Venus Thermosphere and Exosphere: First Satellite Drag Measurements of an Extraterrestrial Atmosphere

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Abstract
Atmospheric drag measurements obtained from the study of the orbital decay of Pioneer Venus 1 indicate that atomic oxygen predominates in the Venus atmosphere above 160 kilometers. Drag measurements give evidence that conditions characteristic of a planetary thermosphere disappear near sundown, with inferred exospheric temperatures sharply dropping from approximately 300 K to less than 150 K. Observed denisities are generally lower than given by theoretical models.