5 CONCLUSIONS AND RECOMMENDATIONS 5.1 INTRODUCTION During the workshop it quickly became apparent that many of the technical problems associated with the environmental effects of SPS propulsion effluents in the upper atmosphere are very broad and poorly defined. Hence, before one plans to do any dedicated field experiments, which are inevitably costly in both time and money, it is appropriate first to undertake a variety of preliminary theoretical and laboratory studies and, where possible, also to use targets of opportunity for initial studies. One problem, the effect on HF skywave propagation due to permanent F- region depletion associated with HLLV launches, surfaced right after the workshop at a briefing given at the Naval Ocean Systems Center (see Appendix F, item F.3). This problem is of such clear importance that it is discussed next, in Section 5.2. Other problems involving H2O/H2 and NO injections, are discussed in Section 5.3 and those involving Ar+ injections, in Section 5.4. During the workshop we were unable to agree on a unique order of priorities for the various items, so they simply are presented in a logical sequence, with appropriate comments on their importance to SPS. It should be noted that the present considerations are not limited to SPS propulsion, but would apply to any very large space transportation project using H2/O2 chemical propulsion and ion propulsion as appropriate. 5.2 PERMANENT DEPLETION OF F-REGION IONIZATION As a result of hydrogen emission from the HLLV second stage rocket and the upward diffusion of the hydrogen, and to some extent also of the water vapor emitted, it is anticipated that there will be a belt of reduced ionization at the latitude of launch. The extent of this region would be several thousand kilometers in the north-south direction, extending around the globe at constant latitude, and beginning at 160-180 km altitude and going to the F2-region peak and above. The mean reduction in ionization may be as much as 50% at night; in the daytime it will be much less, perhaps 10-20%. The critical consequence of such a reduction in ionization is that it drastically reduces the available HF frequency band that can be used for long- range skywave communication at a time when this band is already heavily overcommitted internationally (see Section 3.2.6 and item F.3 in Appendix F). Thus it is clearly of the highest priority to verify the predictions, including atmospheric variability and the overall uncertainty bounds. The specific items that require study are the following: 1. Upward transport of H2 and ^0 injected in the 70-120 km altitude range into the F-region and the associated photochemistry. This includes the dissociation and ionization/deionization, and a resolution of the relative importance of ^0 and H2.
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