly connected to each other.

(Googling Space elevator length gave me 60000 miles = 96560.6 km. Googling "distance to Moon" gave me 384400 km. $384400 \div 96560.6 = 3.98$, thus 4 space elevators)

You need to somehow resolve variable distance of Moon to Earth

As stated in the comments, the Moon changes its distance relative to the Earth by 10 percent. This means its distance can be:

$$(384400 - (0.1 \times 384400)) < \text{ distance } < (384400 + (0.1 \times 384400))$$ $$ 345960\text{km} < \text{ distance } < 422840\text{km}$$

So, you do not only need 4 space elevators, you need to build space elevators using curved cable, which is another level of the whole engineering issue.

You need to resolve losses on the cable

Being in energy field, I have to tell you sad news. The power cables lose their power on the go. We kinda resolve it by pushing the power on high voltage to the cables, but you are still going to lose some power.

Long story short, I believe it is more plausible story to beam energy than to send it via cable

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    $\begingroup$ Actually, I don't think you need multiple space elevators. You could use one elevator to get to geosynchronous (though I'm not sure that works in polar orbit), then just a small thruster nudge toward the moon. The cable won't care the trip takes a year, and it could greatly reduce tensile strength requirements. If you can plausibly solve the problem of construction of and power losses in 400 +/- 20 Mm of electrical cable, or if the readers can be made to believe that those issues have been resolved, then I think you can reasonably just say "it's been built, accept that it's there and move on". $\endgroup$
    – user
    Commented Dec 18, 2014 at 8:27
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    $\begingroup$ Which is my usual "Writer" approach to most of the issues. By being here, I realized that posting answer "Just hand wave it and say it happened" is a) acceptable answer to almost any question here thus b) is utterly wrong ;) $\endgroup$
    – Pavel Janicek
    Commented Dec 18, 2014 at 8:42
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    $\begingroup$ Yes, the point of this site is to cater to those who don't want to just handwave things away. If you are okay with handwaving things away, you can have your unicorn/infantry encounter all you want. :) $\endgroup$
    – user
    Commented Dec 18, 2014 at 8:59
  • $\begingroup$ The minimum distance to the moon at perigee is 356.4 Mm, and the maximum distance to the moon at apogee is 406.7 Mm, both according to Wikipedia's quick facts box on the subject. There are your bounding numbers. $\endgroup$
    – user
    Commented Dec 18, 2014 at 9:43
  • $\begingroup$ An understandable example of calculating power losses in cables is here: electronics.stackexchange.com/questions/58700/… Fill in your numbers to get an idea if it's feasible. $\endgroup$
    – user3106
    Commented Dec 18, 2014 at 10:59
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A few others have addressed the "how" of it, so I'll try to take a stab at the "why" of it. Rather than having a power station on the moon, why not have the cable itself be the power station.

An electric generator consists of a conducting coil moving within a magnetic field. The Earth itself emits a magnetic field. In 1996, NASA had an orbiting spacecraft extend a 20 km conducting wire. The orbit of the spacecraft moved the wire through the Earth's magnetic field, creating a significant current. http://www-istp.gsfc.nasa.gov/Education/wtether.html The problem is that this energy robs kinetic energy from the spacecraft. The more electricity you generate, the more you slow down the spacecraft. For a small body, this isn't viable for power generation, since you would need to continuously accelerate the ship to make up for the drag.

Instead, let's steal kinetic energy from the moon. The tether moves around the earth with the moon, moving through the earth's magnetic field. The tether produces electricity, which we use.

Wouldn't this slow the moon and bring it crashing down on our heads? Well, yeah, but the moon has quite a bit of kinetic energy. A rough calculation gives about 3.6*10^28 joules, or enough to supply the entire world's power supply for the next 65 million years. That would then be a problem for our descendants.

To resolve losses, consider that the strongest power generation will be close to the earth, where the magnetic field is strongest. The connection to the moon is present to make sure that the near end of the cable keeps moving. Perhaps that part of the cable could be made of superconducting material, to further minimize losses.

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    $\begingroup$ Also, the moon is slowly gaining kinetic energy from tidal forces (i.e. from earth's rotation), so if we calibrate it right, we could just extract that much energy which is gained. $\endgroup$
    – Paŭlo Ebermann
    Commented Dec 19, 2014 at 9:12
  • $\begingroup$ Very true. Another thing that I didn't account for, as I just wanted a order of magnitude estimate, is the gravitational potential energy. As the moon is decelerated, it would drop into a lower orbit, transforming some of the potential energy into kinetic energy. The value given should therefore be interpreted as a lower bound on the amount of energy that can be extracted. $\endgroup$
    – Eldritch Cheese
    Commented Dec 19, 2014 at 14:20
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or what type of improvement in technology would be required to make this achievable?

Scientist haven't found a way to control lightnings. If they can achieve that, there could be a way without a cable in the whole distance, and a pole from Moon, and several poles from Earth could make the connection, but in a wireless way. So the cable is impossible, but power connection has a chance. By the way it can't be effective, or cheap. Maybe transports with "batteries" with very huge capacity of power can be a way.

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    $\begingroup$ How would lightning work in space? $\endgroup$
    – Ghanima
    Commented Dec 18, 2014 at 14:02
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    $\begingroup$ That's why you need pole from the Moon, it has to be long enough to earn the atmosphere. $\endgroup$
    – Tamás Juhász
    Commented Dec 18, 2014 at 14:08
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    $\begingroup$ Suggesting a 400.000 km long pole? There might be some minor drawbacks to that idea. $\endgroup$
    – Ghanima
    Commented Dec 18, 2014 at 14:11
  • $\begingroup$ I agree with that, but the question wasn't about effectiveness :) And the lower gravity maybe makes the things easier (wasn't mean to be a word-wit) $\endgroup$
    – Tamás Juhász
    Commented Dec 18, 2014 at 14:24
  • $\begingroup$ The lower gravity of the moon is unfortunately not going to help much, as the pole's end near earth would experience much pull from earth too. $\endgroup$
    – Ghanima
    Commented Dec 18, 2014 at 14:28
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Make the cable non-physical. I mean not solid matter in the usual manner. A string of thin ionized plazma will support magnetic field lines. Those are vibrated to transmit power, and fancy modulation is also used to keep a tight wound torus bundling the field lines together, trapping them and the plazma in mutual feedback. A few superconductive rings made from carbon nanotubes, every few thousand miles, are used for robust containment and initiation.

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Given the capacity to 'beam' power to the moon in so many forms, why a power cable? Or solar on the moons? If you need a midway point...a 'beanstalk' or 'space elevator' to get out of the atmosphere, and then a rotating laser to beam power to the moon from there.

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There is no way to build this cable.

Pavel Janicek came close. Given the situation he described it could be done, although it takes more than 4 cables:

1) You need a space elevator to get into space.

2) You need an orbital ring to actually connect to (monorail fashion, it must be free to move along the ring) as neither body keeps one face pointed at the other. (Yes--the moon does not present a constant face to us! It's called libration--the moon wobbles back and forth. While it's approximately right the end of the cable would wobble badly!) Keeping this ring stable means at least two space elevators, I suspect three would be needed.

3) As he said, the moon moves in and out during it's orbit. His answer of a curly cable wouldn't do it--you're flapping the cable around, that's got to do very bad things. Instead, don't actually connect it. Instead you have cables heading out from both the Earth and the Moon, they are long enough to have overlap even when the Moon is at it's farthest. These cables are coaxial, in effect they are trains riding on each other. This allows it to move in and out without causing disaster.

However, there's one more factor involved. The moon doesn't orbit in the plane of the equator, but 5.145 degrees off from it. I haven't been able to dream up a coupler that can deal with this situation.

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