Being among the most promising candidates for potential extraterrestrial habitats within our Solar System, the Galilean satellites are going to be extensively studied by the upcoming JUICE and Europa Clipper missions. Both spacecraft will provide radio science tracking data, which will allow the satellites ephemerides to be determined to much greater accuracy than is currently the case. Yet, with no flybys of Io, these data sets will be skewed towards the three outer satellites. To mitigate this imbalance, optical space-based astrometry from JUICE will provide a valuable contribution.

To quantify the contribution of JUICE astrometry, we have performed the inversion of simulated optical astrometric observations by JUICE, using suitable a priori covariance to represent the radio science-only solution. Incorporating the astrometry into the ephemeris solution requires the consideration of the offset between Io’s centre-of-figure (COF, which astrometry measures) and the centre-of-mass (COM, which the ephemeris solution requires). We explicitly account for the offset between COF and COM as an estimated parameter in our model.

We assess the contribution of the optical observations to the ephemeris solution as a function of the radio science true-to-formal-error ratio (describing the statistical realism of the simulated radio science solution), as well as optical data quantity and planning. From this, we discuss to which extent space-based astrometry could help to validate the radio science solution, and under which conditions the data could improve the orbital solution of Io.
Significant contributions of astrometry to Io’s orbital solution occur for radio science true-to-formal-error ratios of 4 and higher (for the along-track and normal direction). This shows that optical space-based astrometry can improve and/or validate the radio science solution. Reductions in the obtainable uncertainties for the COF-COM-offset range from about 20 to 50 per cent – depending on the number of observations – using suitable algorithms to select the epochs at which observations are to be simulated. In particular, observations during the high-inclination phase have proven especially beneficial.

Our results show that constraints on the COM-COF offset of Io could be obtained from astrometry at the level 100 m – 1 km, depending on the quantity and planning of the observations. This could provide a novel data point to constrain Io’s interior. Moreover, the astrometric data will provide independent validation – and possibly improvement – of the orbital solution of Io.