Kilometre-scale Jovian moon characterized for a potential JUICE flyby

Abstract

Jupiter’s irregular satellites are thought to be relics of early Solar System planetesimals. However, their small sizes, large distances from Earth and close angular proximity to Jupiter, make them difficult to characterize remotely. Here we report multi-instrument observations of the kilometre-sized Kallichore, the only irregular moon of Jupiter amenable to a close flyby of the European Space Agency’s Jupiter Icy Moons Explorer, with its astrometric and physical properties still poorly constrained. We used Hubble photometry and astrometry, followed by a ground-based multi-site stellar occultation campaign and by astrometric and photometric observations from the 10.4-m Gran Telescopio de Canarias. We could reduce Kallichore’s orbital uncertainty by up to ~80% and determine its shape: an elongated object with a minimum semi-axis ratio a/b = 1.53 ± 0.10, an area-equivalent diameter of \(3.{8}_{-0.3}^{+2.3}\) km and a dark surface with geometric albedo \(3.7_{-2.2}^{+0.7} \%\). No close companions were detected. These new constraints provide a viable pathway towards a Jupiter Icy Moons Explorer flyby once the spacecraft arrives at the Jupiter system in 2031.