The last 14 days of Skylab 1: Orbit determination and analysis

The orbit of Skylab 1 (1973-27A) was determined using 1400 NORAD observations during the 14 days prior to decay on 1979 July 11. The variations in inclination due to atmospheric rotation and 16th-order resonance with the geopotential are analyzed, to give the first values of 16th-order geopotential coefficients determined from resonance, and a value of 1.10 + or - 0.07 rev/day for the atmospheric rotation rate at a height of 210 to 220 km. The daily changes in semimajor axis are used to determine 13 daily values of air density, at heights from 252 km down to 179 km. All agree well with the CIRA 1972 model, and indicate a smaller semiannual variation than in the early 1970s. The variations of eccentricity and argument of perigee take unusual forms, but detailed analysis shows that the variations are in full accord with the theory for an atmosphere with day-to-night variation in density, with the perigee progressing towards the point of minimum density.