Fig. 2. Computed “footprint” of the Skylab I exhaust cloud on a plane at 270-km altitude. Contours of the concentration of exhaust molecules per square centimeter. ble depletion was produced as far away as 2000 km. The depletion lasted for about 4 h. An approximate altitude-vs-range trajectory for the Skylab launch is shown in Fig. 1. Also shown are linear rates of injection of exhaust products (in molecules per kilometer) along the trajectory. These quantities are derived from the paper by Mendillo et al. (1). The gases were injected at a velocity of 4.5 km s-1 relative to the rocket, while the rocket accelerated gradually to a final burnout velocity of 7.3 km s-1. Therefore, for the lower part of the trajectory, the exhaust gases were injected tangentially downward; for the upper part (above 350-km altitude), they were projected forward along the trajectory. Because molecular collision mean-free paths above 350-km altitude are very long, most of the exhaust molecules released above that altitude tended to follow ballistic trajectories that carried them hundreds of kilometers downrange from their points of origin, and terminated at about 270-km altitude, i.e. about four mean- free paths deep in the atmosphere, where the falling molecules were stopped by collisions. The exhaust products were also released with some lateral velocity, amounting to about 1 km s-1 on the average. Accordingly, in the course of their ballistic fall to 270-km altitude, they were dispersed some distance out from the rocket trajectory plane. Through these considerations, we were able to calculate an approximate “footprint” of the rocket exhaust deposition on a horizontal plane at 270-km altitude, i.e. a set of contours of the number of molecules laid down per unit area. The calculated footprint is shown in Fig. 2. Also shown are some of the observational lines-of-sight for the total electron content measurements (Mendillo et al. (1)). The lines-of-sight all passed quite high above the rocket trajectory and above the altitude of the F-layer peak in the trajectory plane (altitude tic marks are indicated on the lines-of-sight in Fig. 2). The two-dimensional computer model uses cartesian coordinates (x.z), where the
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