Determination of Jupiter’s mass from Juno radio tracking data

Juno has been in a highly elliptical (average eccentricity e=0.95), nearly polar, 53-day orbit around Jupiter since July 2016. The solar-powered, spin-stabilized spacecraft hosts a complete suite of scientific investigations (gravity science, magnetospheric science, particles and fields analysis) aimed at shedding light into the planet’s interior and formation [1]. The gravity science investigation relies on Doppler tracking of Juno at Ka-band (32–34 GHz) as it flies by close to Jupiter’s top clouds (∼4000 km altitude on average). Measurements of the spacecraft’s radial velocity (Doppler) with respect to NASA Deep Space Station (DSS) 25 are fitted in the least-square sense to accurate dynamic models of the Juno’s motion to reconstruct its trajectory and estimate Jupiter’s gravity field coefficients [2]. After the first two pericenter passages dedicated to the gravity investigation, Juno had already revealed a north–south asymmetry of the gravity field, which has been tied to the presence of zonal winds at ∼3000 km depth [3,4]. The most recent analysis of the gravity science dataset (hereafter called GRAV), after 11 dedicated perijoves at the midpoint of Juno’s mission, has decreased the uncertainties on the gravity field coefficients, while providing also a determination of the gas giant’s tidal response and pole dynamics [5]. Before the end of the mission scheduled in July 2021, Juno might also unveil fine features of Jupiter’s gravity, such as its frequency-dependent tidal response, the presence of normal modes within the planet, and the depth of the Great Red Spot [6–8]. The GRAV data are sensitive to Jupiter’s gravity field; however, due to the limited timespan of the observations (6–8 h) compared with the 53-day orbital period, it is not possible to decorrelate the monopole component of Jupiter’s gravity field (proportional to the planet’s mass times the gravitational constant GMJ) from the low-degree components [9]. An improvement over the current estimate of GMJ, based on observations of the motion of Jupiter’s satellites from the latest ephemeris reconstruction (GMJ=126,686,534.196±2.7 km3/s2, where the quoted uncertainty is 1-σ) [10], would be relevant for spacecraft navigation, planetary ephemerides generation, and general relativity experiments in the solar system [11]. Before the arrival of the JUICE and Europa Clipper missions to the Jovian system in the 2030s, Juno will be the only spacecraft capable of such a determination