A SETI Live conversation discussing ESA’s Juice observations of interstellar comet 3I/ATLAS during its journey through the inner Solar System, where enthusiasts and experts share insights on the comet’s unique features, composition, and trajectory, along with the significance of these findings for understanding the cosmos and the origins of such celestial bodies.
Credit: Third Party Reference
The discussion also covers the advanced technologies and methods used by the European Space Agency to make these real-time observations.
Why It Matters
Observing an object that formed around another star offers a rare opportunity to compare planetary systems beyond our own. It demonstrates how existing spacecraft can be repurposed to make unexpected scientific discoveries.
Key Science
- Juice observed 3I/ATLAS during perihelion, when Earth-based telescopes could not.
- Spectroscopic observations measured water in the comet’s coma, while imaging tracked the evolution of its dust and ion tails.
- Preliminary results suggest that although 3I/ATLAS formed around another star, its activity, coma, and tails resemble those of many comets in our own Solar System.
The campaign demonstrates how spacecraft, ground-based observatories, and international collaborations can work together to study future interstellar visitors. It is showcasing a remarkable synergy between cutting-edge technology and shared scientific ambition.
By leveraging advanced instruments and innovative research methods, scientists aim to gather unprecedented data on these cosmic travelers, enhancing our understanding of their composition, trajectory, and potential implications for our solar system.
As nations pool their resources and expertise, this collective effort not only fosters international relationships but also paves the way for groundbreaking discoveries that could rewrite our knowledge of the universe and its myriad phenomena.
Credit: Third Party Reference
When astronomers discovered 3I/ATLAS in 2025, they realized they were witnessing only the third confirmed interstellar object ever detected passing through the Solar System. The discovery immediately sparked a global observing campaign, with telescopes around the world turning toward the rare visitor.
In a recent SETI Live conversation, SETI Institute planetary astronomer Dr. Franck Marchis spoke with planetary scientist Dr. Cecilia Tubiana about a unique opportunity to study this rare visitor.
One of the most important phases of the comet’s journey posed a challenge: during perihelion, the point at which 3I/ATLAS passed closest to the Sun, Earth-based observatories could not observe the object because Earth was effectively on the opposite side of the Sun.
Rather than relying solely on ground-based telescopes, scientists turned to the European Space Agency’s Jupiter Icy Moons Explorer (Juice) spacecraft. This satellite is currently traveling toward Jupiter. Although Juice was never designed to study interstellar comets, its position in space placed it in an ideal location to observe 3I/ATLAS during this otherwise inaccessible stage of its journey.
Dr. Tubiana explained that the scientific team quickly saw the chance to observe 3I/ATLAS from space, but they encountered major logistical problems. Usually, planning for deep-space missions takes about nine months, but this effort needed to be set up in just a few months after the comet was discovered.
She noted that the approval of the campaign showed a strong scientific interest in studying material from another planetary system.
Dissecting the Chemistry of an Exocomet
The results discussed during the SETI Live conversation remain preliminary because the observations only recently arrived on Earth. Juice collected its observations between November 5 and November 25, 2025, but the spacecraft’s distance meant the data required roughly three months to reach Earth.
Scientists began analyzing the observations only in the second half of February 2026, and many aspects of the dataset are still being evaluated.
One of the primary goals was to measure the comet’s water production rate. Using the MAJIS imaging spectrometer, researchers identified spectral signatures of water molecules in the comet’s coma and used their intensities to estimate how much water vapor the comet was releasing.
Credit: Third Party Reference
Preliminary analysis shows that the comet produces a lot of water compared to some other comets. Its water output is much higher than that of the short-period comet 67P/Churyumov-Gerasimenko, which was studied during ESA’s Rosetta mission. Still, it doesn’t reach the very high activity levels seen with Comet Halley.
Dr. Tubiana noted that while comet activity often correlates with nucleus size, there are important exceptions. She pointed to Comet Wirtanen as an example of a relatively small but “hyperactive” comet, demonstrating that size alone does not determine a comet’s behavior.
A Familiar Comet from Another System
Beyond measuring water production, Juice used its Ultraviolet Spectrograph (UVS) to study how the comet interacted with solar radiation. Interestingly, NASA’s Europa Clipper spacecraft carries an identical UVS instrument, allowing both missions to conduct coordinated observations of 3I/ATLAS from different locations in space during November 2025.
Images obtained during five observing windows showed the comet evolving familiarly. As 3I/ATLAS approached the Sun, its brightness increased, its coma expanded, and both dust and ion tails became more prominent.
Credit: Third Party Reference
Using specialized ultraviolet filters, the Janus camera team successfully mapped both a traditional dust tail and a distinct ion tail, created by ionized gases being swept away by the solar wind. Researchers also observed plasma interactions that visibly altered the ion tail’s structure.
The observations show that 3I/ATLAS acts like comets in our Solar System, even though it formed around a different star. Instead of finding a completely new type of object, the findings suggest that many processes affecting comet activity are likely similar across different planetary systems.
This similarity enables scientists to compare how small bodies form and evolve beyond our Solar System with those they study nearby.
For Dr. Tubiana, this similarity was one of the campaign’s most important findings. In her words, the object is “different, but not exotic,” sharing a core similarity with the comets astronomers already know.
Building an Interstellar Observation Network
While the Juice observations revealed that 3I/ATLAS behaves similarly to comets in our Solar System, they also highlighted an exciting distinction. Dr. Tubiana noted that data from the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope show that this comet boasts a higher deuterium-to-hydrogen (D/H) ratio.
In simpler terms, a larger portion of the hydrogen in its water molecules is replaced with deuterium, a heavier form of hydrogen, which opens up fascinating possibilities for our understanding of celestial phenomena.
According to Dr. Tubiana, this elevated D/H ratio suggests that the comet likely formed in a colder environment than the one in which our own Solar System formed. She noted that this is the first interstellar object for which such a measurement has been possible, providing scientists with an important clue about the conditions that existed in another planetary system.
The discussion also highlighted how the study of interstellar objects has evolved over the past decade. When 1I/’Oumuamua was discovered in 2017, astronomers had very little time to organize coordinated observations, and many questions remained unanswered because of the object’s rapid departure from the Solar System.
Credit: Third Party Reference
By the time 2I/Borisov arrived in 2019 and 3I/ATLAS in 2025, the scientific community had developed a far more coordinated approach. Researchers around the world now rapidly share data, submit joint observing proposals, and combine observations from spacecraft and ground-based facilities to maximize the scientific return from these rare visitors.
During the discussion, Dr. Marchis emphasized the rapid formation of an international observing community for 3I/ATLAS, which enabled scientists to effectively share data and observations, enhancing the scientific yield during the comet’s short transit through the Solar System.
This collaborative approach is particularly important because interstellar objects spend only a short time within the Solar System before continuing on their journey through interstellar space. Every observation, therefore, represents a unique opportunity that cannot be repeated.
Looking ahead, Dr. Tubiana expressed confidence that future facilities such as the Vera C. Rubin Observatory will discover many more interstellar objects. As detection rates increase, scientists will be able to study a larger and more diverse population of visitors from other planetary systems rather than relying on only a handful of examples.
The 3I/ATLAS campaign shows that spacecraft in deep space can quickly be used to observe discoveries. When combined with observations from ground-based telescopes and advanced facilities, this approach creates a strong framework for studying material from other stars and for understanding how planetary systems form and change in the galaxy.
While only a few interstellar objects are confirmed, many likely enter the Solar System without being noticed. Advancements in technology and observatories are expected to improve discovery rates soon.
Earth-based telescopes couldn’t see the comet during its closest approach because Earth was on the opposite side of the Sun. Juice was in a unique position in space, allowing scientists to study the comet when it was otherwise hard to reach.
Interstellar comets carry material from the early stages of planet formation around other stars. Studying their composition helps scientists compare how planets and small bodies form in different parts of the galaxy.
Reference: https://www.seti.org/news/what-juice-revealed-3i-atlas/
