Irregularities in the Density of the Upper Atmosphere: Results from Satellites

Abstract

WHEN an Earth satellite encounters air drag its orbit contracts and its period of revolution T decreases. For most of the satellites so far launched the rate of decrease of T has been irregular, thus implying irregularities in drag, which could be caused by changes in either the effective atmospheric density near perigee or the effective cross-sectional area S of the satellite perpendicular to the direction of motion. If, like most of those so far launched, the satellite does not point in a specific direction but rotates, S is to be taken as the mean cross-section during one complete rotation. The mean cross-section should remain constant as long as the mode of rotation of the satellite is unchanged, though it would not be constant if, for example, tumbling end-over-end changed to spinning like an aeroplane propeller. A clue to the mode of rotation is provided by the fluctuation in brightness of a satellite; if this remains almost the same for many months it is unlikely that the mode of rotation has changed appreciably.