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It is said that the Moon plays a significant role on stabilizing the obliquity of Earth (or call it the axial tilt). Without it, the tilt of the planet is subject to great variations causing unpredictable climatic conditions on the planet. This is not ideal for life.

Scenario: An advanced civilization discovers a perfect planet that is just like Earth expect that the planet doesn't have a natural satellite. They really want to settle the planet and they are planning for a long term investment.

How can they minimize the changes in the planet's obliquity over long periods of time?

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Probably the most straightforward solution is to grab some nearby asteroids and build yourself a new moon.

It would take a lot though, It's estimated that our entire asteroid belt combined together would mass about the same (or in fact a little under) the mass of our moon. However if you want a long term safe environment harvesting all the rogue, orbit crossing asteroids and compressing them into a moon would kill two birds with one stone (sorry for the pun). It keeps you safe from asteroid impacts and fixes your planetary tilt.

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  • $\begingroup$ Could we steal a moon from another planet? $\endgroup$ – NauticalMile Jun 12 '15 at 17:26
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    $\begingroup$ A little under? The high estimate of mass for the entire asteroid belt is 4% the mass of our moon. Where is your estimate from? $\endgroup$ – Samuel Jun 12 '15 at 22:08
  • $\begingroup$ Oops, my information obviously far off, that's what you get for not paying attention :) (I really thought it was bigger than that) So, yeah, you're gonna need to get more mass as well, perhaps from the oort cloud. $\endgroup$ – DX101 Jun 13 '15 at 13:26
  • $\begingroup$ If we put it close enough, it could have a larger impact on Earth? $\endgroup$ – Vincent Jun 14 '15 at 21:52
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If the aliens (or us, for that matter) have superscience, then one answer would be to grab a moon from a Gas giant planet in the system and place it in orbit around your planet. Playing a game of planetary billiards with comets from the system's Kuiper belt and using flypasts to add or subtract orbital velocity from the moon you want gets the job done, although it takes a long time and an insane amount of computational power. The constant stream of comets through the inner solar system wold make for a spectacular night sky, but interplanetary transportation would be rather hazardous.

Plan "B" would be to forget the moon and use the incoming comets directly to apply a gravitational torque to the planet. This might be sub optimal if there is a lot of space infrastructure, since the traffic control system would have to ensure clear space for the incoming comet to pass the planet, and the water vapour and dust from the comet's tail would have long term effects on the orbital infrastructure and possibly the planetary climate (reflecting sunlight away and dropping the average temperature).

With a bit more superscience, Plan "C" might be to create a very small artificial moon using ultra dense materials. A slug of Neutronium the mass of the Moon would be incredibly tiny (and probably unstable). A miniature black hole might be created to do the job (one of lunar mass would probably be stable enough to last for geological ages), and careful observers would see an "Einstein ring" in the sky every time the miniature black hole passed in front of a star. Finally, if the superscience is sufficiently advanced, they might simply warp space near the planet to create the same effect.

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