On the long-term behavior of LAGEOS II semi-major axis and thermal thrust perturbations

We analyzed the orbit residuals of the two LAGEOS (LAser GEOdynamic Satellite) satellites and of LARES (LAser RElativity Satellite) after modeling the main gravitational and non-gravitational perturbations acting on their orbits. The purpose of this activity is to study the residuals in the different Keplerian elements and then comparing them with the predictions of the non-conservative force models -- not included in the a priori models used for the Precise Orbit Determination -- which we are developing with the aim of improving the global dynamic model of the orbits of these satellites. Among these non-conservative forces there are those related to the thermal thrust produced by the pressure of solar and terrestrial (albedo and infrared) radiations. Due to the thermal inertia of the different elements that make up the surface of these passive satellites, as for that linked to their cube corner retroreflectors, a non-uniform distribution of temperature on their surface originates, which causes an anisotropic emission of radiation with significant long-term effects on different orbital elements. The different weight of these forces, strictly influenced by the rotational state of the satellites -- both in orientation and in rate -- seems to be the main cause of the inversion observed in the decay of the semi-major axis of the LAGEOS II starting from mid-March 2012, approximately 19 years after the launch of the satellite. This behavior, apparently unexpected and far from its previous interpretation, will be described and discussed in the light of the thermal thrust and spin models of the satellite that we have developed. This study and research activity is part of a broader activity in the field of fundamental physics that aims to use these geodetic satellites as proof masses to test and compare the predictions of General Relativity with those of other alternative theories of gravitation in the context of the SaToR-G (Satellite Test of Relativistic Gravity) experiment.