Juice Spacecraft Uncovers Surprising Water Vapor Emissions from Comet 3I/ATLAS

In a noteworthy exploration, the European Space Agency’s Jupiter Icy Moons Explorer (Juice) has provided new insights into interstellar comet 3I/ATLAS shortly after the comet’s closest approach to the Sun in November 2025. During this pivotal moment, Juice deployed five of its scientific instruments to analyze the comet’s activity, with researchers spending weeks sifting through the extensive data collected.

Initial findings indicate that 3I/ATLAS was expelling approximately 2,000 kilograms of water vapor per second, equating to the volume of around 70 Olympic swimming pools daily. This emission, while not unprecedented, sits on the higher end of expected values for comets near the Sun, such as 67P and Halley’s Comet. While the water vapor output from 3I/ATLAS remained stable during subsequent observations on November 12 and 19, detailed analysis of this data is ongoing.

Further investigations from Juice’s Submillimeter Wave Instrument (SWI) revealed that the majority of the emitted water vapor was directed toward the Sun. Remarkably, it appears that much of this vapor does not originate directly from the comet’s solid nucleus but instead from icy dust particles that have dispersed into the comet’s surrounding coma. Researchers are examining this vapor, including a comparative analysis with findings from other telescopes that detected a high ratio of ‘semiheavy’ water, suggesting the comet may have formed in extremely cold, ancient conditions influenced by ultraviolet radiation from youthful stars.

The Ultraviolet Imaging Spectrograph (UVS) on Juice captured light emitted by gases surrounding the comet, illustrating an expansive envelope of gas and dust extending over 5 million kilometers from its nucleus. Such outflows are typical of active comets, whose tails can often reach lengths of up to 10 million kilometers.

Juice’s high-resolution camera, known as JANUS, further corroborated these findings, visually documenting the comet’s behavior and structure despite its significant distance of over 60 million kilometers. The images revealed the comet’s coma and dual tails, revealing intricate interactions with solar radiation and particles.

Additionally, the Navigation Camera (NavCam), primarily intended for navigating around Jupiter’s moons, inadvertently contributed to planetary defense efforts by capturing detailed images of 3I/ATLAS. This allowed ESA’s Planetary Defence team to improve their trajectory calculations for the comet, vital for tracking its orbit and understanding the potential impacts of its dust and gas emissions.

As scientists continue to analyze the data from Juice’s instruments, Olivier Witasse, ESA Juice Project Scientist, described the encounter with the comet as an extraordinary opportunity for furthering our understanding of such rare celestial visitors. While Juice is set to reach Jupiter’s moons in 2031, it will make another pass by Earth in September 2026, allowing scientists to use its suite of instruments once again.

The excitement surrounding Juice is palpable, with researchers eager to unveil more secrets of Jupiter and its icy moons in the years to come, bolstered by the promising results already obtained from time spent observing 3I/ATLAS.