Dottorato in ASTRONOMIA

Titolo della tesi: The Italian Spring Accelerometer on-board BepiColombo: Cruise Phase Data Analysis and Mercury Orbital Phase Simulations

On October 20, 2018 the BepiColombo mission was launched from the Centre spatial guyanais a Kourou, in the French Guyana with the Ariane 5 launcher. The BepiColombo mission is a joint mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) and represents a pioneering project in planetary exploration, aiming to solve the mysteries of Mercury, the innermost planet of our solar system. With innovative instruments and sophisti- cated technologies, BepiColombo will capture high-resolution images, analyze the planet’s surface and magnetosphere, and contribute to the knowledge of the solar system’s history and evolution. Many of the scientific objectives of BepiColombo strongly rely on the accurate and precise orbit reconstruction of the probe. Thanks to advanced Ka band transponder of the Mercury Orbiter Ra- dioscience Experiment (MORE) and to the Italian Spring Accelerometer (ISA), it will be possible to reconstruct the spacecraft orbit around Mercury with high accuracy, a first step to reach the scientific goals of the Bepicolombo RadioScience Experiments (BC-RSE). The exploitment of the full capabilities brought by the state-of-the-art tracking system requires an accurate knowledge of the dynamical environment of the probe. The harsh environment of Mercury due to the proximity to the Sun, makes the dynamic environment to be hard to modeled with the accuracy required by the tracking system. For this reason, the Italian Spring Accelerometer (ISA) has been selected as a payload among all the scientific suite of the BepiColombo mission. ISA data will be injected into the orbit determination software as a direct measurement of the non-gravitational acceleration, avoiding the non-trivial modeling of such perturbations. After giving an overview of the mission and of the orbit determination procedure for the MORE experiment, the thesis is developed in two distinct parts. In the first part, the analysis of the ISA data collected during the BepiColombo cruise phase, in particular during the Earth flyby, the second Venus flyby and the first two Mercury flybys, is shown. ISA is one of the few instruments that gathered data around each closest approach. The instrument scientific team was able to validate all the tools developed for data calibration and analysis by comparing the actual data with the expected computed reference accelerations. Thanks to the eclipse events during the Earth flyby and both the Mercury flybys, ISA clearly detected the drop in acceleration due to the sudden lack of solar radiation pressure. The magnitude of the drops was fully compatible with the models. During the Venus flyby, an acceleration spike was detected across the closest approach due to an outgassing event. The data were compatible with flight dy- namic analysis and with the Mercury Plasma/Particle experiment (MPPE) observations. During both Mercury flybys, thanks to the low closest approach altitude, ISA detected for the first time the gravity gradient acceleration exerted by Mercury on the spacecraft. The comparison between the mathematical model and the observation let the ISA team estimate the position of the spacecraft center of mass, which was compatible with the formal uncertainty of the estimation produced by flight dynamics. In the last part of the thesis, the numerical simulation of the MORE gravity and rotation experiment during the first year of the BepiColombo scientific phase around Mercury, using ISA synthetic data in the orbit determination procedure, is shown. The ingestion of ISA data into the orbit determination software requires a smart pre-processing and selection of a suitable cali- bration strategy of the accelerometric data. For this reason, the goals of the numerical simulation were to find a calibration strategy that allows the radio science experiment to fulfill all the scientific goals. Moreover, the developed orbit determination setup allows to perform the integration of the equation of motion of a reference point of the spacecraft different from its center of mass. The validation of this setup was a mandatory step to be done before the start of the scientific phase. Numerical simulation results showed that the chosen calibration strategy let to achieve an unbiased orbit solution and to fulfill all the radio science experiment scientific goals.