Juice reveals what the interstellar comet 3I/ATLAS is hiding: five early findings that open new questions about the origin of the icy traveler from beyond the Solar System
The interstellar comet 3I/ATLAS became an exception the moment it entered the Solar System. Discovered on 1 July 2025, it was very quickly confirmed that this was not a body formed alongside the Sun, but an object that came from interstellar space. This made it only the third confirmed interstellar object recorded in our cosmic neighborhood, after 1I/ʻOumuamua and 2I/Borisov. But what makes this case even more interesting is the fact that the European spacecraft Juice also happened to be on its path, ESA’s mission to Jupiter and its icy moons, equipped with instruments that proved unexpectedly suitable for studying an active icy comet.
A few days after 3I/ATLAS passed perihelion, that is, the point of closest approach to the Sun, Juice observed the comet in November 2025 from a distance of about 60 million kilometers. The campaign itself was not routine: the spacecraft was in a warm flight phase after flying past Venus, and the teams had to assemble an observation of an object that appeared without warning and that, from Earth’s perspective, could not be tracked at all times. The data reached Earth only in February 2026, after months of waiting, and now the first analyses are beginning to form a picture of what 3I/ATLAS was really like as it passed through the inner part of the Solar System.
What the measurements suggest so far is not a spectacle in the sense of completely overturning the existing physics of comets, but perhaps something scientifically even more interesting: it turns out that a body formed outside the Solar System looks surprisingly familiar in a series of key characteristics. It is precisely this combination of similarities and chemical peculiarities that makes 3I/ATLAS one of the most important small bodies observed in recent years.
Strong water evaporation immediately after perihelion
The first and most striking early finding concerns the amount of water vapor the comet was releasing. According to the preliminary analysis of the MAJIS instrument, on 2 November 2025, just four days after passing through perihelion, 3I/ATLAS was ejecting about 2000 kilograms of water vapor per second. In an everyday comparison, this corresponds to roughly seventy Olympic swimming pools of water vapor per day. Such a figure does not mean it is an absolute record-holder among comets, but it places it in the higher part of the scale for active comets located relatively close to the Sun.
It is important to understand what such a measurement actually means. Comet nuclei are made of a mixture of ice, dust, and volatile compounds. As they approach the Sun, the surface and shallow layers begin to heat up, the ice sublimates, and gas from the interior carries dust particles out with it. The amount of gas a comet loses is therefore one of the basic indicators of its activity. For comparison, ESA’s summary indicates that the well-known comet 67P/Churyumov–Gerasimenko was releasing about 300 kilograms of water per second under comparable conditions, while Halley was significantly stronger, at about 20,000 kilograms per second. In other words, 3I/ATLAS was not an extreme case like Halley’s Comet, but it was by no means a weak or inert passerby either.
It is additionally interesting that measurements on 12 November did not show a sharp drop in activity, even though the comet was already moving away from the Sun. This points to the possibility that heated material continued to feed the coma after perihelion, or that an important role was played by already ejected icy dust that continued releasing water. Such nuances are important precisely because they help distinguish whether the activity is predominantly tied to the nucleus itself or to the cloud of particles surrounding it.
Most of the vapor came from the Sun-facing side, but not necessarily from the nucleus itself
The second important insight was provided by the SWI instrument, which also detected water vapor, but from the spatial distribution of the signal it was concluded that most of the release was taking place on the side of the comet facing the Sun. This is expected in the basic thermal sense: the sunlit side receives more energy and releases volatile ingredients more quickly. But the preliminary data suggest something even more interesting, namely that a large part of the water vapor may not have been coming directly from the solid nucleus, but from icy dust grains that had already left the nucleus and continued evaporating within the coma.
Such a scenario is important for understanding the physics of active comets. If a significant share of water comes from floating icy grains, then the coma is not just a passive consequence of nucleus activity, but also an active chemical and thermal environment in which the material continues to evolve after separation from the parent body. This changes the way spectroscopic measurements and composition estimates are interpreted, because part of what is seen no longer represents only the surface of the comet, but also the processing of material in the surrounding cloud of gas and dust.
Particular weight is given to this topic by the comparison with measurements of the ratio of ordinary water to so-called semi-heavy water, that is, H2O and HDO. In astronomy, the deuterium-to-hydrogen ratio is one of the most important chemical traces of origin, a kind of signature of the conditions in which the ice formed. Earlier observations using ALMA and the James Webb Space Telescope opened the possibility that this ratio in 3I/ATLAS is unusually high, which could mean that the comet formed in a very cold and very old environment, exposed to strong ultraviolet radiation from young stars. If the SWI data prove to be consistent with those findings, 3I/ATLAS could become a rare direct sample of the chemistry of another planetary system, and not just another interesting icy object passing by.
The tail and coma stretched for millions of kilometers
The third finding shows how spatially developed the comet was. The UVS instrument recorded the light of oxygen, hydrogen, and carbon atoms, as well as the signal of dust around and behind the comet. According to initial estimates, gas and dust extended more than five million kilometers from the nucleus of 3I/ATLAS. For comets this is not unprecedented, but it clearly indicates that the body was very active and leaving behind an enormous trail of material.
For the wider public, that information may sound abstract, but in scientific terms it is exceptionally important. The larger and more structured the cloud of gas and dust is, the more information it carries about composition, the dynamics of material ejection, and interaction with solar radiation and the solar wind. UV radiation is particularly useful because it reveals fragments of molecules created by the breakdown of water, carbon dioxide, and other compounds. In this way, scientists do not merely observe a “bright blotch” around the comet, but reconstruct the processes that created that blotch and determined its shape.
Such enormous tails and dispersed trails of material also serve as a reminder that comets are not compact snowballs passively orbiting through space. They are dynamic, changing bodies that can eject enormous quantities of gas and dust in a short time. In the case of 3I/ATLAS, this is especially interesting because the object being observed spent billions of years outside our system and has now, at least in its external manifestations, reacted to the Sun in a way that is very similar to native comets.
In appearance and behavior, 3I/ATLAS resembles an “ordinary” comet
Perhaps the most vivid result came from the JANUS camera. High-resolution images showed a coma hiding the nucleus and two tails: one directed away from the Sun, which is typical of a gas tail under the influence of the solar wind, and another that follows the comet’s path through the Solar System, corresponding to the distribution of dust. Finer structures were also observed in the coma and tails, such as rays, jets, and filaments, pointing to complex interaction processes between the ejected material, solar radiation, charged particles, and the magnetic field.
It is precisely this visual “normality” that may be the greatest surprise. When interstellar objects are discussed, the public often expects something radically different, almost exotic in every respect. But 3I/ATLAS, at least according to Juice’s observations so far, did not look like a cosmic intruder behaving according to entirely different rules. On the contrary, its activity, the structure of its coma and tails, and its response to solar heating are very close to what astronomers already know from comets formed in the Solar System.
That, however, does not diminish the importance of the discovery, but increases it. If it turns out that the fundamental physical mechanisms are similar in other planetary systems as well, then models of comet formation and evolution can be applied far more broadly than could previously be confirmed. At the same time, chemical differences, such as the possible unusually high abundance of deuterium or carbon dioxide, remain traces of special conditions in the source system. In other words, 3I/ATLAS looks familiar on the outside, but may carry a different chemical signature in its interior.
Trajectory data are also important for planetary defense
The fifth finding moves beyond the narrower framework of pure cometary science and enters the field of planetary defense. Although 3I/ATLAS posed no danger to Earth, its trajectory provided a valuable test for methods of precisely determining the orbits of bodies that are not constantly visible from Earth. Juice’s navigation camera, from a different position than Earth-based telescopes and during a period when the comet was difficult to observe from Earth, enabled additional measurements of its position and motion.
ESA had already shown earlier that observations from Mars orbit can significantly improve the prediction of 3I/ATLAS’s trajectory. Data from the ExoMars Trace Gas Orbiter reduced the uncertainty of the comet’s location by approximately a factor of ten. Observations from Juice extended that logic: when an object is tracked from deep space, from a different geometry and under different observing conditions, it is possible to determine its orbit more precisely, but also to estimate how much it is affected by the evaporation of material. This is especially important for comets, because jets of gas and dust can produce small but measurable deviations from the trajectory the body would have if it were completely passive.
Such experience has very concrete value. Planetary defense does not refer only to the detection of potentially dangerous asteroids, but also to the development of methods for the rapid, reliable, and multiply confirmed determination of the orbits of all fast and unusual bodies passing through the inner Solar System. 3I/ATLAS served as a kind of exercise in an exceptionally demanding case: the object was interstellar, fast, geometrically unfavorable for part of the observations, and additionally active like a real comet. That is exactly why the data obtained carry significance that goes beyond the boundaries of a single isolated scientific story.
Why this encounter was rare and scientifically valuable
The success of the 3I/ATLAS observations is not reduced only to the content of the data, but also to the fact that they happened at all. ESA had to adapt, in a short time, the plans of a mission primarily intended for Jupiter and its icy moons. The observations were limited by thermal conditions and organized in several short sessions, while the entire data package remained stored on the spacecraft until it became possible to send it to Earth at a higher transmission speed. The operational execution itself therefore also served as a kind of dress rehearsal for future fast and complex campaigns when Juice reaches the Jupiter system.
In the broader scientific sense, 3I/ATLAS brings something that laboratories on Earth cannot offer: direct insight into material formed around some other star. Such objects are not mere curiosities. They are archives of the early phases of planetary system formation, preserved in deep freeze for billions of years. When such an object enters the Solar System and in doing so develops an active coma, scientists get a rare opportunity to remotely analyze gases, ice, and dust that formed in a completely different cosmic environment.
That is why even the preliminary results carry such weight. The large amount of water vapor, the spatially developed coma, the two tails, and the behavior resembling an “ordinary” comet suggest that the basic physics of sublimation and coma development may not be a peculiarity of our system. At the same time, open questions about isotope ratios, the richness of carbon dioxide, and the possible very ancient origin of 3I/ATLAS indicate that the chemical story is more complex and potentially far more exotic than the comet’s appearance alone reveals.
What comes next for Juice and for the study of 3I/ATLAS
Scientific teams are only now continuing with the full analysis of the data, and some of the results are expected in scientific papers over the coming months. This means that the current conclusions are not yet the final word, but the first layered sketch of an exceptionally valuable set of observations. Even that sketch, however, already shows that Juice took advantage of a unique opportunity and extracted from a side episode on the way to Jupiter data that will be analyzed for a long time.
For the mission itself, this is an additional signal that the instruments are working convincingly and far from the final destination. According to the plan, Juice should reach Jupiter in 2031, where it will study the planet and its large icy moons in detail, primarily Ganymede, Europa, and Callisto. Before that, a new gravity-assist maneuver awaits it on its return toward Earth in September 2026. After the experience with 3I/ATLAS, every new activation of the instruments will also carry an additional dose of expectation. Because if a spacecraft intended for icy worlds has already managed to extract so much from a single rare interstellar comet, then the prospects for what it will discover in the Jupiter system are even greater than at the beginning of the mission.
Sources:- European Space Agency (ESA) – overview of how the Juice mission organized and carried out the observation of 3I/ATLAS, with observation dates, distance, and operational details (link)- European Space Agency (ESA) – official overview of ESA’s observations of the interstellar comet 3I/ATLAS from its discovery onward (link)- NASA Science – basic overview of comet 3I/ATLAS, its discovery, trajectory, perihelion, and observations by multiple space missions (link)- NASA Goddard / JWST – summary of results pointing to a very CO2-rich coma of comet 3I/ATLAS (link)- European Space Agency (ESA) – analysis of the improvement of 3I/ATLAS’s trajectory with the help of data from Mars orbit and the value of such observations for planetary defense (link)- European Space Agency (ESA) – Juice factsheet confirming the mission’s goals, arrival at Jupiter in 2031, and return toward Earth as part of gravity-assist maneuvers (link)
Find accommodation nearby
Creation time: 1 hours ago