Analysis of JUICE 3GM gravity experiment at Callisto and Ganymede

Abstract:

The JUpiter Icy Moons Explorer (JUICE) is a ESA mission that will launch in 2022 and arrive in the Jovian system in 2029 to investigate Jupiter's icy satellites. The mission will perform a series of flybys of the icy moons Europa, Callisto and Ganymede before beginning a 280-day orbit around Ganymede: 150 days in Elliptical Orbit (GEO) followed by 130 days Circular Orbit at 500 km (GCO-500). A suite of field and particle instruments will characterize the plasma and magnetic environment around the celestial bodies. The radio science experiment 3GM (Gravity and Geophysics of Jupiter and the Galilean Moon) comprises of two separate and independent units: a Ka-band Transponder (KaT) enabling two-way coherent range-rate and range as accurate as ~0.012 mm/s @ 60 s and 20 cm respectively, and an Ultra Stable Oscillator (USO). The two units pertain to two different scientific objectives: the KaT gathers data pertaining to geodesy and geophysics, while the USO gathers data relating to atmospheric science. The KaT will allow us to improve our current knowledge of the interior structure of the three moons, by measuring their gravity fields, and to expand our knowledge of the Jovian satellite ephemerides by solving for the spacecraft and moons orbits. This is achieved by means of a precise orbit determination. The 3GM simulations carried out for the GCO-500 reveal that Ganymede’s gravity field can be determined up to degree and order 40, depending on the magnitude of the gravity spectrum. The Love number k₂, modelling the gravitational response to eccentricity-driven, time-variable gravity gradient from Jupiter, is determined with an accuracy of σ_k2 ∼10^-4. The obliquity, φ, and the libration at orbital period, ϕ, can be retrieved with a level of uncertainty of 1 and 2 μrad, respectively. A brief analysis of 3GM improvements with a mission extension to a circular orbit at 200 km (GCO-200) is also included in this work. The Callisto gravity experiment is analyzed under different mission scenarios to identify the possible scientific outcome. The baseline trajectory (141a) is composed of 12 flybys and will allow to estimate the moon's gravity field at least up to degree and order 5 and the σ_k2 > 0.08. The backup trajectory (230la), with launch foreseen in 2023, is composed of 17 Callisto flybys which will allow to obtain a better estimation of k₂, σ_k2 < 0.05.

.