Jupiter's gravity field determination with Juno: the challenges of accurate modelling of spacecraft dynamics

After three years spent orbiting Jupiter, the Juno spacecraft provided a new and detailed insight on the interior structure of the planet thanks to its state-of-the-art radio tracking system. The onboard and ground radio science instrumentation provides very accurate Doppler measurements of two-way radio links established simultaneously at X- and Ka-band (7.2-8.4 GHz and 32.5-34 GHz), with accuracy as low as 10 micron/s (in the spacecraft's two-way range rate) at 60-s integration time. The first Doppler data collected from Juno enabled measurements of Jupiter's gravity field with unprecedented accuracy, suggesting the presence of a diluted core and revealing a large North-South asymmetry, which has been explained by the surface winds extending to a depth of thousands of kilometres. Data from the two gravity orbits could be explained with a zonal field: indeed, a rotating fluid planet in hydrostatic equilibrium would exhibit only zonal static gravity coefficients. However, the current dataset is not fully compatible with this simple gravity model. Additional empirical accelerations are required to reduce the residuals to the noise level, indicating the presence of a non-zonal and/or non-static component of Jupiter's gravity. Different sources can be proposed for these accelerations, including, but not limited to, a combination of: 1) small-scale atmospheric structures, 2) deep-rooted gravity anomalies, 3) normal modes. To accurately investigate the underlying physics, and confidently relate the estimated accelerations to Jupiter's gravity, the dynamical model of Juno's spacecraft must be accurately characterized, as well as any instrumental effect related to the tracking system. Thus, an accurate modelling of non-gravitational accelerations (solar pressure, anisotropic thermal emission), spacecraft induced effects (antenna motion and the bending of Juno's solar panels), and known instrumental effects (related to the ground stations or to the spacecraft radio system) have been assessed to avoid any contamination of the gravity solution obtained so far.