The orbital decay of the semi-major axis of LARES and the LARASE contribution to SLR measurements for applications in the fields of space geodesy and geophysics.

Lucchesi D.M., Anselmo L., Bassan M., Magnafico C., Pardini C., Peron R., Pucacco G., Stanga R., Visco M.
  Venerdì 30/09   15:00 - 18:00   Edificio Psicologia 2 - Aula 2E   III - Astrofisica
The new laser-ranged satellite LARES (LAser RElativity Satellite) is expected to provide new refined measurements of relativistic physics as well as significant contributions to space geodesy and geophysics. However, because of its height, about 1450 km with respect to the 5900 km of the two LAGEOS, LARES is subject to a much stronger perturbation provoked by the neutral drag than that on the two LAGEOS. From a Precise Orbit Determination (POD) of LARES over a time span of about 3.7 years we have been able to measure an orbital decay in the residuals of its semi-major axis of about 1 m/yr, that corresponds to a transversal mean acceleration of about $(-1.457 \times 10^{-11}$ m/s${}^{2}$. This POD has been obtained analyzing LARES normal points with the GEODYN II (NASA/GSFC) software. Neither the neutral drag nor the thermal effects have been included in the dynamical models of GEODYN II. By means of a modified version of the SATellite Reentry Analysis Program (SATRAP) of ISTI/CNR, the neutral drag perturbation has been computed over the same time span accounting for the measured decay and considering the real evolution of the solar and geomagnetic activities for several atmospheric models. In particular, assuming as reference for the unmodeled transversal acceleration due to the neutral atmosphere the above value, the drag coefficient estimated by SATRAP is comparable to the average value estimated by GEODYN II in a least square fit of the tracking data. This means that the current best models developed for the atmosphere behavior are able to account for the observed decay, within their errors and range of applicability. A further analysis is needed in order to extract from the observed decay a possible smaller contribution related with other unmodeled effects, as the thermal ones, acting on the satellite. In this context it will be necessary to fix the contribution of the signature of the drag and of the thermal effects in the residuals of the other orbital elements of LARES. This study falls within the activities of the LARASE (LAser RAnged Satellites Experiment) research program.