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How would I gain energy from a rotating planet?

A rotating object, especially something as large as a planet or a star, has kinetic energy. However, my question is how would you go about harvesting the energy the object has from rotation to go towards other purposes such as powering several thousand space stations? I'm making a hard science fiction short story, so if we could keep it as realistic as possible that would help. Thank you in advance.

3 Answers
3

The answer is the tides. Your space stations need to be numerous and large enough to cause tides on the planet below. Tidal forces slow the rotation of the planet, so if you set up some tidal stream generators, you are essentially taking the rotational energy of the planet and turning it into electricity.

This is not a great idea though, because not only do tidal forces slow a planet's rotation, they also push the moon further away. In real life, with our moon, this is only a few centimeters per year. We'll have our moon for billions of years. The rate the moon moves away is proportional to how strong the tides are. The stronger the tides, the faster the moon moves away. If you set something like this up to generate as much energy as possible, you would want the strongest tides possible. This would also mean your space stations would get pushed away quickly. To push the stations back to their original positions requires some other energy source; solar sails, chemical rockets, something. Which raises the question, "Why not cut out the middle-man and just use that other source for energy?"

If you're orbiting space stations around an Earth-like planet, tidal power plants beaming energy to the space stations would do the trick. Converting tidal power to electricity/radiant energy slows the planet's rotation.

If not, you're going to need a lot of space elevators. Transferring mass past the zero-g point to the counterweight (where it can then be given a gentle push and let go) will generate energy. A star would be trickier, but solar energy would work fine in that case.

You could have a payload that undergoes a slingshot maneuver around a planet which allows it to convert some of the planets kinetic energy into its own via gravity. You then use that extra speed that you gained to generate power. If you could somehow get two planets, You could somehow slingshot the object around both planets to keep increasing its speed and have power stations between the planets that drain the gained speed so that you can keep this going to keep harvesting energy.

Assuming you have enough resources, you could expand this to literally have a line of iron/steel poles that fly between the planets passing through massive space station power planets that consist mainly of coils which will programmed to only drain a fixed about of kinetic energy so that the iron/steel poles can slingshot forever.

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