Noctilucent clouds can apparently form only at the high latitude mesopause because only there is the supersaturation of water vapor sufficient for their formation. Our preliminary estimates of the increase in optical depth at visible wavelengths due to noctilucent cloud formation from deposited water vapor is only 1x10 in the noctilucent cloud zone (a very small fraction of the earth's surface). If the optical depth over the whole surface of the earth were increased by that amount, the estimated temperature decrease would be only about 10”&; the threshold for perceptible climate change is about 0.IK. However, immediately following launch the increase in optical depth can be quite large perhaps to t 25 along the trail axis 1 hour after launch. However, the ice particles in the trail quickly spread and evaporate. Thus, one might expect a substantial contrail to form near the launch point and to dissipate in less than one day. Our ionospheric calculations showed that on the global scale there would be only a small increase in electron density in the D-layer due to increased NO generated by reentry. Such increases could be very large (order to magnitude or more) in the region near the entry trail for times up to about 1 day. 1 - NASA-Ames Research Center, Moffett Field, CA 94035 2 - R and D Associates, Marina del Rey, CA 90291 3 - Jet Propulsion Laboratory, Pasadena, CA 91103 4 - San Jose State University, San Jose, CA 95129
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