Jupiter’s irregular moons are considered time capsules from the early Solar System, but most are so small and distant—and appear so close to Jupiter in the sky—that astronomers struggle to pin down their orbits and physical properties. Now, a new multi-instrument campaign has delivered unusually detailed constraints on Kallichore, a kilometre-sized irregular satellite of Jupiter that is uniquely promising for a close encounter.
The work targets the European Space Agency’s Jupiter Icy Moons Explorer (JUICE), planned to arrive in the Jupiter system in 2031. Until now, Kallichore’s astrometric uncertainty and shape estimates were too loose to confidently assess the feasibility of a flyby. Using a carefully coordinated set of observations, the researchers aimed to refine Kallichore’s trajectory and determine key physical parameters.
The team began with Hubble Space Telescope data, combining photometry and astrometry to improve the object’s measured position and brightness. Hubble’s resolution is crucial for separating a faint, fast-moving target from the glare of Jupiter and background sources, providing a baseline for subsequent ground-based measurements.
Next came a global ground-based stellar occultation campaign. By timing when Kallichore passed in front of distant stars, the observers extracted direct constraints on its size and geometry. Stellar occultations remain one of the strongest tools for kilometre-scale bodies, because they can reveal silhouette-scale information even when imaging fails.
To further sharpen the dynamical picture, the researchers added astrometric and photometric observations from the 10.4-m Gran Telescopio de Canarias. This multi-site strategy reduces systematics related to local weather, instrument calibration, and observing conditions, thereby tightening the orbit more effectively.
The results are striking: the orbital uncertainty was reduced by as much as ~80%. With the improved trajectory, the team could also infer Kallichore’s shape from combined constraints, finding it to be an elongated object with a minimum semi-axis ratio of a/b = 1.53 ± 0.10.
From the occultation-derived effective dimensions, Kallichore’s area-equivalent diameter is reported as 3.8-0.3+2.3 km. The surface appears dark, with a geometric albedo of 3.7-2.2+0.7%, consistent with primitive, low-reflectivity material expected for captured small bodies.
Importantly for mission planning, the study found no evidence of close companions—an essential factor when evaluating encounter risk and navigation. Together, the findings outline a viable pathway toward a JUICE flyby, turning Kallichore from a poorly known dot into a quantified target that spacecraft operations can design around.
Subject of Research:
Jupiter’s irregular satellites; characterization of the moon Kallichore for a potential JUICE flyby.
Article Title:
Kilometre-scale Jovian moon characterized for a potential JUICE flyby.
Article References:
Rizos, J.L., Gómez-Limón, J.M., Kilic, Y. et al. Kilometre-scale Jovian moon characterized for a potential JUICE flyby. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02929-z
Image Credits:
AI Generated
DOI:
https://doi.org/10.1038/s41550-026-02929-z
