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To: Don Lloyd who wrote (7748)	8/26/2001 6:53:29 PM
From: Maurice Winn	Respond to of 74559

Don, I haven't read all the posts yet [and have to go play golf right now] but the main reason earth's odd shape figures less prominently in position maintenance is that the gravitational anomalies contributed by earth's odd shape are small compared with the dirty great moon which pulls tides around the earth and yanks satellites around the place. The sun, which is also noticeable in gravitational effects around here, [also pulls the oceans around for example], has less effect than the moon. Plus, there are an infinite array of other relatively trivial gravitational effects. Plus, corrections in position can't be done super precisely so there is always a bit of hunting in systems stability. So, the gravity which is being corrected [rather than overcome as CB sees it] is that which pulls the satellite out of line rather than back towards earth [which is what CB thinks]. As a satellite goes around, the effects for the most part are like tides, back and forth and mostly don't need correction, but there are still residual effects which do need correction. For example, as the satellite is heading towards the moon, it is pulled out of orbit, but then, a little while later, it is going away from the moon and it is pulled back into orbit. That's the way tides work too. Always pulled towards the moon so as the earth turns, the water goes one way, then the other. So, they have little rockets on the satellites which push the satellite back into position. To minimize satellite mass, the ejection speed of the mass which is used to correct the orbit needs to be as high as possible; momentum = mass x velocity so the higher the speed a given mass is ejected, the more oomph transmitted to the satellite to push it back on track. By accelerating ions, they can be made to go really really fast. So a small mass can be used to move the satellite. But mostly that's new technology and there is a balance between cost, tried and true and a huge array of variables in satellite size, mass, and every other bit of the satellite. Once the satellite is up there, that's it! So they need to be robust. Anyway, I still think they could use the photovoltaic wings to fly them [gee, they could use the photovoltaics to generate electricity to accelerate ions for position location - yeah... I should patent that one]. They just need to carry some air with them and blow it out onto the photovoltaic wings [and they could use the air to burn fuel]. Life's a giggle, Mq

To: Don Lloyd who wrote (7748)	8/26/2001 7:04:18 PM
From: Ilaine	Read Replies (1) \| Respond to of 74559

To teach children the two directions of force being exerted on a satellite, centrifugal and centripetal, children are given a string tied to something like a ball of paper, and told to swing it around. The ball of paper wants go straight, due to inertia, but the string, representing the pull of the earth, prevents this from happening. Yes, you are right, the pull of the earth keeps the satellite from flying away - but only because the satellite is going at the proper velocity to maintain its orbit. If the satellite was going faster, it would fly away. If the satellite was going slower, it would fall to earth. liftoff.msfc.nasa.gov Further, earth does not rotate around its own center of gravity, both the earth and the moon orbit around their common "center of mass", which on average lies 4688 km from the center of the earth and 379712 km from the center of the moon. There are five points between the earth and the moon where a spacecraft would be in balance with the gravities of the earth and the moon and their orbital motion. These are called "Lagrange points" and are quite a bit further away than the orbit of a LEO. Because the earth isn't exactly round, and isn't the only body in the solar system, satellites won't follow a nice Kepler orbit without a little help once in a while. I guess that's what you are suggesting, that once a satellite gets into a Kepler orbit, it will say there.