Environmental disturbances on missions for precise tests of relativistic gravity and solar system dynamics: the bepicolombo case

BepiColombo is an ESA/JAXA mission directed toward Mercury, which has been successfully launched on 20 October 2018. The spacecraft will perform accurate general relativity tests both during the 6-years cruise phase and the orbital phase around Mercury. The on-board microwave instrumentation enables very precise ranging and Doppler measurements even when approaching superior solar conjunctions, offering the possibility to improve the classical relativistic test of time delay and frequency shift of radio signals. Random accelerations caused by unpredictable solar irradiance fluctuations could negatively affect this experiment. No measurements of these fluctuations are available for the purposes of the experiment and we show that no direct compensation strategies based on the on-board accelerometer can be applied because of the extremely low frequency components of the disturbance. We analyse the effect of the dynamical noise introduced by fluctuations in the solar irradiance showing by means of numerical simulations that suitable stochastic models of the noise source can still guarantee an estimate of the relativistic time delay with improved accuracy with respect to current knowledge.