require some type of transfer from a lower orbit to GEO. In most cases, ion propulsion is considered the most attractive option for this transfer stage, although cryogenic tugs could offer a desirable alternative if the satellite can withstand the higher acceleration. In the paragraphs that follow, the status of the orbit transfer stage for the solar-photovoltaic SPS is first summarized, followed by a summary for that of the solar thermal SPS. A discussion of general requirements and the overall state of technology in this area is then provided. (1) Solar-Photovoltaic SPS For the solar-photovoltaic SSPS, Ref. All (the most recent document and also the most comprehensive) describes plans for assembly in low earth orbit (LEO), then transfer to GEO via ion propulsion using a partially extended SSPS for the propulsion power supply. For this approach, three significant issues are cited, namely: (1) Development of a large diameter ion thruster (2) Selection of an ion engine propellant (3) Development of an alternative power source if SSPS power cannot be used for the thrusters. "Near term" studies, where "near term" is defined as extending up to the time the Shuttle will be available for in-space development activities, would include the following: • Study high performance stage alternatives with respect to: Propulsion systems Power sources Propellants • Study the potential of large cryogenic propellant tugs having: One and 1/2 stages Two stages • Refine mission plans and hardware definition for Orbital Technology Verification Shuttle missions, including: Flight tests on orbit transfer of large flexible bodies and radiation sensitive materials.
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