Combining JUICE and Europa Clipper range, range-rate and VLBI observables to Improve the Galilean moons ephemerides

ESA’s JUICE and NASA’s Europa Clipper (EC) are the next two missions to the Jupiter system, focusing on three of the Galilean moons: Europa, Ganymede, and Callisto.

JUICE will spend four years in the Jovian system and after tens of flybys of Europa, Ganymede, and Callisto will enter into orbit around Ganymede where it will nominally remain for nine months, until its end of mission. EC will also spend about four years in orbit around Jupiter, performing more than 50 flybys of Europa, the main mission target, but it will also fly by Ganymede and Callisto several times. Combining the data of the two missions will enable a better global estimation of the moons’ gravity fields and ephemerides.

During the Jupiter tours of both missions and JUICE’s Ganymede orbital phase, radiometric tracking data will be acquired at Earth ground stations, enabling precise spacecraft orbit determination, and joint estimation of the main dynamical parameters of the Jupiter system. The two missions rely on different radio links: JUICE is endowed with a triple two-way radio link configuration in two frequency bands (X/X, X/Ka and Ka/Ka) which will allow for a full calibration of dispersive noise sources. EC is capable of X/X and X/Ka links, with X/X being the nominal configuration during Europa flybys.  

Range, range-rate, as well as VLBI (lateral positioning) tracking data, from both missions, will allow to retrieve the static gravity field and tidal parameters of the moons, together with their orbital position. This will also provide crucial information about Jupiter’s gravity tidal parameters, in particular the imaginary part of its Love numbers at the frequency of the Galilean moons. A better characterization of tidal interactions between Jupiter and the Galilean moons can unveil crucial information about the stability and the evolution of the Laplace resonance, governing the dynamics of the three innermost Galilean moons.

In this study, we analyze the attainable uncertainties for the parameters characterizing the ephemerides reconstruction of the Galilean moons using range, range-rate, and VLBI simulated observables. VLBI data mainly provide the spacecraft angular position with respect to reference radio sources (quasars) tied to an inertial frame (“plane of sky”), while range and range and range-rate (being computed along the line of sight) especially constrain the spacecraft state within their orbital plane. Including VLBI data is thus expected to be particularly effective in improving the uncertainty of the moon ephemerides in the out-of-plane direction. We will quantify the synergy between the different radiometric observable types, assess their respective contribution to the moons' ephemerides, the imaginary part of Jupiter’s Love number and analyze the sensitivity of the estimation solution to various parameters (observation planning, expected data quality, etc.)