Re: Long term viability of geosynchronous orbits

From: Jonathan W (
Date: Fri Dec 07 2012 - 23:00:12 UTC

  • Next message: Andrew Coyle: "Re: Long term viability of geosynchronous orbits"

    George (and everyone else),
    Thank you for your very through responses.  I knew that the atmospheric
    drag at 22,300 miles is very low, but I didn't realize that it is
    practically zero.  I also appreciated the comments on why we don't have any
    tiny moons at this moment in time.
    On Fri, Dec 7, 2012 at 1:52 PM, George Roberts <> wrote:
    > I thought that by now Ted would say something but I guess he hasn't been
    > reading this thread.
    > 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.
    > 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
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