Observations of 3I/ATLAS with JUICE-SWI

The Submillimeter Wave Instrument (SWI) aboard the JUpiter Icy Moon Explorer (JUICE) conducted a series of targeted observations of the interstellar comet 3I/ATLAS, discovered on July 1, 2025, by the ATLAS survey in Chile. As the third confirmed interstellar object to traverse our solar system—following 1I/ʻOumuamua and 2I/Borisov—3I/ATLAS offers a unique opportunity to study the composition and physical properties of material from outside the Solar System. JUICE-SWI executed dedicated observations of the comet between November 2 and November 19, 2025, during which the SWI instrument performed four 30-minute observations at a distance of approximately 80 million kilometers. The observations targeted the ground-state rotational transitions of ortho- and para-water in the 600 GHz and 1200 GHz frequency bands, using frequency-switching mode to maximize on-source integration time. We present the data reduction, calibration procedures, and  results from the SWI observations. These include derived rotational line intensities and the water production rate. We discuss the implications of the measurements in the context of interstellar comet composition, comparing them with other observations. The results provide early insights into the volatile content of an interstellar visitor and contribute to our understanding of the chemical diversity of cometary material across stellar systems.