destroyed. At the same time, the H2O and H2 molecules are destroyed as fast as they diffuse into the F2 layer. Although the O+ ions are replaced rapidly, the reaction sequence leads to a net production of H atoms. The H atoms are not destroyed, and they may accumulate in the upper thermosphere. Figures 6a, b, and c include sets of contours of H2O, H2, and electron concentrations at noon, 24 h after the launch. They show that the H2O and H2 molecules are destroyed almost as fast as they diffuse into the F2 region, and, in the daytime, the electrons are replaced as fast as they are removed. At night the electrons are not replaced, and their concentration at the F2 peak is decreased to about 70% of the normal value. It is possible that the H atom production could be a significant environmental problem. Its net effect could be to increase the density of the upper thermosphere. Within the first 30 h after the HLLV launch, 1.3 x IO31 H atoms are produced above 100-krn altitude from the added H2O and H2. This can be compared with the normal global inventory of H atoms above 100 km, which is of the order of 4x 1032, and it implies that each HLLV launch could increase the thermospheric H inventory by about 3% per day if other sources and loss rates remained unchanged. From each launch the H production would continue for about 5 days, leading to a maximum upper limit global H atom inventory change of about 15%. However, an increase of thermospheric H-atom concentration would probably lead to an increased exospheric escape rate. The consensus of a number of studies on the global H-atom escape rate gives a value of about 1 x 1032 atoms per day. Thus, the normal thermospheric residence time for an H atom is of the order of 4 days (or perhaps 1-10 days). The subject of thermospheric H-atom inventories and escape rates needs to be studied more carefully. The present calculations suggest that a doubling of the upper thermospheric density might be possible if a schedule of frequent HLLV launches were initiated. The possible consequences of a density doubling could be serious, though presently a matter of vague speculation. Thermospheric wind patterns could be affected, as well as relativistic electron precipitation rates and global climate. The HLLV would also deposit large quantities of water vapor in the mesosphere. At the mesopause (~85-km altitude), the normal temperature is about 180 K, and the saturation concentration of water vapor in equilibrium with ice at that temperature is 2.6xl012 molecules cm-3. According to the design rate of output of water vapor in the HLLV exhaust (4.0xl029 molecules per linear kilometer), we would expect a large contrail to form at 85-km altitude with crosswise dimensions of at least 14 km. A contrail of this size would persist for at least several hours. The contrail might, however, be very persistent, and a cumulative effect is possible over the course of many HLLV launches. Under normal conditions, water vapor is a major contributor to radiative cooling at the mesopause, due to molecular rotation transitions in the wavelength range 20-100 p.m. The normal contribution of H2O to the cooling rate is about 1 K per day. A local increase of H2O concentration would lead to an increase in the cooling rate and a lower equilibrium temperature. It is possible that a tenfold increase in the mesopause H2O concentration, resulting from many HLLV launches, would lead to a temperature decrease sufficient to produce a permanent ice cloud. The photochemical lifetime of mesospheric H2O vapor is about 40 days. 5. HLLV ORBIT CIRCULARIZATION AND DEORBIT MANEUVERS With the main HLLV second-stage bum confined to altitudes below 120 km, the
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