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. Jonathan On Fri, Dec 7, 2012 at 1:52 PM, George Roberts <gr@gr5.org> wrote: > 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 > > _______________________________________________ > Seesat-l mailing list > http://mailman.satobs.org/mailman/listinfo/seesat-l > -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mailman.satobs.org/mailman/private/seesat-l/attachments/20121207/5791f265/attachment.html _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l
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