The motivation as to why one would want to give a tidal locked moon a spin is out of the scope of this question. I only want to focus on the feasibility. Suffice to say there could be a few hypothetical reasons such as a night/day cycle for prospective inhabitants.


If there is a moon that is tidally locked, could humans with present or near-future technology create spin on that moon such that the mass of the planet it orbits and the distance the moon is to the planet remain unchanged. That is to say, the only variable we can change is the equipment/technology on the moon to generate spin.

Further Clarification

  • Present/near future technology (+50 years max)
  • Cannot change mass of planet (in the long run)
  • Cannot change distance of moon from planet
  • Technology includes but is not limited to: propulsion, magnets, megastructure gears, ect
  • Unlimited Budget
  • 100 year time limit
  • near circular orbital eccentricity
  • mass of moon is flexible (you can stipulate if your answer works better on small moons or large moons)
  • Resulting spin cannot make moon uninhabitable, overly/deadly seismic or otherwise
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    $\begingroup$ You say that this cannot change the mass of the planet. To which precision? Technically, even something "trivial" such as building a rocket and sending it to the moon will change the mass of the planet (and the moon). I feel that with your conditions taken at face value, the only answer that we could arrive at is that "that's not possible" because you don't really allow us to do anything, but that isn't particularly satisfying. To what extent are these "clarifications" immutable? $\endgroup$ – a CVn Sep 3 '17 at 16:39
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    $\begingroup$ Intuitively it seems that adding additional gravitational pulls to the system could change the spin of the moon. But, 100 years seems far too short for this for of change with current technology. I suspect an early idea would be to blast a large chunk off and create a spin in the process. But you say that's not what you are looking for if I understand correctly. $\endgroup$ – DPT Sep 3 '17 at 16:41
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    $\begingroup$ @MichaelKjörling Assuming most of your materials come from the planet domestically, then the mass of the planet wouldn't change on balance, but I still see what you mean. I added a little clarification to the clarifications. Of course if we are burning lots of fuel then maybe mass would be lost by becoming energy. If it's minor changes in the planet's mass, that's acceptable, you can assemble or build things on the planet. That clarification is mainly there to prevent simply hollowing out the planet so that it's gravity become very weak. $\endgroup$ – Arash Howaida Sep 3 '17 at 17:01

There are two contradictory conditions. With current and near future technology, spinning up a moon within 100 years simply isn't possible. We cannot command enough energy to bring in enough asteroids or comets to either strike the body and impart momentum, or arrange flypasts in an attempt to use gravitational torque to spin the body.

To so so in under 100 years requires massive amounts of energy, which can be provided by far future technology. Researcher Paul Birch wrote several papers on "How to spin a planet" (including changing the orbits) using a stream of ultra high speed pellets to impart momentum. these were powered by "light sail windmills" near the Sun, and using a small percentage of the Sun's total luminosity allowed the system to spin up Venus and move it to a different orbit in a period of decades.

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Scematic of how to spin a planet

The major difficulty in using this system to spin a tidally locked moon like the Galilean moons of Jupiter is the stream of pellets will be carrying an ungodly amount of momentum and kinetic energy, and cannot ever intersect the path of any planets, moons, asteroids, spacecraft etc. between the windmills around the Sun and the target. As you get farther away from the sun, this obviously becomes much more difficult, and will require a massive amount of computational power to work out how to launch the stream and take into account intervening bodies, gravitational deflection, transit time and the effects of an irregular series of "pushes" to spin the moon.

So if you are willing to wait a century or two for technology and the space infrastructure to mature enough to build these devices, then it will be quite possible to spin or move moons and planets around to your heart's content.

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The only thing I can think of, for large moons, is asteroid bombing on a very low, almost tangential orbit.

That would change mass of the satellite, possibly in a noticeable way.

That would surely produce seismic activity, but that could be concentrated in areas far from settlements (if any) and reduced using many smaller impacts.

Also orbit would be changed somewhat by impacts (they would change both angular and linear momentum), but effect can be corrected using several impacts on different orbital positions of satellite (i.e.: have impacts where angular momentum changes adds up, while linear momentum changes even out).

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