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Instead of trying to tether the Moon, can we somehow harness the Moon's kinetic energy, the same way we do wind? If so, how is this possible?

HDE 226868
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MDMoore313
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    You mean tidal power? – Ville Niemi Dec 19 '14 at 00:51
  • Almost, it seems a bit inefficient to convert power that's already been converted, if it is at all possible to harness it directly. It's like converting solar energy to grass to feed cows to feed humans, cut out the middle man, ya know? – MDMoore313 Dec 19 '14 at 00:52
  • Except tidal power is collected from the moon over the entire ocean and focused on to a coast line. – Samuel Dec 19 '14 at 01:01
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    Tidal power isn't harnessing the moon's kinetic energy, but its gravitational pull. – Adam Miller Dec 19 '14 at 02:21
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    If we harnessed the Moon's kinetic energy, that would mean taking some of it away, and that means making it shift orbits. I know it has a lot, and it probably wouldn't miss much, but that might not be a wise decision in the end... – Adam Miller Dec 19 '14 at 02:22
  • @AdamMiller Understatement of the day! – HDE 226868 Dec 19 '14 at 02:23
  • Are you referring to using it for something besides a gravity slingshot? Those are a well understood way to bleed kinetic energy from a planetoid. – Cort Ammon Dec 19 '14 at 04:44
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    @AdamMiller Actually you're wrong. Tides are generated by using kinetic energy from the moon. Gravity is the medium used to move them but the energy comes from the moon's rotation. – Tim B Dec 19 '14 at 09:26
  • @TimB do you mean the moon's orbit? It's tidally locked to Earth, so it doesn't rotate. – HopelessN00b Dec 19 '14 at 12:24
  • @Villie actually tides are not as heavily affected by the moon as we thought. It plays a role, but the sun (directly or indirectly, everything on earth is powered by the sun) is the bigger contributor. I only say this because I was about to mak the same comment before I double checked myself :) – dsollen Dec 19 '14 at 15:02
  • @HopelessN00b Actually it does rotate, it rotates at the same speed as which it orbits. The kinetic energy from tides comes from both though so you are sort of right. – Tim B Dec 19 '14 at 17:15
  • @dsollen - Lunar tides are stronger than Solar Tides. The sun's gravity is stronger, but Tides are related to the difference in gravity across the earth, effectively a D^3 relation. Neap Tides are when the Lunar and Solar Tides are in opposition, and are smaller but still present. – Oldcat Dec 19 '14 at 19:49
  • yes, but wind is a major factor and temperature are also important factors, and those both ultimately trace to the sun as well ;) – dsollen Dec 19 '14 at 19:50
  • The energy of the tides really comes from the rotation of the Earth, not the orbits of the Moon or the Earth. The tides slow down the planet's rotation and convert that into moving water arround (which ultimately ends up as heat), a much less noticeable flexing of the planet as a whole (also ending up as heat) and a gradual change in the orbits of Earth about the Sun, and the Moon about Earth. The Moon's orbit is gaining energy, not losing it. – smithkm Dec 21 '14 at 03:21

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Sure, it's possible, as most things are. It's not plausible, though, and a very not good idea™, unless you intend to wipe out our planet.

In essence, there is no such thing as "free" energy - energy has to come from somewhere, so if you take it from the moon, and transfer it somewhere else, the result is that the moon has less kinetic energy. In an orbital system like the Earth and moon, lowering the kinetic energy of the moon would cause it to orbit the Earth closer, quickly resulting in stronger and potentially destructive tidal forces, and eventually resulting in the moon crashing into the Earth (if you kept harvesting energy from it). You also run the risk of creating an unstable orbital system, which in the long term would result in the moon either "being ejected" from Earth orbit, or again, crashing into the Earth.

Furthermore, long before developing the technological capabilities to harvest the moon's energy, we'd have a much better energy option anyway, in the form of large solar panels harvesting the vast energy output of the sun, for an example we can easily envision today - and the future may well bring even better options we can't foresee, as it tends to do.

As to how the moon's energy could be harnessed, it would probably be similar to the theoretical models for harvesting the (rotational) kinetic energy of a black hole. The fundamental principle is electromagnetic induction and there are a couple good posts explaining how that would work - one at physics.SE and another at Standford's site. The general approach would be the same with the moon, though as it's tidally locked and doesn't rotate, you take a slightly different approach - imagine a colossal magnet on the moon, and a colossal coil of wire on the Earth (or in Earth orbit). As the moon orbits the Earth and passes by the magnetic coil, it would induce an electrical current in the coil, which could be harnessed pretty easily.

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HopelessN00b
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    I think you're underestimating the kinetic energy of the moon. Taken its mass (7.4E22 kg) and its orbital velocity /w respect to earth (1 km/s) its kinetic energy would be 3.7E28 J or approx. 1E25 Wh. That's 70 Million times the world energy consumption (140 PWh/a, 2008). So we could draw some power off the moon without risking it to either get it lost in space or crash it into the earth. – Ghanima Dec 19 '14 at 09:27
  • @Ghanima Sure, but do you really think we'd be able to draw off enough energy to make setting up a colossal EM induction system without running that risk? I'm not sure, but I don't see even recouping the energy it would take to get such a system in place, let alone building it. – HopelessN00b Dec 19 '14 at 12:23
  • I think the key point is that by the time were able to do this we would instead start using solar panels, or even set up nuclear facilities on the moon were we can run them safely and ship the energy back. It wouldn't be cost efficient, plus helping to doom the world and all :) – dsollen Dec 19 '14 at 14:42
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    @dsollen Very true. For our particular planet. But the general case of extracting kinetic energy from an orbiting body is kind of interesting. Say that humanity eventually colonizes Europa and manages to conquer the indigenous super-intelligent civilizations of squids that live on/in it. The idea of providing power to our colony of super-intelligent squid slaves by extracting kinetic energy from Io might be appealing. Perhaps even more so than solar or nuclear or whatever else. What do we care if Io crashes into Jupiter? It's not like the giant scorpions that live on Io are useful to us. :) – HopelessN00b Dec 19 '14 at 14:52
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    Who told you about the squidians or their massing tripod death-rays for an earth evasion!? They are a classified! We had to create a classification above Top Secret for that! Bah, now that you let it out our reptile overlords that control us through the Illuminati (which they created) will certainly hunt you down and eat you! Ah well, I suggest a bath of garlic tonight, it adds to the flavor. You will be fondly remembered...until they erase our memories of you. – dsollen Dec 19 '14 at 14:58
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One possible method of using the Moon's motion to generate power is by using a solar sail as a power-generating kite. It's not really harnessing the Moon's kinetic energy, but using the Moon's reliable orbit as a good base and harnessing kinetic energy from the solar wind.

Adam Miller
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