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I was working on a side project to design an alien fleet that is wandering in the universe, and harvesting energy from planets for their needs. While I can think of ways to make that happen (geothermal, coal/fossil, "alien technology"), I was actually wondering if it is possible for people on earth to do similar things.

Specifically, I wonder if it is possible for people on earth, with modern technology (21st century), to utilize the kinetic energy from earth by decelerating the rotation?

Utilize here not just mean to help with launching rockets, but more referring to a machine/mechanism that can decelerate the planet rotation, and gain electricity/thermal energy from this process.

If it is not feasible right now, what would be a plan to make that happen, if the technology level is not a hard constrain? Lower technological level is more preferred.

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    $\begingroup$ Before harvesting energy by decelerating a planet, you would better ask how we can harvest energy of an already spinning planet. $\endgroup$ – kikirex Jun 13 at 21:33
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    $\begingroup$ @kikirex I thought these two are the same thing, as I can't think of a way that we gain energy while the planet preserve it's spin speed. $\endgroup$ – Chenxi GE Jun 13 at 21:47
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    $\begingroup$ So....no easy, cheap solar power for these fiends? They really, really need to mess with our calendars? May the owls deafen them with their incessant hooting. $\endgroup$ – user535733 Jun 14 at 0:21
  • $\begingroup$ @user535733 well these alien bastards love to mess around! $\endgroup$ – Chenxi GE Jun 14 at 17:19
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Actually, it is possible and people do it. Tidal turbines generate power off of the tide rising and falling. This is indirectly generating power off of the Earth's rotation, and here is a diagram explaining why.enter image description here

The earth has such a massive amount of kinetic energy that tapping into it a little is not noticed, but building thousands of tidal turbines would seriously start to convert the kinetic energy of the earth into electricity.

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    $\begingroup$ This is generating energy from both the Earth's rotation and the moon's orbit, it's similar to natural tidal friction which converts some rotational energy to heat, also causing the moon's orbital radius to increase, so the decreasing angular momentum due to the Earth's rotation slowing is compensated for by the increased ang. momentum of the moon. Due to conservation of ang. momentum, it shouldn't be possible to directly convert energy from rotation to some other form like electricity without increasing ang. momentum of some other system. $\endgroup$ – Hypnosifl Jun 13 at 22:59
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    $\begingroup$ The external torque is the movement of water back and forth on the face of the earth. Since the water is "sloshing" back and forth, it is possible to tether it with a turbine, slowing the earth and generating electricity. $\endgroup$ – Bilbo Baggins Jun 13 at 23:09
  • $\begingroup$ But this sloshing only happens because there is an external torque from the moon on the combined system consisting of both the water and the non-water portions of the Earth. The total angular momentum of the Earth-moon system can be treated as constant (I assume tidal friction from the Sun and other planets is negligible), so If the angular momentum of the non-water portion of the Earth is decreasing over millions of yrs, it's not being compensated for by an increase in the angular momentum of the oceans as a whole, it's just being compensated for by increased angular momentum of the moon. $\endgroup$ – Hypnosifl Jun 13 at 23:26
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    $\begingroup$ The gravity of the moon is creating torque on the oceans, and the turbines are applying the torque in the oceans against the earth's rotation and generating electricity. $\endgroup$ – Bilbo Baggins Jun 13 at 23:35
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    $\begingroup$ Yes. Without the gravity of the moon, it would be impossible. $\endgroup$ – Bilbo Baggins Jun 14 at 0:17
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Doable, yes. Feasible, no. This shouldn't surprise you, because if it was feasible, presumably Elon Musk would have done it.

This sounds really weird, but as it happens, there's a way to gain kinetic energy, which can then presumably be transferred to thermal/electric energy, which functions as a result of using the Earth's movement and slows it down. And the good news is that its possible with today's technology. It's referred to as the Gravity Assist. (Technically, it doesn't actually slow down the Earth's rotation, it slows down the Earth's orbit around the Sun. Fine. But it's a far better system because it takes more energy to rotate the Earth around the Sun than to spin the Earth, so more energy is gained.)

It has some downsides, namely the incredibly large energy expenditure needed to be able to get a ship into position to use the gravity assist, and that the alignment needs to be right, and that you'll accelerate the ship to a vector you don't necessarily want. But if you somehow managed to set up the system, you can use the Earth's movement for energy.

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This is both doable and feasible, even with near-future technology.

As described in David Brin's short story "Tank Farm Dynamo" (1983), you can let an orbiting satellite have a large wire loop that interacts with the Earth's magnetic field to produce electricity. This slows down the satellite, making its orbit decline, but it will also slow down the Earth's rotation a tiny, tiny fraction.

A much Larger version of this would be to wrap a wire spool all the way around the Moon, generating electricity as the Moon travels through the Earth's magnetic field, slowing both the Moon's orbit and the Earth's rotation in the process.

What I can't tell is if the energy generated would be of a magnitude to justify the expense, compared to mining Helium 3 and using it for fusion power.

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    $\begingroup$ Compared to mining Helium 3 from the Moon? Probably. Compared to mining Helium 3 from a source that would actually makes sense like Saturn, or (even better) breeding it? Harder to say. $\endgroup$ – Eth Jun 14 at 10:34
  • $\begingroup$ @Eth: I mentioned the Moon since that was where I put the spool. $\endgroup$ – Klaus Æ. Mogensen Jun 14 at 13:26
  • $\begingroup$ Or even building an insanely tall wind power station on moon. The wind power station is so tall that it reaches the earth (dense) atmosphere and generates electricity. More unrealistic than your design though. $\endgroup$ – Chenxi GE Jun 14 at 17:25

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