remote sensing from satellites. Studies called for in this context include long-term measurements of water vapor concentrations and cloudiness in the mesosphere, and the description of condensation and evaporation of water vapor emitted in high-altitude rocket exhausts. The injection of water and hydrogen, as well as that of nitric oxide (NO) due to atmospheric heating from reentering spacecraft, may enhance the airglow and may also affect the long-range propagation of relatively low frequency radio waves (VLF and ELF) by changing the ion chemistry in the ionospheric D-region. Enhancing the hydrogen concentration in the upper atmosphere will increase the drag on low-altitude orbiting satellites; the significance of this effect is not yet established. In going from a low-altitude parking orbit to geosynchronous orbit, the current SPS concept calls for the use of argon ion engines. The quantity of argon injected into the atmosphere above 500 km is very large indeed, relative to the ambient atmospheric mass, and it is not yet established how rapidly the ion beams will be stopped or what the lifetime and energy loss and gain processes of the ions are. The injection could lead to significant changes in the earth's radiation belts, possible changing the radiation environment of spacecraft in orbit. The structure of the report is the following. Section 1 reviews the injections of different materials in different altitude ranges, amplifying Table S.l and attempting a quantitative estimate of the relative significance of the different injectants. It also reviews how the study was conducted. Section 2 treats problems due to the very large propulsion injections in the 70-120 km altitude region. Section 3 discusses the general problem of hydrogen and water injections due to SPS in the ionosphere. Section 4 reviews effects on the magnetosphere in the passage from the parking orbit (LEO = Low Earth Orbit, at 500 km) to geosynchronous orbit (GEO at 36,000-km altitude, or 6.5 earth radii away). Section 5 presents the conclusions and recommendations of the study. Appendix A lists the membership of the present workshop, Appendix B gives a brief summary of the SPS transportation system, Appendix C defines abbreviations and acronyms used here, and other appendices furnish various technical details.
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