Passing through our solar system, the interstellar comet designated 3I/ATLAS is expelling roughly 2,000 kilograms of water vapor every second, enough to fill 70 Olympic sized swimming pools daily.
Discovered on July 1, 2025, 3I/ATLAS is only the third interstellar object ever detected entering our solar system.
This output places 3I/ATLAS among the more active comets recorded in the inner solar system.
For comparison, Comet 67P released approximately 300 kilograms per second at a similar distance during ESA’s Rosetta mission, while Halley’s Comet vented roughly 20,000 kilograms per second at its peak.
European Space Agency, “Five Things Juice Has Revealed About Comet 3I/ATLAS,” ESA Science & Exploration (2026).
Passing through our solar system, the interstellar comet designated 3I/ATLAS is expelling roughly 2,000 kilograms of water vapor every second, enough to fill 70 Olympic sized swimming pools daily. The European Space Agency’s Juice spacecraft captured the phenomenon in November 2025 during an opportunistic observation campaign while en route to Jupiter.
Discovered on July 1, 2025, 3I/ATLAS is only the third interstellar object ever detected entering our solar system. Although Juice was launched in April 2023 primarily to investigate Jupiter’s icy moons after its arrival in 2031, its suite of onboard remote sensing instruments proved ideal for observing the comet’s volatile activity.
An unplanned observation window
When mission planners realized Juice would pass near 3I/ATLAS, they organized an ad hoc observation campaign. The team deployed five onboard instruments: the JANUS optical camera, the MAJIS imaging spectrometer, the Ultraviolet Imaging Spectrograph, the Submillimeter Wave Instrument, and the Particle Environment Package. Thermal operational constraints limited the viewing window to six 45 minute sessions and one four hour window, yielding 126 scientific data files totaling more than 11 gigabits.
On November 2, 2025, four days after the comet reached its closest approach to the Sun, MAJIS detected water vapor escaping at 2,000 kilograms per second. This output places 3I/ATLAS among the more active comets recorded in the inner solar system. For comparison, Comet 67P released approximately 300 kilograms per second at a similar distance during ESA’s Rosetta mission, while Halley’s Comet vented roughly 20,000 kilograms per second at its peak.
Plume structure and chemical dynamics
Data gathered by the Submillimeter Wave Instrument showed that most of the outgassing occurred on the comet’s sun-facing side, originating primarily from icy dust grains scattered throughout the surrounding coma rather than directly from the solid nucleus. Subsequent observations on November 12 and 19 showed that the rate of water release remained steady as the object receded from the Sun.
Measurements from Juice’s additional instruments further clarified the comet’s physical structure:
Ultraviolet spectrograph: detected atomic oxygen, hydrogen, and carbon gas, alongside a dust trail extending over 5 million kilometers from the nucleus.
detected atomic oxygen, hydrogen, and carbon gas, alongside a dust trail extending over 5 million kilometers from the nucleus. JANUS camera: captured high-resolution imagery from a distance of 60 million kilometers, revealing an extended coma and two distinct tails, one pointing away from the Sun, and another trailing along the comet’s orbital path.
captured high-resolution imagery from a distance of 60 million kilometers, revealing an extended coma and two distinct tails, one pointing away from the Sun, and another trailing along the comet’s orbital path. Trajectory tracking: navigation cameras assisted ESA’s Planetary Defence team in refining precise orbital models for the object.
Testing instruments ahead of Jupiter
Because of Juice’s orbital position following the campaign, the recorded data took several months to beam back to Earth, arriving in February 2026. The observation effort served as a test for instruments that will spend years analyzing Europa, Ganymede, and Callisto in the 2030s.
According to Giuseppe Piccioni of Italy’s National Institute for Astrophysics (INAF), analyzing the volatile composition of 3I/ATLAS offers rare insight into circumstellar chemistry beyond our own solar system, detailing physical and chemical processes that occurred around another star billions of years ago.
Source: Piccioni, G., Palumbo, P. et al. European Space Agency, “Five Things Juice Has Revealed About Comet 3I/ATLAS,” ESA Science & Exploration (2026).
