Skylab 1 rocket (1973-27 B): Orbit determination and analysis

Abstract

The final-stage rocket which projected Skylab into orbit on 14 May 1973 itself entered a nearly circular orbit at a height near 400 km, inclined at 50° to the equator. The rocket, designated 1973—27 B, remained in orbit until 11 January 1975 and, being 25 m long and 10 m in diameter, was the brightest of the artificial satellites then visible. Its orbit has been determined at 62 epochs from some 5000 optical and radar observations. The average orbital accuracy in perigee height and orbital inclination is 90 m, though some orbits with Hewitt camera observations have much smaller s.d. (down to 10 m) . As the orbit contracted under the influence of air drag, it passed slowly through the 31:2 geopotential resonance, when the track over the Earth repeats every 31 revolutions at intervals of 2 days. The variations in inclination and eccentricity during the resonance phase have been successfully analysed, after some difficulties, and values have been obtained for six lumped 31st-order harmonic coefficients in the geopotential; these will provide a crucial test of the accuracy of future geopotential models. The variation in inclination before and after resonance has been analysed to determine the average atmospheric rotation rate A (rev/day). Results are: A = 1.04 ± 0.05 at height 380 km between June 1973 and June 1974; and A = 1.34 ± 0.09 at height 305 km for October-December 1974; and A = 1.06 ± 0.06 at height 200 km in January 1975. A rapid atmospheric rotation at heights of 300—350 km, deduced previously by combining results from many satellites, is thus confirmed by a single satellite. The observed variations in eccentricity have been compared with those predicted by the (untested) theory for orbit contraction in an atmosphere with strong (threefold) day-to-night variation in density. The comparison confirms the accuracy of the theory and of the CIRA 1972 atmospheric model.