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Let's say that for the purpose of terraforming, we need to speed up the rotation of a planet (like Venus), a moon or an asteroid to near Earthian day. How would we go about doing this? Can this be accomplished from within the body itself?

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The key thing to doing this is to accelerate some of the mass of Venus outside of the planet, or alternatively bringing in some mass. Thus, there are 2 main things that could be done to alter the rotation speed. These ideas are explained in Wikipedia.

  1. Carefully plan a series of asteroid impacts in a direction where they can increase the rotational energy of the planet.
  2. Shoot out some part of Venus, most likely it's thick atmosphere, in to space beyond the escape velocity.

In addition, the wind speed of Venus is higher than the land speed, so putting up some large wind blockers would help to convert some of that energy into rotational energy of the planet. However, this would be a fairly minimal effect, as this is already being done in some scale, although not as quickly as is desired.

So, how else might this be accomplished? Xkcd has a nice comic showing one way, spinning around in circles can cause the motion of the planet to change.

enter image description here

Furthermore, in a what if scenario, they gave one way to change the rotation speed of the Earth, driving around in circles. That would work as well, although very slowly. But in theory a train could be built that circled the planet, solar powered, that would slowly increase the rotational speed of the planet, although it would be quite slow.

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    $\begingroup$ It doesn't matter how long the train runs. As soon as it gets up to speed, it's done all it can to change the rotation rate. As soon as the train stops, the Earth's rotational rate returns to what it was. The total angular momentum of the Earth and the train is conserved. $\endgroup$ – Mark Adler Nov 21 '16 at 5:28
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    $\begingroup$ Consservation of angular momentum: en.wikipedia.org/wiki/… I'm disappointed in Randall Munroe. $\endgroup$ – HopDavid Jan 27 '17 at 15:56
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Venus is almost completely tidal locked to the sun (meaning one side always faces the sun). The only way to speed its rotation is to move it away from the sun, (though that would probably mess with our gravity) And possibly by adding well placed a moon, tidal locked to Venus like our moon is to us might help out pull the sun.

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  • $\begingroup$ Can you explain more how these would help? $\endgroup$ – Nathan Tuggy Nov 21 '16 at 2:05
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Well, if we have unlimited engineering capability...Move Mercury into orbit around Venus. The tidal effects could potentially impart spin to both objects, generating magnetic fields in both and creating the possibility for a thickening atmosphere on Mercury and a thinning on Venus. But would the lack of a body at the current orbit of Mercury disrupt the orbital resonance of the Solar System?

Alternatively, the composition of the atmosphere interacts with the solar wind to create an externally induced magnetosphere, protecting the atmosphere. If you could temporarily disrupt that process, it could allow some of the atmosphere to escape. I don't know enough about this magnetosphere to suggest a way to disrupt it, however. Fun thoughts!

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  • $\begingroup$ Changing the rotation by moving a planet? You just solved the problem by introducing an even bigger problem. $\endgroup$ – Roman Reiner Jan 27 '17 at 5:43
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Well, first of all it is critical to understand that angular momentum is conserved. This is a very fundamental law of physics and, by Noether's theorem, is a consequence of (or is equivalent to) the rotational invariance of space: the laws of physics are unchanged by rotations.

What this means is that the only way you can do something like this is to acquire angular momentum from somewhere else: you can't do something magic within the planet itself which will cause its angular momentum to change.

So what are the things you could do?

Well, without violating the conservation of angular momentum, you are allowed to redistribute it within the body. So, for instance, perhaps the atmosphere has very high average wind speeds which circle the planet: well, you could dump all of that angular momentum into the planet by, perhaps, building giant walls or something. Unfortunately the mass of the atmosphere is generally so low that this doesn't help much: the Earth's atmosphere has a total mass of about $5\times 10^{18}\,\mathrm{kg}$, compared to the Earth's mass of $6\times 10^{24}\,\mathrm{kg}$: even though the atmosphere is right at the edge of the planet where a given wind speed corresponds to the most angular momentum, this isn't going to help much.

Summary: not totally insane (needs giant walls), but doesn't help.

So, OK, what about finding lots of large chunks of matter and throwing them at the planet in such a way that they dump angular momentum into it. Well, you could do this, I suppose. The problem is that they also dump a lot of energy into it. And you'll need a lot of large chunks of matter: whole moons' worth, really.

Summary: fairly insane (you need to be able to move moon-sized chunks of matter around and throw them at planets. Could work but result is that you have a lava planet, so you now need to wait millions of years for it to cool. Probably not a good approach.

Well, what about the standard moon trick: find a suitable moon and put it in orbit around the planet. The angular momentum of the planet-moon system is now conserved but tidal effects will exchange orbital angular momentum and angular momentum of the two bodies. The planet will end up tidally-locked to the moon, and will thus spin with the same period that the moon orbits. The problem here is that this probably is very fast, and the process takes millions or billions of years.

Summary: fairly insane (you have to be able to move moons around), doesn't get you very high spin rates, takes many millions of years. probably not a very good approach.

OK, well, we could do the inverse of the throwing-things-at-the-planet trick: we could land on the planet and build giant guns, and throw huge amounts of the planet into space, in such a way that we increase the angular momentum of the remaining chunk of planet. You'd need to get the energy to do this from somewhere.

Summary: less insane than the previous two approaches. Could work in reasonable time. Removes the surface layer of the planet, which may be a problem.

And finally the proper mad science solution. This whole increasing-the-angular-momentum thing is kind of boring. If the planet has any angular momentum at all we don't actually need to change it: we can just change the size of the planet. So all you need to do is crush the planet down to a sufficiently small size and it will spin as fast as you like.

Summary: completely insane (requires planet-crushing technology). Works in reasonable time. Surface gravity on crushed planet may be uncomfortably high. Crushing likely causes lava-planet problems again. Danger of ending up with neutron star or black hole.

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Put simple nuclear rockets on Venus, attached to Venus on Rocky mountains. Keep them running for many years. This might speed rotation of planet. Or use 🚀 rockets attached to Venus to turn entire planet into giant spaceship, to put planet in habitable zone orbit.🙂. Terraform Venus and live there. Do similar to other planets.

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    $\begingroup$ (-1) Hi, thank you for your answer. Unfortunately, right now it's not clear how any of what you're proposing could work, so the whole answer consists mostly of complete speculation. Consider adding some references or back-of-the-envelope calculations to support your claims. $\endgroup$ – TooTea Mar 18 at 10:38

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