Samos 2 (1961 alpha 1): Orbit determination and analysis at 31:2 response

Samos 2, 1961 alpha 1, launched on 31 January 1961, was the first satellite to enter a Sun-synchronous orbit at an inclination of 97.4 deg. The initial perigee and apogee heights were 474 km and 557 km respectively, the initial period was 94.97 min and the satellite decayed 21 October 1973 after more than 12 years in orbit. Samos 2 passed through the condition of 31:2 resonance in June 1971 and orbital parameters were determined at 22 epochs from 1974 observations using the RAE orbit refinement program, PROP, between mid April and mid September 1971. The variations of inclination and eccentricity during this time were analyzed and values for six lumped 31st order harmonic coefficients in the geopotential were obtained. These lumped harmonic coefficients were compared with those derived from the individual coefficients of order 31 and appropriate degrees, from the most recent Goddard Earth Model, GEM 10C. The decrease in inclination between launch and 1971 was examined. It is found to be caused mainly by a near resonant solar gravitational perturbation.