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To: Ilaine who wrote (7813)	8/27/2001 6:10:42 PM
From: Maurice Winn	Respond to of 74559

<>>Mq, let me see if I have this straight. 1. It is your position that gravity has nothing to do with why LEOs are far smaller and lighter than GEOs.> Pretty much true [from an orbital mechanics point of view]. Certainly large-mass satellites don't get pulled off track at LEO heights any more than small specks of dust do. The launching process is something else and the balance of payload and launch costs and final altitude isn't something I know about [though I have that male answering syndrome]. I do know it costs a lot more to heave a big payload up than a small one and the higher they go, the more it costs. It's one of those square of the variable situations. <2. It is your position that LEOs only use their thrusters to compensate for the gravitational pull of the sun and the moon, and never use their thrusters to compensate for the gravitational pull of the earth nor to maintain their velocity.> The moon and sun are the dominant effects, but all gravitational, solar wind and other variables [including the earth's wonky gravitational shape] which push and pull satellites around have to be compensated for inasmuch as they aren't tidal in nature, meaning the satellite wobbles around as it approaches and goes away from the moon but it isn't simply kept on a straight track. I don't know exactly how they do it but I suppose they have a statistical position for the satellite and when it reaches various limits within an envelope it is nudged back on track after a few orbits when the misalignment is confirmed. They certainly don't keep it on a purely circular orbit. It's more like the track in an old vinyl record - for very low orbits it's very like that! Round and round, little variations in the groove, but staying on a gradually descending orbit until suddenly it zooms out of the music groove and plunges to the centre. The little variations off-track during a particular orbit are ignored because the dominant ones self-correct. But there are some bigger effects such as when the moon is perpendicular to the plane of a satellite's orbit, rather than edge on [when the moon's effects are purely tidal and require no correction]. When the moon is facing the plane of the orbit it would always pull the satellite steadily off course. That would happen once a month [which is how long it takes the moon to go around the earth]. I'm making this up as I go, but I think it's correct and I am not joking this time. Think of the orbits yourself to confirm this. But even that effect is tidal because two weeks later, the moon will be back on the other side of the orbital plane, pulling the satellite back again. But a week of pulling sideways must must move the satellite quite a bit off track. F = ma and F = G [msat mmoon]/d2 means a = need slide rule... but it would be quite noticeable. Just out of interest, notice that the moon takes a month to go around the earth, a GEO takes precisely a day and LEOs rip around every 100 minutes, but none of those times are a function of size. That is the key fact which I think you are misunderstanding. A Globalstar satellite at GEO height would take precisely one day. At the moon's height, it would take exactly one month. None of them need to be kept from falling down [though orbits deteriorate for various reasons, especially with atmospheric interference, which there is very little of at 1414 km, which is the Globalstar level]. <Is that a fair statement of your position? <<> I think I've given you a fairly good [if brief] description of my position and the satellites' positions. I have to go study golf balls some more ... Mq