# A planetary dynamo

Is it possible for a planet and moon to generate electricity in some way? Could a combination of magnetic fields and iron content in the crust or core produce electromagnetic induction due to the rotation of either planet or moon, and also the orbit of the moon? Here are a few ideas:

The moon orbits over both poles of the planet, causing an alternating field to pass through it.

The magnetic field of the planet is perpendicular to its axis of rotation, and the moon orbits around its rotational equator in the opposite direction of the spin. This would cause an alternating field through the moon.

The moon orbits like our moon, where it is always facing towards the planet. One of the moon's magnetic poles is pointing towards the planet. As it orbits, the planet experiences an alternating field.

Are any of the above plausible? If so, would they actually generate a voltage in the core of either planet or moon? Are there any other possibilities?

• When you say "generate electricity" what do you mean? Actually creating electrons...no...making electrons move yes. But what effects do you want? Aug 24, 2016 at 12:31
• An overall charge or voltage, enough that you could theoretically use the planet to generate a small current.
– Aric
Aug 24, 2016 at 12:57
• So for example if I laid a cable along the ground I should see current moving along it? Aug 24, 2016 at 12:59
• basically like that. So the iron in the planet's core creates a voltage which can be used to make a current.
– Aric
Aug 24, 2016 at 13:32

There are other possibilities:

The magnificent Jupiter-Io Flux Tube

Now you gotta consider that you probably cannot live on the planet itself in this case1, but beside that there's a huge potential energy.

Briefly: Due to the nature of Jupiter and IO a so called flux tube exists between them through which huge amounts of electricity are discharged into jupiter's atmosphere which there eventually end up as lightning.
Harnessing power from this phenomena has been used in scifi for some time, alas at this moment I couldn't find a single passage about it..

1Alas considering that you'd need the technology to harness this power, you'd likely also have the technology to colonize such planets/moons

Tap the gravitational effect of the movement of the moon relative to the planet.

People have been using this method for centuries, from tide mills to modern hydro-power.

While this is less flashy than what you had in mind, it's very simple and reliable.

• I am considering literal electromagnetic induction, so using magnetic fields instead of gravity. I like the idea though,
– Aric
Aug 24, 2016 at 13:29
• @AricFowler, once a month the moon passes through the magneto tail, in a polar orbit that could be twice a year at best. I don't think a direct orbitally induced induction is possible without massively ramping up the magnetic field strength somehow. Aug 24, 2016 at 13:37

There is something called an "electrodynamic tether" in the aerospace industry, which almost matches your description. If a moon were low enough to interact with ionospheric plasma and had electrically conductive regions exposed (a metallic core, brines, etc.), it could drive enormous currents at the cost of lowering the moon's orbit. The circuit passes through the body and is completed by the plasma.

The only situation in which I could imagine this happening is with a metallic moon (perhaps a captured metallic asteroid) the orbit of which has only just gotten low enough for the effect to be significant. If the current runs continuously, the orbit will decay very rapidly. The moon can not be large. It must be small enough to be held together by electrostatic forces; if it were held together by gravity so far below the Roche limit it would be torn apart.

It would also be possible to drive moon outwards by actively supplying correspondingly huge currents.

You can read more about electrodynamic tethers on Wikipedia (https://en.wikipedia.org/wiki/Electrodynamic_tether) or in "Space Mission Engineering: The New SMAD", pp. 782-783. (Edited by James Wertz, David Everett & Jeffery Puschell.) It is not mature technology.