As it crossed our Solar System, the interstellar object 3I/ATLAS released up to the equivalent of 70 Olympic swimming pools of water each day. This phenomenon offers scientists a rare opportunity to study materials formed around other stars, potentially much older than our Sun.
The discovery was made by the European Space Agency (ESA) Juice probe, currently on its way to Jupiter and its icy moons. In November 2025, Juice opportunely managed to observe 3I/ATLAS with its MAJIS and JANUS instruments as the comet traversed our Solar System.
3I/ATLAS as seen by Juice's MAJIS instrument. Credit: ESA/Juice/MAJIS
Like local comets, 3I/ATLAS began ejecting material as it approached the Sun. The heat caused the ice to sublimate, transforming directly into gas. This gas escapes, forming a characteristic coma and tail, accompanied by a brighter glow than expected.
MAJIS observations detected infrared emissions of water vapor and carbon dioxide. Giuseppe Piccioni from INAF explained that these repeated detections indicate an active release of volatile ices just after the closest approach to the Sun. The estimated flow rate reaches two tons per second, or 70 Olympic swimming pools per day.
These measurements were unplanned and difficult: Juice only had short windows to spot the comet, whose emissions were very faint. The data only reached Earth in February 2026, after a long wait due to the probe's orbital position. Pasquale Palumbo, a researcher at INAF, highlighted that the images reveal for the first time the comet's intense activity near the Sun.
The images from JANUS show an extended coma, a tail, as well as structures such as rays and jets. These data will make it possible to study the comet's evolution on short and medium time scales. For the team, this is a foretaste of what Juice will accomplish around Jupiter.
This interstellar comet offers a unique opportunity to study materials preserved since the formation of other planetary systems. By analyzing its composition, scientists hope to learn more about the elements that were available during the birth of planets around distant stars, long before our Sun.