When interstellar comet 3I/ATLAS swept past the Sun in November 2025, ESA's Jupiter-bound spacecraft Juice happened to be in the right place — and made the most of it. Five science instruments were switched on, collecting what may be a once-in-a-lifetime dataset on a visitor from another star system.
The numbers are striking: 2,000 kg of water vapour per second, or roughly 70 Olympic swimming pools per day, just four days after perihelion. Most of this outgassing came from the Sun-facing side of the comet — and much of it not from the nucleus itself, but from icy dust grains suspended in the surrounding coma. The gas and dust tail extended at least 5 million kilometres from the nucleus.
The water tells a deeper story. ALMA and Webb had already found an unusually high ratio of heavy to normal water in 3I/ATLAS — a potential fingerprint of formation in an ancient, extremely cold environment bathed in UV radiation from young stars. Juice's SWI instrument is now checking whether its own measurements align with that picture.
Perhaps the most striking finding, though, is the simplest one: despite its interstellar origin, 3I/ATLAS behaved just like an ordinary Solar System comet during its solar encounter. "Extreme but not exotic," as ESA put it. As a bonus, Juice's navigation camera — built to guide the spacecraft around Jupiter's moons — was used to refine the comet's trajectory and begin estimating how much material it shed along the way.