Thesis title: Tracking systems and orbit determination for relativistic gravity experiments with the missions BepiColombo and JUICE: simulations and early results
BepiColombo is an ESA/JAXA mission currently in its 7-year cruise phase towards Mercury. The Mercury orbiter radioscience experiment (MORE), started its scientific investigation during the superior solar conjunction (SSC) in March 2021 with a test of general relativity (GR). Other solar conjunctions will follow during the cruise phase, providing several opportunities to improve the results of the first experiment. The MORE radio tracking system enables precise ranging and Doppler measurements almost at all solar elongation angles, thus providing an accurate measurement of the relativistic time-delay and frequency-shift experienced by a radio signal during a SSC. A similar experiment, performed by Cassini in 2002, provided the most accurate estimate of the post-Newtonian parameter γ so far, with an accuracy of 2.3×10^(-5). The final objective of the experiment is to place new limits to the accuracy of the GR as a theory of gravity under weak-field conditions. As in all gravity experiments, non-gravitational accelerations acting on the spacecraft are a major concern. Because of the proximity to the Sun, the spacecraft will undergo severe solar radiation pressure (SRP) acceleration, and the effect of the random fluctuations of the solar irradiance may become a significant source of spacecraft buffeting. The first aim of my work is to provide a realistic estimate of the outcome of the six cruise SSC experiments of BepiColombo, by including the effects of random variations in the solar irradiance in the dynamical model. Numerical simulations show that, with different assumptions on the solar activity and observation coverage, the accuracy attainable in the estimation of γ is in the interval [6-13]×10^(-6). The analysis, then, focused on the first BepiColombo GR test (March 2021). A precise data calibration process, dynamical model and observation model have been developed, at the level required by the performance of MORE. This analysis indicates an estimate of γ about 3 times more accurate than the one obtained by Cassini. However, the orbit determination fit showed that unmodeled non-gravitational forces were acting on the spacecraft, whose nature is still unclear. As indicated by the first BepiColombo SSC, unmodeled non-gravitational accelerations are a serious hindrance for precise test of gravitational theories in the inner solar system. Carrying out the experiment in more stable dynamical environment (as in the case of Cassini) would allow to fully exploit the accuracy of the novel tracking systems. A suitable opportunity is provided by a set of superior conjunction experiments happening during the cruise phase of the ESA's JUICE mission to the Jupiter system, planned for launch in September 2022. After a 9-year interplanetary cruise the spacecraft will reach the Jovian system in mid 2031. The 3GM (Geodesy and Geophysics of Jupiter and the Galilean Moons) radio-science package onboard the spacecraft relies on almost the same radio-tracking system of MORE, but in this case the spacecraft will travel far away from the Sun, thus reducing the effect of random SRP variations. I analyzed the three opportunities to perform superior conjunction experiments during the cruise phase, with the goal of assessing the uncertainty attainable on the estimate of γ. Numerical simulations indicate that 3GM can estimate γ with an accuracy at the level of 4.8×10^(-7), which represents a constraint on the value of γ 50 times tighter than the one obtained by Cassini and about four times better than the expected result from BepiColombo at the end of the mission. This would be a major leap forward in tests of relativistic gravity.