# Plausible way to make a planet spin faster

Imagine that you want to colonize Venus. But a day on Venus lasts 4 months, and that's terrible. The big brains at the Science Palace have decided to make Venus spin faster, so that the length of day would be correct (one of the big brains suggested putting opaque shields into orbit that would artificially create night-time, but he got demoted).

What is the lowest tech level at which this idea is plausible, and what would it look like? Would there be any fun side effects of the method/its results?

• Why would you want to colonize the hellscape that is Venus, of all places? – Frostfyre Jan 27 '17 at 16:45
• One of the closest bits of real-estate in the solar system an it can hold an atmosphere – Lio Elbammalf Jan 27 '17 at 16:47
• kinda related and as far as you are asking about technology, relevant - How can I move a planet? – MolbOrg Jan 27 '17 at 22:07
• See also this Q about spinning up a planet, though a faster result was intended. The same mechanisms would (more easily) provide the modest spin you ask for here. – JDługosz Jan 28 '17 at 8:25
• Related, but on a much smaller scale than your question, the XKCD solution. Venus' atmosphere is much more dense, so drag would disintegrate the asteroids more quickly than Earth's atmostphere – Punintended May 18 '18 at 18:00

The amount of angular momentum in a planet is incredible; planets are like giant gyroscopes.

The lowest tech level required would be "Dyson Sphere," because you'd need MASSIVE amounts of energy to attempt to add angular momentum to the planet.

Also note, due to conservation of angular momentum, you're going to need to "steal" momentum from an absolutely massive ... mass. We're talking, somehow transferring the angular momentum from Mars.

As for "fun side effects," let's say you have a Dyson Sphere, or a Dyson Ring to collect power. You use this to build an absolutely massive space ship; basically, it's engines surrounding a huge conglomeration of asteroids. Maybe you even turn Ceres into your vehicle. You fly Ceres to Mars, and use tractor beams on the north pole. This causes angular momentum to transfer to the ship, which you then whirl on to Venus, and use tractor beams on the south pole to re-transfer the momentum from the ship to Venus. Do this a couple trillion times and Mars will be tidally locked to the Sun, while Venus is spinning a bit faster.

Of course, once you've spent that much fuel, you might just put ion thrusters on ships, tractor beam them to the poles, and then fire rockets to rotate the ships (the angular momentum comes from the particles ejected into space).

This is a MASSIVE undertaking; we're talking about fantastic amounts of energy required. Equal, perhaps, to the energy required to initiate warp drive on a ship, to boldly go somewhere with habitable planets.

Or, eat a bit of crow and rehire the guy with the sunshield idea; it'll be way cheaper than a Dyson Ring, and makes a great place to put orbital solar power stations.

The rotational energy of an object is

$$E_{\mathrm{rot}} = \frac12 I \omega^2$$

where I is the moment of inertia and ω is the angular velocity.

The moment of inertia of Venus is about 5.88×1037 kg·m², and its angular velocity is about 3×10−7 rad/s (sidereal rotation period of 5832.6 hours); this results in a rotational energy of 2.63×1024 J. If the rotation of Venus were to be accelerated so that it had the same sidereal rotation period as Earth, that is, 23.9345 hours, its rotational energy would increase to 1.56×1029 J.

The total amount of energy produced by the human civilization in one year is about 14000 Mtoe (millions of tons of oil equivalent), or 5.86×1020 J. Accelerating the rotation of Venus so that it would have the same sidereal rotation period as Earth would require 267 million times more energy than what the entire human civilization produces in a year.

I think that it is safe to say that a project which requires 300 million times more energy than what the entire world produces in one year won't be undertaken any time soon.

• OP's question is not about time it happens but about how it possible can be done, you'll better to compare ti to the energy possibly available in the star system than with earth production, as long as currently, we can't move even a smallest rock in the system. – MolbOrg Jan 27 '17 at 22:05

Human civilization already has a technology to do this (with a few caveats).

The idea is to direct a large enough planetoid to collide with the planet to give it much needed angular momentum. This is probably how Earth acquired its faster spin in the past. The caveats are:
1) there should be a suitable planetoid in the system
2) with today's technology (even scaled up) it will take a lot of time to change planetoid's orbit
3) after the collision it will be quite a while (perhaps millions of years) before the planet surface will become suitable for colonization.

• This was the same idea that King of Snakes posted here and then deleted it after comment feedback convinced him that it was not going to work. Since you can’t read deleted posts, I'll copy my own comment here: – JDługosz Jan 28 '17 at 8:29
• An asteroid will not so-affect a planet. Scale error. Something more like the impact that formed the moon? Well, that blows the whole thing apart and the amount of angular momentum delivered is simply the solar orbital speed. Imagine something as big as the planet coming from the opposite direction and somehow spinning the planet at the speed at which the surfaces pass: you’re on to something, but why would it spin up and not splat and fall into the sun? – JDługosz Jan 28 '17 at 8:29
• And user @renan noted: « This has been discussed before. A meteor impact that would cause an infinitesimal change on the angular speed of an Earth sized planet would destroy its surface and atmosphere. Please read what-if.xkcd.com/26 » – JDługosz Jan 28 '17 at 8:32
• @JDługosz - valid points. However, by "planetoid" I didn't mean a small asteroid, like Chicxulub one, the one I'm talking about should be much bigger. And yes, in the process the planet will lose much of its atmosphere and will have to build a new one. This one of the reasons for my caveat #3. – Alexander Jan 30 '17 at 17:24
• That’s still too small. Think vaporizing the entire planet, not just losing the atmosphere. See this video talk to get some idea. – JDługosz Jan 30 '17 at 19:50

Okay, I don't know if it can be considered "plausible", owing to the notion that even our best unmanned probes don't do well on Venus for any length of time, however using the smallest amount of handwavium possible, consider this:

I propose that if we have progressed technologically to the point that Venusian solar shades in planetary orbit are possible, we likely also have materials suitable for ground-to-orbit deployment of a 'cable' maybe made of a yet undiscovered monomolecular fiber, capable of enormous tensile strength. Such a material could be used to anchor several (very large) cables, reaching from the surface to a low orbit, joined at the orbital end and attached to a system that slowly lifts the 9.2MPa surface atmosphere ( perhaps like a siphon) to a directed port that uses the atmosphere escaping to provide thrust in a low-thrust 'spinward' direction. The result would be a thinning of the atmosphere, and a slow but steady thrust that *with time * will affect the planet's rotation. I would advise however that since Venus does not have its own magnetosphere and relies on a solar 'envelope' of magnetism to deflect particles, thinning the atmosphere may produce undesireable results

• mass of the atmosphere is not significant enough to make your plan to succeed. – MolbOrg Jan 27 '17 at 22:01
• did you read the first sentence I wrote? – Joe Jan 28 '17 at 14:54
• Yes, I did and I did it again. U suggest something like en.wikipedia.org/wiki/Aeolipile , so not enough mass of the atmosphere (there probably is a case when it might be sufficient. the case is if atmosphere exhaust is 0.9999999... c (speed of light)) – MolbOrg Jan 28 '17 at 23:13
• If you don't want to use the atmosphere, or if you cannot accelerate your atmospheric propellant enough to do it before depleting it, you can also use crust material as propellant with a mass driver instead. – Eth May 22 '18 at 17:39

It would take a loooooooooooooooong time, but you could put up a bunch of surfaces that were dark on one side, and reflective on the other side. Have the dark side face sunrise, and have the shiny side face the sunset(that's backward). Light hitting the shiny(dark) side would slowly increase the rotation of the planet.

(information was based on conjecture and faulty memory, thank you for the correction JDługosz)

http://io9.gizmodo.com/5875747/the-light-mill-a-toy-that-shows-how-we-might-one-day-power-space-ships

• I don’t think the principle would work on that scale, in a thick atmosphere, or where the atmosphere is following the surface rotation. Think conservation of angular momentum. – JDługosz Jan 29 '17 at 9:17
• the atmosphere moves relative to the planet, so it would have no effect on the rotation of the planet or this setup. All this setup does is transfer the energy of the light hitting the reflective side of the surfaces to the rotational speed of the planet. In fact, the surfaces could, theoretically, be made in a way to allow the atmosphere to pass through them almost unrestricted. – Brian Lami Feb 2 '17 at 11:49
• That would spin in the opposite direction to the light mill, and is not the same principle it illustrates. – JDługosz Feb 2 '17 at 19:21
• I stand corrected. My directions were backward, but my other points, I believe, remain valid. (Thank you for the correction. – Brian Lami Feb 3 '17 at 23:27
• Note that the light mill is a black and white spinning thing but is not an example of this effect. Also, light pressure has actually been used to slow the spin of spacecraft! – JDługosz Feb 4 '17 at 5:18

The atmosphere rotates faster than the planet, and is massive. But Venus has no intrinsic magnetic field. Instead, one is extrinsically induced by the interaction of the solar wind with the elements in the atmosphere. This is what keeps the atmosphere from escaping to space.

Break that. Use airship drones in the upper atmosphere to alter the atmospheric composition enough to disrupt the extrinsic magnetosphere, allowing the atmosphere to escape to space until the rotation of the planet catches up to the atmosphere.

• The atmosphere rotates faster? Really? How about a link? So why would removing some air cause the planet to spin faster? – JDługosz Feb 2 '17 at 19:23

"In theory" you could give the planet a large moon and use the moon's orbit to impart more angular momentum to the planet, but this would take centuries, and tech that allows you to move a sufficiently large moon into a stable and mathematically correct orbit of your target planet.

This would also likely cause massive tectonic issues for the planet, assuming the planet has a still liquid mantle and spinning core or whatever. If it didn't, I think there is a good chance you'd just alter its orbit, not its rotation, and/or the planet would break up under the strain.