Fundamental physics measurements with laser ranged satellites: the Larase experiment and its perspectives and goals

Passive laser-ranged satellites, launched for geodynamics and geophysics purposes, not only have contributed to significant measurements in space geodesy that enabled, among several aspects, a deeper knowledge of the Earth's geopotential, but they also provided an outstanding test bench to fundamental physics, as in the case of the measurement of the gravito-magnetic and gravito-electric precession that are predicted by Einstein's theory of general relativity (GR). Indeed, the physical characteristics of such satellites - such as their low area-to-mass ratio - as well as those of their orbits, and the availability of high-quality tracking data provided by the International Laser Ranging Service (ILRS), allow for precise and accurate tests of gravitational theories. The precise orbit determination (POD) represents the first issue to be fulfilled, to be followed by a solve for the unknown parameters. The POD requires two main features: i) high-quality observations and ii) high-quality dynamical models. The first point is provided by the ILRS by means of the very precise Satellite Laser Ranging (SLR) technique. With regard to the second point, a big effort has been done to develop models for the non-gravitational forces on passive satellites, with significant results in the literature in the case of the two LAGEOS satellites. The aim of LARASE (LAser RAnged Satellites Experiment) is to go a step further in the tests of the gravitational interaction in the field of Earth, i.e. in the weak-field and-slow motion limit of GR, by the joint analysis of the orbits of the two LAGEOS satellites and that of the most recent LARES satellite. One of the key ingredients to reach such a goal is to provide high-quality updated models for the perturbing non-gravitational forces acting on the surface of these satellites. Therefore, in the context of the LARASE collaboration, we started an activity dedicated to revisit, extend and improve current models for the non-gravitational perturbations in the case of LAGEOS-type satellites. We discuss the spin modelling problem and its intimate relationship with the thermal thrust forces; also the atmospheric drag impact on the orbit will be discussed, especially in the case of LARES due to its much lower altitude with respect to that of the two LAGEOS. Also the effects of the Earth's tidal perturbations will be addressed for the LARES satellite. Finally, we present our recent results on the data analysis of the orbit of the two LAGEOS satellites and on that of LARES with our results on relativistic measurements in the case of the orbit of the two LAGEOS satellites with updated constraints on non-Newtonian gravitational dynamics.