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To: Maurice Winn who wrote (7726)	8/26/2001 2:10:38 PM
From: Ilaine	Respond to of 74559

>>It's the photovoltaic wings which make the satellites stay up, not the fuel on the satellite being used to push them up.<< ROFL! The wings don't keep the satellite up, Mq. How would they do that? Flapping? No, they provide solar power.-g- Satellites aren't aerodynamic like planes because there isn't any air to speak of. Globalstar's satellites have an altitude of 1410 km., which, technically, is outside of the outermost layer of earth's atmosphere, the exosphere (640 km. to 1240 km.) but there are molecules of gas excaping from earth's gravitational pull and some of them undoubtedly do strike LEOs occasionally. Not enough to fly on, though.-g- >>GEOs have bigger photovoltaic wings because the atmosphere is thinner way out there.<< Oh, brother. Nothing called atmosphere beyond 1280 km. GEO altitude is 36,000 km. What keeps satellites "up" is a number of things, but the most important is velocity. All satellites must maintain the proper velocity for their altitude, otherwise they either go flying out into space or crash to earth. Satellites which are closer to earth, like LEOs (lower earth orbit) need a faster velocity (maybe 17,000 mph). Satellites which are further from earth, like GEOs (geosynchronous orbit) need a slower velocity (more like 7,000 mph.) When a satellite is placed into orbit by a rocket, the rocket is supposed to release the satellite at the proper velocity for its altitude, and angular momentum comes into play. The combination of "push" and "pull" keep the satellite in place, but of course gravity does its thing, and must be overcome. The mechanism that maintains a satellite in its proper orbit (station-keeping) is very complex, but the most important elements are sensors which track the satellite's position, and thrusters. The physical mechanism for station-keeping is typically expulsion of hydrazine gas, but Globalstar uses something called a plasma propulsion system, which apparently propels plasma, not gas. Globalstar also uses something called kinetic wheels, which provide torque and kinetic momentum. Nevertheless, Globalstar's satellites have a useful life of only 7.5 years. Early satellites (Koralev, Sputnik I and II) did not have photovoltaic panels. They relied on battery power (silver-nickle), and most of the weight of the satellite was the power supply. Sputnik III used solar power but not solar wings. The useful life of a satellite is determined by the fuel which is used to maintain orbit. The amount of fuel a satellite can carry is not infinite, and when they run out of fuel, it is not presently feasible to refuel them. Smaller satellites need less fuel, but also carry less fuel. The people who design satellites take into account multiple factors, including size (more precisely, mass.) There are multiple tradeoffs involved. As I explained before, the closer the orbit, the greater the gravitational pull. Bigger satellites are pulled "harder" than smaller ones, but can also carry more fuel. On the other hand, the bigger the satellite, the more expensive it is to get it into orbit. Apparently sometimes it's more economical to launch smaller, faster and cheaper satellites and replace them more often. This is especially true when the satellites are not complex, like Globalstar's, which are far less complex than Iridium's satellites. I didn't think I needed to explain this in detail to you, but if you think GEOs need bigger wings because the atmosphere is thinner, you need more help than I can give you.