How can humans (like that that are on Earth now) stop planet from rotating (tidaly lock it, to make an eyeball planet)? Is that technologicaly possible? Can moons of that planet help them? They want to stop the rotation as fast as possible. How fast they can do it?
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$\begingroup$ Which humans? Those from our time? $\endgroup$– L.Dutch ♦Commented Apr 29, 2020 at 19:44
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1$\begingroup$ Please explain what you mean by "eyeball planet". Also, what sort of tech level are we considering? $\endgroup$– TitaniumTurtleCommented Apr 29, 2020 at 19:44
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1$\begingroup$ The question would seem to demand the application of huge ammounts of energy - the issue would be to do this on a reasonable tiemscale without tearing the planet apart. Possible for mankind now - no, we can't access that kind of energy nor control it enough. What technologies would be required to accomplish it? That's a question which would generate more usefull answers for you I suspect. $\endgroup$– Escaped dental patient.Commented Apr 29, 2020 at 19:56
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1$\begingroup$ This is a lot more difficult if you want to make it survivable to humans. Out of curiosity, do you? $\endgroup$– HalfthawedCommented Apr 29, 2020 at 20:23
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$\begingroup$ Hi, galaxy001, I was writing an answer but that one of Engineer Toast in @Renan link fit better. Just as complement, total mass of an asteroid impact to brake Earth rotation is compared with sum of the masses of Pallas and Vesta, the 2nd and 3rd biggest bodies of asteroid belt. $\endgroup$– Rodolfo PenteadoCommented Apr 29, 2020 at 20:33
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2 Answers
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That depends on how destructive they are willing to be. The fastest way to tidally lock a planet would be to hit it with a few large objects. Planets become tidally locked with the sun when the rate of their axial rotation is the same as their orbital rate. Every time the planet completes one orbit, it also completes one rotation. As earth completes a rotation 365 times per orbit, its rotation would have to be slowed down to 1/365th of its current speed.
If you want to be less destructive, you can change the tidal forces on the planets by adding orbiting objects or speeding up/slowing down existing objects. Unfortunately, the slowdown would take place on a geographic timeline, so your humans are probably best dropping a few well-placed rocks on their planet.
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You do not want the planet to "stop" rotating, you want its rotation to exactly match the revolution time around its sun.
The difference is negligible in the case of the Earth - you need to go from about 365 rotations every year to exactly 1, rather than 0, rotation.
The involved energy is 1/2 multiplied by the planet's rotational inertia multiplied by the square of its angular velocity.
And a planet's rotational inertia (its moment) is 0.4 * M * R * R, where R is the radius in meters and M is its mass in kilograms.
You need to produce all that energy, and you need to deliver it to the planet without wrecking it.
Once solved the energy problem, the show-stopper if the planet has a liquid center is that the energy can only be delivered to its surface, while the nucleus will keep spinning. This will probably have disastrous results (earthquakes, etc.) even if the interface viscosity is low.
Then, the rotation supplies a small centrifugal force, and removing it will change the shape of the planet. Very little, but again enough to trigger considerable quakes.
