Direct physical and astrometric characterisation of a kilometre-scale Jovian moon for a potential JUICE flyby

Jupiter’s irregular satellites are thought to be relics of the early outer Solar System, yet their small sizes and large distances make them challenging to characterise, limiting their feasibility for in situ exploration. Kallichore, a kilometre-scale member of the retrograde Carme group, is currently the only irregular satellite potentially accessible to a close flyby by ESA’s JUICE mission, now en route to Jupiter with arrival scheduled for 2031. However, its poorly constrained orbit and size pose a major challenge for encounter planning and execution. We report Hubble Space Telescope astrometry and photometry, a stellar occultation by Kallichore, and follow-up astrometric and photometric observations with the 10.4-m Gran Telescopio de Canarias. Three positive occultation detections allow us to determine the astrometric position of Kallichore with sub-milliarcsecond precision, corresponding to a sky-projected positional accuracy of about 1 km. From the occultation geometry and complementary photometry, we constrain an area-equivalent diameter of 3.8 (+2.3 / −0.3) km, infer an elongated shape with a minimum semi-axis ratio of a/b = 1.53 ± 0.10, and estimate a geometric albedo of 3.7 (+0.7 / −2.2)%. No close companions were detected. This represents the first direct physical characterisation of the smallest outer Solar System satellite measured from Earth. Our astrometric measurements reduce Kallichore’s orbital uncertainty by up to 80%. These constraints on both orbit and physical properties establish a viable pathway toward a future JUICE flyby. If Kallichore originates from primordial trans-Neptunian populations as a captured fragment, such an encounter would provide access to one of the most pristine remnants of the early outer Solar System.