Kilometre-scale Jovian moon characterized for a potential JUICE flyby
arXiv:2607.28357 · doi:10.1038/s41550-026-02929-z
The paper presents new observations of Jupiter’s irregular moon Kallichore, using Hubble, ground‑based telescopes and stellar occultations to refine its orbit, size, shape and surface reflectivity, enabling a potential JUICE spacecraft flyby in 2031.
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 kilometre-sized Kallichore, the only irregular moon of Jupiter amenable to a close flyby by ESA's Jupiter Icy Moons Explorer (JUICE), whose astrometric and physical properties were still poorly constrained. We used Hubble photometry and astrometry, followed by a ground-based, multi-site stellar occultation campaign and astrometric and photometric observations with the 10.4-m Gran Telescopio Canarias. We reduced Kallichore's orbital uncertainty by up to approximately 80% and determined its shape: an elongated object with a minimum semi-axis ratio of a/b = 1.53 +/- 0.10, an area-equivalent diameter of 3.8 (+2.3/-0.3) km and a dark surface with a geometric albedo of 3.7% (+0.7/-2.2). No close companions were detected. These new constraints provide a viable pathway towards a JUICE flyby once the spacecraft arrives in the Jupiter system in 2031.
Published in Nature Astronomy on 24 July 2026