3I/ATLAS Astrometry and Orbital Dynamics from JUICE NAVCAM

Background: The interstellar comet 3I/ATLAS was discovered in July 2025. Its particular orbit, passing through the Solar System on the opposite side relative to Earth, made it hardly observable during its perihelion passage, where the cometary activity is the strongest. In November 2025, ESA’s Jupiter Icy Moons Explorer (JUICE) performed a dedicated observation campaign of the comet during its cruise phase, reaching 0.4 AU distance at closest approach. Many remote sensing instruments participated and collected data of the interstellar visitor.

Idea: After the successful pointing, images of the Navigation Camera (NAVCAM) were downlinked and analysed astrometrically. Due to the orbital geometry of JUICE relative to 3I/ATLAS, the measured positions carry significant weight in the orbital solution of the comet, allowing a resolution on the order of a few hundred kilometres of its position in space.

Purpose: The astrometric data can be used to dynamically assess the orbit of 3I/ATLAS around perihelion passage, especially to measure the effect of non-gravitational effects on the comet’s trajectory due to its outgassing.

Results: The NAVCAM images provided a successful performance demonstration in JUICE’s early mission phase. The analysis shows good resolvability of non-gravitational effects in the orbit. Depending on the types of gasses involved in the outgassing at a certain heliocentric distance, the trajectory experiences slightly different perturbations. Observations by other instruments onboard JUICE (MAJIS, SWI) confirm H₂O outgassing around perihelion, whereas earlier spectroscopy measurements from Earth detected high CO₂ abundances in the coma at farther heliocentric distances. NAVCAM astrometry is used to discriminate the dominating volatile from orbital dynamics.

Outlook: The dynamics of the interstellar comet suggest a varying outgassing profile throughout time, indicating the need for more complex formulations of non-gravitational acceleration models for highly eccentric orbits. Using interplanetary spacecraft for the tracking of Small Bodies in the Solar System has proven to be a valuable asset for precise orbit determination in cases when Earth-based observations are limited.