Air Density in the Upper Atmosphere from Satellite Orbit Observations

Abstract

FROM the rate of change of period of a satellite, it is possible to derive the density of the atmosphere at the altitude of the perigee of the orbit. Now that some ten successful satellite launchings have taken place, giving orbits with various perigee heights, the variation of air density with height can be derived over a considerable range of altitude. Many perigee heights have been less than 230 km., the exceptions being Explorer IV (1958 ζ) at 260 km; Explorer I (1958 α) at 365 km., Vanguard I (1958 β 2) at 656 km., and the recently launched Vanguard II (1959 α) at 558 km. The air density at greater heights is therefore less well established. From an analysis of data from six different satellites, the smoothed set of values in Table 1 has been derived. The accuracy, estimated by fitting a quadratic variation with height to the logarithm of the density, is of the order of 20 per cent at the lower heights and 50 per cent at the greater heights.