I was speculating that geostationary satellites could be around possibly for billions of years (i.e. much beyond the point where plate tectonics would have erased all traces of human civilization on Earth) and might make interesting artifacts for future aliens to find. But I'm realizing I'm on very shaky ground for all sorts or reasons. I hadn't appreciated that these satellites are in an unstable equilibrium radially. With a slight decrease in radius, atmospheric drag and tidal effects cause radius to reduce further, and vice-versa for an increase in radius - albeit very very slowly. I understand there are various gravitational disturbances (Sun and Moon) that cause the orbits to move a few degrees off off the equatorial plane and also that the Earth is slightly flattened which causes orbits to slowly precess. And finally the Earth is gravitionally lumpy and satellites tend to drift to one of two stable longitudes. I guess my first question is: are these local points of longitudinal stability sufficient to lock the orbital period and overcome the unstable radial equilibrium?

  • $\begingroup$ Earth's lumpyness maybe could have longitudinal stable points. But the Moon must have a greater and more uneven influence, passing by 10-11 times further away from Earth than GEO. And the Moon has a latitudinal pull too because of its inclined orbit. Someone might show a real simulation, but no, nothing can remain stable in GEO for even thousands of years. $\endgroup$
    – LocalFluff
    Jan 3 '17 at 11:21
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    $\begingroup$ Billions of years is a very long time (even millions of years is a long time). Do you require the orbit to remain geostationary for that length of time, or are you happy with the spacecraft simply remaining in orbit? Compare Earth's rotation - Change in rotational velocity on Wikipedia. $\endgroup$
    – user
    Jan 3 '17 at 12:30
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    $\begingroup$ dropbox.com/s/u418iblbfk7w0ew/…. It states: "From 1434 objects for which orbital data are available ... 471 are actively controlled, 747 are drifting above, below or through GEO, 190 are in a libration orbit and 15 are in a highly inclined orbit. ... there are 50 uncontrolled objects without orbital data ... the total number of known objects in the geostationary region is 1484". As you say, Syncom3 is one of the drifting ones - drifting west at 0.23 degrees per day (presumably this changes a lot as it drifts past the stable 'libration' longitudes) $\endgroup$
    – Roger Wood
    Jan 7 '17 at 2:09
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    $\begingroup$ Thanks @uhoh. It is certainly an interesting topic. I don't feel very qualified to answer the question, but maybe later I can summarize the understanding from the various comments. $\endgroup$
    – Roger Wood
    Jan 10 '17 at 3:06
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    $\begingroup$ If the semi-major axis of the orbit has changed that means either the orbital energy has changed (solar pressure?) or the gravitational field vs. radius has changed (lumpy earth?). $\endgroup$
    – Roger Wood
    Jan 10 '17 at 3:06

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