On October 30, 2025, the comet known as 3I/ATLAS, the third confirmed interstellar object to visit our Solar System, reached its closest approach to the Sun (perihelion).
This remarkable milestone has excited astronomers worldwide, offering a rare window into the chemistry, origin, and behavior of a comet formed beyond our own stellar neighborhood.
Discovery & First Glimpse.
3I/ATLAS was discovered on July 1, 2025, by the Asteroid Terrestrial‑impact Last Alert System (ATLAS) telescope in Chile.
The “3I” designation marks it as the third object of interstellar origin ever observed (after 1I/ʻOumuamua and 2I/Borisov).
At discovery, it appeared faint (magnitude ~17-18) and only slightly extended—but follow-up observations revealed a coma, indicating the object was actively sublimating despite being far from the Sun.
A Unique Composition.
Early spectroscopic data revealed something unexpected: the comet’s coma showed a dominance of CO₂ gas with only minimal water vapor, even at roughly 3 astronomical units (AU) from the Sun—where water ice would normally begin to sublimate.
One explanation: the sublimation of CO₂ may cool the comet’s surface, delaying the onset of water ice sublimation. Another possibility: as an interstellar object, 3I/ATLAS may have formed under entirely different chemical and physical conditions—resulting in unusual ice composition and behaviour.
Orbit & Behaviour — A One-Time Visitor.
3I/ATLAS is on a hyperbolic orbit, meaning it will not return to our Solar System—it came from interstellar space and will leave again.
As it approached perihelion, astronomers noticed non-gravitational accelerations—tiny deviations in its orbit caused by jets of gas and dust streaming from its surface, acting like miniature thrusters.
Data from the global network of citizen astronomers using the Unistellar Network helped detect a sudden surge in brightness just before the comet passed behind the Sun (from Earth’s perspective). This brightening exceeded predictions, hinting at an outburst or increase in surface activity.
The Nickel-Iron Mystery.
One of the most puzzling findings: Spectra from the European Southern Observatory’s Very Large Telescope (VLT) revealed a higher nickel-to-iron ratio than typical Solar-System comets. While most comets release nickel and iron in roughly solar proportions, 3I/ATLAS showed strong nickel emission but weak iron—although the ratio began to normalise as it moved closer to the Sun.
This suggests that either the temperature-dependent release of these metals or the original chemical pathways in the comet’s birthplace differ from what we are used to. This offers a rare glimpse into interstellar matter and helps frame future studies of cometary composition beyond our own system.
Why This Matters for Science.
Studying 3I/ATLAS is more than just a curiosity—its interstellar origin means it carries chemical fingerprints from a system entirely different to ours. By comparing its composition and behavior with comets native to our Solar System, scientists hope to unravel how planetary systems form and evolve under different conditions.
Additionally, this event serves as a rehearsal for future missions such as the Comet Interceptor by the European Space Agency (ESA)—which is being designed to intercept the next interstellar visitor. As Dr. Franck Marchis put it: “We are learning how to observe, interpret, and react quickly so that next time, we’ll be ready to send a spacecraft.”
What’s Next.
With 3I/ATLAS now heading back into the depths of interstellar space, it will fade from view for Earth-based telescopes. The wealth of data collected—from major observatories and citizen scientists alike—will nonetheless yield insights for years to come.
For space-watchers, the takeaway is clear: when the next interstellar visitor arrives, we’ll be better prepared—thanks to what we’ve learned from this extraordinary encounter.