QUESTIONS AND ANSWERS I. ABOUT THE SYSTEM 1.1 Will an orbiting satellite the size of SPS be stable at GEO or will it de-orbit like the Skylab and be a potential danger to people on the ground? The atmospheric density at geostationary orbit (GEO) is so low that synchronous satellites are generally considered to have an indefinite lifetime. However, the SPS would have a much smaller mass to area ratio than any previous satellite at this altitude and thus would be more subject to atmospheric drag. An investigation of orbital decay of the SPS components-^- found that decay of the satellite over its 30-year lifetime could be expected to lie between 0.25 and 2500 meters, i.e., less than 1 part in 10,000 in the worst case. Other components at geostationary orbit (construction bases, etc.) would be influenced even less since they have higher mass to area ratios. There are perturbations from other causes such as solar radiation pressure, lunar/solar gravity gradients, and the equatorial ellipticity of the earth. These are somewhat larger than the atmospheric drag effect (although still small) and will be accommodated with planned station-keeping. A more significant problem is presented by the components in low earth orbit such as the staging base and the electric orbital transfer vehicle during loading and servicing operations. Both of these components would experience decay of such magnitude that essentially continuous orbit maintenance will be necessary. Loss of orbit maintenance capability would result in irreversible decay in a matter of weeks. Thus, all the subsystems involved (guidance, propulsion, stabilization, power) will be highly redundant and rapidly repairable so as to make uncontrolled orbit decay nearly impossible. It will also be necessary to keep sufficient reserve propellent onboard to continue operations in case of launch failure of the resupply vehicles. Launch vehicle range safety will require that launch failures do not result in land impact. Since this corresponds to current practice, no unique requirements are foreseen for SPS launch vehicles simply because of their size. ^Memorandum EW4-79-126 from Johnson Space Center (EA4, Associate Director for Program Development) to NASA Headquarters (RES-1/ Manager, Space Utilization Systems), Re: SPS System Orbital Decay, 2 August 1979.
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