which is present in the debris but not in meteors, is deposited in the ablation region. The probable deposition of such materials and their possible effects are not known. A recent paper (Kessler and Cour-Palais, 1978) estimated the potential for the formation of a permanent belt of satellite debris around the earth. As visualized in their study, it is highly probable that debris fragments may strike a large satellite in the next decade) some evidence indicates that this may already have occurred). Such an impact would produce many more fragments in a variety of orbital inclinations. According to their calculations, given the present launch rate and estimated population of existing satellites and debris, the near-earth orbital environment could be approaching a critical point around the year 2000 wherein such debris collisions would lead to a chain reaction type of process. The end result would be the creation of an artificial debris cloud around the earth. Not only would such a belt be a very real threat to an SPS and other satellites, but an SPS could exacerbate the situation. Although the conclusions of Kessler and Cour-Palais are somewhat tentative, they must be taken seriously as a possible environmental effect of global significance. 2.3 WATER VAPOR IN THE MESOSPHERE AND LOWER THERMOSPHERE (Ellsaesser) At present there are very few measurements of the water vapor concentration in the mesosphere, most of which are rocket measurements at high latitudes (Arnold and Krankowsky, 1977; Rogers et al., 1977; and data from the AFGL SPIRE flight [J. S. Garing, AFGL, private communication]), suggesting a water vapor mixing ratio of 5 ppmv. Radford et al. (1977) using groundbased microwave radiometry, obtained a mixing ratio as high as 15 ppmv, but this seems much too high to understand on physical grounds. Figure 2 shows all presently available data. The longest and most generally accepted series of ^0 observations above the tropopause are those of the MRF (British Meteorological Research Flights) and of Mastenbrook (1968, 1971, 1974). These indicate a decrease in mixing ratio for the first 1-3 km above the tropopause, both polar and tropical, to values of 3-5 ppmv (parts per million by volume, i.e., molecular rather than mass mixing ratio) near 19-20 km. Above 20 km there is a fairly consistent tendency toward both higher mixing ratios and greater uncertainty in the data. Mastenbrook (1974), Harries (1976), and Penndorf (1978) have all interpreted these and other observations as showing a constant mixing ratio from the lower stratosphere up to 28-35 km, with the suggestion of an increase at higher levels. Below 20 km these two series of observations show seasonal cycles decreasing in amplitude with distance above tropopause, and a tendency for a biomodal distribution. Most soundings are of the "dry" type showing mixing ratios 3 ppmv, while perhaps 20% of the soundings are of the "wet" type showing mixing ratios above 10 ppmv. These series also support a long-term trend with almost a doubling between 1952 and 1973 and a decrease since then of at least twice the rate of the earlier increase. Beyond these variations, these data series show remarkably little variability.
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