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I was thinking about space-elevators and large masses being put into geosynchronous orbit, when I considered the possibility of a natural satellite being in a synchronous orbit. I did a little digging (5 minutes on Google) and I found this.

As a follow up question, what would have to happen for this to occur without the meddling influence of man?

The only way I can really think that this could happen, is if these things all happened..

  1. Natural satellite has to be in an equatorial orbit.
  2. It has to be in a lower orbit than the "synchronous orbit distance"
  3. It must be massive enough that tidal forces can slow it until the orbit has a radius equal to the synchronous orbit distance.

And even given all the above, I don't see a way that orbit could last long. (Even at the moon's 3.8 cm/year, it would still only be in a synchronous orbit for several months at most.)

Is my thinking correct?

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  • $\begingroup$ The tidal transfer will stop when dual lock is achieved. It relies on a difference in the orbital and rotational periods. $\endgroup$ Commented Oct 3, 2015 at 1:42
  • $\begingroup$ @dmckee So hypothetically, a natural satellite could have a semi-permanent synchronous orbit? $\endgroup$
    – CoilKid
    Commented Oct 3, 2015 at 1:48
  • $\begingroup$ You could rig initial conditions to have that outcome. No idea of the initial conditions are naturally realizable. $\endgroup$ Commented Oct 3, 2015 at 1:51

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1 is necessary for a geostationary orbit (which would be useful for an elevator location), but is not necessary for geosynchronous orbit (which your question asks).

2 is not advantageous. A (massive enough) satellite lower than geosynch altitude will induce tides that lower the altitude further, probably ending in a collision. Our moon is currently above a geosynch altitude, but it is slowing the earth's rotation. The end result will be a moon in a synchronous orbit, just that altitude will be farther out.

3 is the big one. The closer the objects are in mass, the faster that the tidal interactions can stabilize the system.

A synchronous orbit would be stable. The moon's orbital increase is not a constant value. It is actually driven by the difference between it's angular speed and that of the earth's rotation. If the difference is zero, then there is nothing to drive it in or out.

As an example, Pluto and Charon are in a mutually synchronous orbit (see 3 above).

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  • $\begingroup$ +1 Great, simple answer that gives considerable insight into the complicated question of stablility without the gory details. Great example at the end, too. $\endgroup$ Commented Oct 3, 2015 at 7:49
  • $\begingroup$ I was trying to think of the correct term so I could apply it to other planets, which is why I used "synchronous". Looks like I picked the wrong one though :) Thanks for the answer, very clear and simple. $\endgroup$
    – CoilKid
    Commented Oct 3, 2015 at 16:53

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