Long term viability of geosynchronous orbits

From: George Roberts (gr@gr5.org)
Date: Fri Dec 07 2012 - 19:52:31 UTC

  • Next message: Jonathan W: "Re: Long term viability of geosynchronous orbits"

    I thought that by now Ted would say something but I guess he hasn't been 
    reading this thread.
    
    GEOSYNCHRONOUS VERSUS GEOSTATIONARY
    
    First of all, the orbits are called geosynchronous, not geostationary. 
    Geostationary refers to a particular geosynchronous orbit that doesn't 
    deviate north or south from the equator.
    
    Geostationary is unstable even over one year due to the moon and requires 
    fuel to maintain.  Pretty quickly dead satellites start to drift north and 
    south by up to some amount (23 degrees maybe?) then drift back into 
    geostationary again, then back to non stationary.
    
    The orbit form earth looks like the satellite moves north and south along a 
    line perpendicular to the equator.
    
    This doesn't answer the question about viability as geosynchronous can be 
    stable for I'm sure thousands of years.
    
    DRAG
    Another issue discussed was drag at that altitude.  Yes, there's drag but 
    it's probably too small to worry about.  Someone correct me.
    
    TIDES
    I've read that ignoring drag, anything orbiting completely inside 
    geostationary distance will have it's orbit decay due to tides.  Anything 
    outside that distance will increase it's orbit due to tides.  The moon is a 
    good example.  It is outside that distance (by a factor of about 10) and has 
    been moving farther and farther away from the earth despite drag.  The 
    energy to move the moon to a higher orbit came from the earth's rotation - 
    the earth is rotating slower and slower as the moon's orbit is lifted higher 
    and higher.  But the closer you are to geosync, the smaller the effect. 
    Which leads us to graveyard orbits.
    
    GRAVEYARD ORBITS
    http://en.wikipedia.org/wiki/Graveyard_orbit
    When a geosynch sat gets down to 3 months left of fuel they usually send it 
    into a Graveyard orbit which is *higher* than geosynch.  The goal is for all 
    dead geosynch satellites to go there but only 1/3 or so make it.  The reason 
    for moving it higher versus lower is so that it is out of the way of new 
    geostationary sats on their way to their new orbit.
    
    HOW LONG?
    But none of this answers the question, how long would a geosynch sat last? 
    I don't know the answer.  I suspect it's much less than 100 million years. 
    If it was that long then I would expect us to have lots of other small moons 
    up there.  I suspect it's more like thousands of years but I really don't 
    know.   Maybe 100,000 years.
    
    A two body orbit is amazingly stable.  Add a third body (like the moon) and 
    things are very unstable.  There aren't very many (any?) stable orbits left 
    inside the orbit of our moon.  Otherwise we would have more moons. 
    Including only Earth, Moon, Sun, Jupiter and trying to find a stable orbit 
    inside the moon's orbit that lasts more than a million years is probably 
    impossible.
    
    So I don't think this photo-disc-message will last long enough for aliens to 
    find it.  It would have been better to put it on the moon.
    
    - George Roberts
    http://gr5.org
    
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