trail appeared along the flight path, but this quickly dissipated except for a short segment [~ 10 km (6 mi) in length] near the mesopause height. The segment elongated over a 50 km (30 mi) horizontal distance and became quite distorted. Benech and Dessens state that the rocket deposited 260 g H2O, 520 g HC1, and 1600 g A12O3 along the 10-km segment of the trail that persisted. It is interesting to note that 1 kg of H2O can be dispersed into 0.1 /urn radius ice spheres to produce a cloud with an area of 30 km2 (12 mi2) and a vertical optical thickness of about 10 4, that is, a small noctilucent cloud. The 1600 g of A12O3 alone probably would not be sufficient to form a visible cloud; the ejected particles are typically much smaller than 0.1 gm radius (14) and are therefore inefficient optical scatterers. On the other hand, A12O3 particles are excellent ice sublimation nuclei. The H2O and HC1 rocket vapors could combine on the alumina surfaces to form a condensate of lower vapor pressure than pure water. The supersaturation required to nucleate 0.1 /urn A12O3 particles is also lower than that required to nucleate the 0.002 gem (or smaller) meteoric dust particles normally found in the mesosphere (7). Benech and Dessens suggest, in addition, that after sunset A12O3 particles may be cooler than the ambient air due to efficient radiative cooling. This would give A12O3 a further advantage as an ice nucleating agent. A variety of water release experiments have been made to test for artificial cloud formation. Fogle and Haurwitz (15) reviewed the August 1964 attempts to form ANLCs over Fort Greely, Alaska using 2 kg of pure H2O. Although the water was injected directly into the mesosphere under conditions "favorable” to condensation, no clouds were observed. The results are inconclusive, however, because the height of injection was never actually determined, and the water was released in bulk with little likelihood of extensive dispersion. A review of several other water release experiments and rocket cloud observations in the thermosphere (above 100 km) (60 mi) is available in the La Jolla SPS Workshop Proceedings (16). These results are not particularly relevant to the mesospheric cloud problem for several reasons. In the thermosphere, rocket exhaust expands essentially into free space, and its initial energy density is a key factor in determining the degree of condensation in the trail (16). In the mesosphere, however, the expanding exhaust rapidly achieves pressure equilibrium with the atmosphere, and is confined to a slowly-growing, well-defined contrail. Ice particles in the thermosphere are not in thermal equilibrium with the surrounding air (16). In fact, a complex interplay of kinetic, radiative, and latent heat processes determines the temperatures of the evolving ice crystals. Mesospheric water particles, on the other hand, quickly attain the temperature of the ambient environment (6), and therefore have similar, readily determined physical properties (e.g., vapor pressures and growth rates). Ice particles also fall much faster in the thermosphere than in the mesosphere, leading to quite different cloud histories. In summary, past observations of mesospheric clouds associated with rocket launches seem to indicate that tenuous short-lived clouds may be generated by solid- fueled rockets, which emit copious quantities of sublimation nuclei in addition to water vapor (11-13). In contrast to these small rocket events, however, the SPS HLLV would deposit roughly 108 g of water between 80 and 90 km (48 and 54 mi) (almost 109 g of water vapor would also be deposited above 90 km, but this water vapor would be widely dispersed before it could diffuse downward to the mesopause). It would be remarkable indeed if transient cloudiness did not result from such a massive deposition of water. Surprisingly, persistent mesospheric clouds have, to our knowledge, never been observed following any of the large Saturn V liquid-
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