Non-gravitational perturbations on the semimajor axis of LAGEOS.

Orbital analysis of the passive, geophysics-dedicated satellite LAGEOS has revealed an unmodeled semimajor axis decay corresponding to an average along-track acceleration of -3.4 x 10 to the -12th m/sq s, modulated by several long-periodic terms with amplitudes of the order of 10 to the -12th m/sq s. Fourier analysis of the data shows that the main periodicities are at 1050, 560 and 280 days, namely related to the orientation of the orbit plane and its configuration with respect to the sun. While the secular component of the perturbation can be explained by charged and (at a lesser extent) neutral particle drag, other mechanisms are needed to explain the long-periodic variations. Three possible mechanisms are discussed and, whenever possible, asssessed quantitatively: (1) radiation pressure by earth-reflected sunlight; (2) radiation pressure recoil due to anisotropic thermal emission by the satellite itself; and (3) uncertainty in the available models of solid and oceanic tidal perturbations. Although knowledge of many relevant parameters is poor, these tiny nongravitational forces appear to account satisfactorily for the most prominent periodic components of the observed semimajor axis decay. However, the dynamical model is not improved below an acceleration uncertainty of about 6 x 10 to the -13th m/sq s on timescales between about 1 month and about 1 year, and an even greater uncertainty affects shorter timescales.