ON THE ALONG-TRACK ACCELERATION OF THE LAGEOS SATELLITE

The Earth-induced Yarkovsky thermal drag is known to be the principal drag mechanism for LAGEOS. In combination with charged and neutral particle drag this thermal drag can account for all of the observed average along-track acceleration as well as its long periodic variation, if we allow for an alignment of the satellite's spin axis with the Earth's polar axis, decreasing the colatitude of the spin axis by 50% every 6 years. This motion is probably due to the interaction of eddy currents in the satellite and the Earth's magnetic field. The spin axis appears not to precess in longitudinal direction, indicating that gravitational torques hardly affect the satellite. Calculations confirm that the related principal moments of inertia of the spacecraft differ much less than generally assumed. A combination of two other force models, operating only during the partly eclipsed orbital periods, is likely to explain most of the anomalous residual fluctuations in the along-track acceleration. The first model is the Yarkovsky-Schach effect, a variant of the aforementioned thermal drag, involving heating by the Sun. The second embodies anisotropic reflection of sunlight by LAGEOS' surface; one hemisphere of the satellite probably has a specular albedo, which is about 5% larger than the other. By applying these two models, in addition to particle and Yarkovsky thermal drag, the standard deviation of the residual acceleration could be reduced by 61%.