# 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

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.

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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