Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Precise orbit determination methods for the VERITAS mission to Venus

On June 2, 2021 the VERITAS mission to Venus has been selected by NASA as one of the two winners of the 2019 Discovery competition. VERITAS is designed to substantially push forward our understanding of Venus and rocky planets in general. The main question driving the VERITAS scientific investigation is the unveiling of the dynamical evolution path that led Venus, in principle so similar to the Earth, to its current state of unhinhabitable planet. Many of the scientific objectives of VERITAS strongly rely on the accurate and precise orbit reconstruction of the probe. The exploitment of the full capabilities brought by the state-of-the-art tracking system requires the accurate modeling of the dynamical environment of the probe. In this thesis the orbit determination problem of VERITAS is analyzed and and developed in detail on both the technical and scientific point of view. After giving an overview of the the VERITAS mission and the orbit determination procedure, the thesis is developed in three distinct but inter-dependent parts. In the first part we develop a technique for the calibration of the Venus atmospheric noise (ionospheric and tropospheric) on the VERITAS two-way Doppler tracking data and validate it with an application on NASA's Mars Reconnaissance Orbiter. We show in the case of Mars, but the generality of the method allows its application to the Venus case as well, that a large fraction of the tracking data that were previously discarded could be reintroduced in the orbit determination dataset, leading to an improved solution. In the second part, we develop the capability of combining the Doppler tracking data with the synthetic aperture radar observations of the Venus surface (tie points) with the aim of determining the rotational state and tidal deformation of the planet to an unprecedented level of detail. With an extensive set of numerical simulations we show that the combination of the two datasets would allow a determination of the rotational state of the planet, and thus of its moment of inertia, improved by nearly an order of magnitude with respect to the Doppler only solution. We apply these results to recent models of Venus interior and show that VERITAS will provide much improved constraints on the interior structure of the planet with respect to the current state of the art. In the last part of the thesis we analyze in detail the effect of the non gravitational accelerations that will act on the probe during the orbital phase and develop an advanced modeling capability, based on ray-tracing techniques. Applying this advanced modeling capability, available to the general public through the release of an open-source python software library pyRTX, we show that the high accuracy of the VERITAS tracking system would require a very precise modeling (beyond the current capabilities of the widely used JPL MONTE orbit determination code) of the non gravitational accelerations to avoid significant errors in the orbit reconstruction and thus on the derived scientific products.