Interstellar space · ATLAS (Chile).3I/ATLAS (C/2025 N1)

On 1 July 2025, the NASA-funded ATLAS network (Asteroid Terrestrial-impact Last Alert System) detected from its station in Río Hurtado (Chile) an object on a hyperbolic trajectory. Confirmed as interstellar, it was designated 3I/ATLAS (C/2025 N1): the third visitor from outside the Solar System on record, after 1I/ʻOumuamua (2017) and 2I/Borisov (2019). Unlike ʻOumuamua, it showed cometary activity early, coma and jets, so astronomy classified it as an interstellar comet.

Observing campaigns (Hubble, JWST, VLT, Subaru, and even the Parker Solar Probe) revealed unusual chemistry: a CO₂/H₂O ratio of 7.6 measured by JWST at 3.3 AU from the Sun, among the highest ever observed in a comet and 4.5 sigma above the trend for long-period and Jupiter-family comets, with little water in the gas; nickel atoms with very little iron before perihelion; and several jets of material in specific directions. Part of that oddity faded with time: after perihelion, VLT spectra also showed iron, and the nickel-to-iron ratio evolved toward values comparable to Solar System comets; in December 2025 carbon monoxide was the most abundant molecule, followed by water and CO₂, and JWST detected methane in an interstellar object for the first time. It passed perihelion on 29 October 2025, at 1.36 AU from the Sun (between the orbits of Earth and Mars) and passed Earth at 1.8 AU, with no risk at all.

Astrophysicist Avi Loeb publicly catalogued a growing list of 3I/ATLAS anomalies, which reached more than two dozen, and argued that several were not predicted by standard cometary models. Among them: a nickel-to-cyanide ratio far higher than in thousands of known comets, reminiscent of that in industrial nickel alloys; a sunward jet (anti-tail) that does not appear to be a mere perspective effect; a nucleus estimated to be far more massive than those of ʻOumuamua and Borisov; and, above all, a non-gravitational acceleration detected near perihelion with a substantial sideways component, not simply directed away from the Sun as one would expect from a comet's sublimation. Loeb noted that, to explain that acceleration by evaporation, the object would have had to shed a considerable fraction of its mass without visibly disintegrating.

The nuance matters, and Loeb himself stresses it: these anomalies do not necessarily point to a technological origin. His explicit conclusion is that they should be documented and studied in detail, keeping open both the unusual-interstellar-iceberg hypothesis and the exotic one, without asserting the latter. The scientific consensus (NASA, ESA and most of the cometary community) holds that it is a genuine interstellar comet: unusual in its chemistry and dynamics, but natural, and it recalls that non-gravitational accelerations are routine in active comets.

3I/ATLAS shows how the study of an anomalous astronomical object should work: an object detected in time by a publicly funded institutional network, immediately followed by the world's largest telescopes (Hubble, JWST, the VLT, Subaru and even the Parker Solar Probe), with open, peer-reviewed data, and still with properties that surprised the models. It is the mature counterpoint to ʻOumuamua: that one passed too fast and too faint to characterize, leaving a vacuum that speculation filled; this time astronomy could measure, photograph and debate the object in real time.

It serves above all as a case study in separating the real anomaly from over-interpretation. There are confirmed, measured oddities (a CO₂/H₂O ratio among the highest on record, nickel with very little iron before perihelion, jets in specific directions, a non-gravitational acceleration with a sideways component) that science acknowledges and still studies. The decisive thing is what the community does not do with them: neither NASA nor ESA jumps from 'unusual' to 'artificial', because the diversity of objects from other planetary systems is enormous and we have barely begun to sample it, and because mechanisms like non-gravitational acceleration have cometary precedent. And most instructive: even the person pushing the exotic question hardest, Avi Loeb, deliberately stopped at 'document and explore' the anomalies rather than asserting they are technology. That discipline (anomaly yes, exotic conclusion no until the data demand it) is the epistemic standard that holds for all UAP material: taking what surprises seriously without turning surprise into confirmation.