make reliable estimates of the SPS impacts on the upper atmosphere and subsequently on the biosphere. The task of performing a preliminary assessment is especially difficult at this time for three reasons: (1) the basic physics and chemistry of the upper atmosphere and the processes that couple it to the lower atmosphere are under intensive investigation and much of our present knowledge* has resulted from relatively recent advances in ground- based (i.e., incoherent scatter radar) as well as rocket-borne instrumentation. Hence, to a certain extent, our ability to predict the consequences of perturbing the natural atmosphere depend upon progress in basic research on the natural atmosphere itself; (2) analytical (theoretical models) and experimental tools that can be used to investigate possible immediate consequences of perturbing the upper atmosphere are still in a fairly early stage of development and theoretical predictions have only been partially verified by experiment and observation; (3) the detailed information regarding the reference SPS design and especially the transportation system, including rocket propulsion systems, propellant exhaust products, launch schedules, etc., is now just becoming available. 3.4.1.1 Vehicle Effluent Effects The area that has received the greatest attention (both theoretical and experimental) recently has been that of ionospheric depletion due to deposition of rocket exhaust in the upper atmosphere. The initial evidence that related rocket exhaust emissions to ionospheric depletion came about quite accidentally. In 1973, Skylab II was launched into orbit with an unusually long burn of the second stage of the Saturn V rocket, which passed through the ionosphere near the area where total electron content (TEC) observations were routinely made by ground-based radio observations as shown in Fig. 3.4. Careful analysis by Mendillo et al. (Ref. 3.3) showed that the passage of the rocket through the F-layer could reasonably account for the observation that a 50% reduction in the TEC occurred, lasting for nearly four hours *For a good review of our present state of knowledge regarding the upper atmosphere, see Reference 3.2.
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