Air density at heights between 130 and 160 km, from analysis of the orbit of 1968-59b

Abstract

The satellite 1968-59B was a heavy sphere, of mass 272 kg and dia. 0.61 m, launched by the United States Air Force on 11 July 1968 into a polar orbit with an initial perigee height of 150 km. Despite the low perigee, the satellite remained in orbit for 38 days because of its exceptionally large mass/area ratio, and offers the opportunity of finding values of air density at heights lower than has been possible in previous studies of satellite orbits. Analysis of the orbit gives 28 values of air density, which show that the density increased from 1.49 × 10 −9 kg/m 3 at a height of 155 km to 7.9 × 10 9 kgm 3 at 130 km height. These values are in surprisingly good agreement with the COSPAR International Reference Atmosphere 1965, being about 9 per cent lower than CIRA. When the values of density are converted to a fixed height near 150 km, the day-to-day variations are found to be remarkably smooth between 12 July and 13 August, the deviations from the mean being less than 4 per cent; but there was a significant increase in density at the time of the increased solar activity centred at 15 August.