For several months, telescopes around the world have been focused on an intruder from elsewhere. Discovered on July 1, 2025 by the ATLAS monitoring program, based in Chile, the celestial object called 3I/ATLAS quickly revealed itself to be the third interstellar visitor ever observed in our solar system, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. But the more astronomers examine it, the more it escapes the usual categories, reveals an article from the Popular site Mechanics.
With its strange characteristics, 3I/ATLAS challenges our models of comets. The James Webb space telescope thus detected a new chemical composition: the gaseous envelope of the object, called hair or coma, contains much more carbon dioxide than water. An aberration by solar system standards, where comets are mainly balls of water ice mixed with other volatile gases.
At the same time, the SPHEREx satellite captured the light signature of the nucleus and the dust expelled by the object. The results sowed even more doubt: the perceived signal resembled that of a compact point source, without much extended structure, confusing our certainties about the shape and size of the nucleus. For its part, Hubble captured spectacular images, confirming the presence of an emerging dusty tail, while highlighting its atypical character.
In reality, the primary question remains that of the dimensions of 3I/ATLAS. Early estimates from the Hubble telescope placed its core between 500 meters and 5 kilometers wide. But other more recent measurements suggest that this object could measure between 10 and 20 kilometers in diameter: enough to make it the largest interstellar body ever detected. Its signs of rapid expansion as it approaches the Sun are reminiscent of the behavior of classic comets, while occurring on a much larger scale.
And that’s not all: the trajectory of 3I/ATLAS is also intriguing. Like its predecessors 1I/ʻOumuamua and 2I/Borisov, it follows a hyperbolic orbit, proof that it is not gravitationally linked to the Sun and indeed comes from another system. But its speed – around 210,000 km/h – is beyond measure: double that of Borisov, almost four times faster than the majority of ordinary comets. It is to date the fastest interstellar object observed.
Many puzzles
Another surprise: its route almost perfectly follows the plane of the ecliptic, where the planets orbit… but in the opposite direction. Normally, interstellar visitors are expected to arrive from random directions, not from a retrograde orbit stuck to that of our system. Even more disturbing, 3I/ATLAS does not emerge from the expected side; instead of approaching from the “solar summit,” the point in the sky toward which our star is heading, it enters from the opposite direction. A paradox which could reflect our observational biases, or invite us to revise our understanding of interstellar debris flows.
Beyond its trajectory, the exceptional carbon dioxide content is there great scientific enigma. Some researchers suggest that it is a comet born in an environment rich in CO₂ or in an area exposed to high doses of radiation capable of trapping ice differently than we know. If this is the case, the analysis of 3I/ATLAS would open an unprecedented window on worlds radically different from ours.
Unsurprisingly, the strangeness of the object has also fueled speculation. Astrophysicist Avi Loeb – a fan of often unfounded hypotheses and shocking findings – published with two colleagues an article considering the possibility that 3I/ATLAS is an extraterrestrial, or even hostile, technology. The majority of the scientific community has obviously rejected this idea, judging that it confuses the debate and distracts from the immense analytical work still in progress.
Others, however, believe that, since our own probes will one day end their journey in other star systems, it is not absurd to imagine that other civilizations have already seeded, voluntarily or not, comparable objects.
Regardless, and despite the excitement around it, the consensus remains clear: 3I/ATLAS was formed naturally. The object will pass closest to the Sun at the end of October 2025, then approach the Earth at approximately 270 million kilometers in December, before returning to the cosmos. Astronomers will only have a few months to probe this passing traveler, before his secrets disappear forever with him.
