3I/ATLAS: What We Know So Far

In July 2025, a telescope in Chile caught a faint speck hurtling through space at impossible speed. It was logged as 3I/ATLAS, the third confirmed interstellar visitor after ʻOumuamua and Borisov, a cold rock passing through our system on a one-way ticket from the stars. But as more data came in, it started behaving in ways that didn’t fit any rulebook. It brightened too early, too fast, and then, as if trying to make a point, it grew a tail that pointed directly at the Sun.

Astronomers at the SETI Institute and ESA insisted it was a comet, albeit an unusual one. Others quietly disagreed. Harvard astrophysicist Avi Loeb called it “another object that refuses to obey.” He’d said the same about ʻOumuamua years earlier, when its strange acceleration sparked the first murmurs of “maybe it’s not natural.”

The story of 3I/ATLAS began with what the press called a “burst of energy”, a sudden, vertical spike in brightness that made it seem as if the object had powered on. The light curve rose at a rate proportional to r_h^{-3.8}, far steeper than any known comet. Observations from the James Webb Space Telescope revealed that nearly 87% of its volatile emissions were carbon dioxide, with barely 4% water vapour. In other words, it was venting dry ice into the void, an extreme rarity among comets, which typically rely on water sublimation closer to the Sun.

NASA dismissed talk of “energy bursts,” framing it as ordinary physics: pristine interstellar ices reacting to heat after billions of years in darkness. Yet the timing was unnerving. 3I/ATLAS began outgassing from over six astronomical units away, about the distance of Jupiter’s orbit, long before most comets even stir. A volatile this active, this far out, challenges conventional cometary thermodynamics. A few researchers proposed a natural but unlikely explanation: an abundance of CO₂ trapped near the surface, exposed by micrometeor impacts. Others quietly noted that it looked more like a system waking up than melting down.

Then came the tail. In late July, the Hubble Space Telescope captured an unmistakable anomaly, a bright, narrow extension not trailing away from the Sun but pointing toward it. Most assumed it was an optical illusion known as an anti-tail, a trick of perspective. But follow-up observations from multiple vantage points confirmed it was real: a genuine sunward jet. The official model claims this was caused by anisotropic sublimation, essentially, one side of the object venting so violently that it ejected ice grains toward the Sun faster than they could evaporate.

By September, the sunward feature vanished and a conventional tail appeared in its place, perfectly anti-sunward. The shift was textbook… almost too textbook. Loeb noted the transition’s precision resembled “a controlled reorientation” more than a random thermal phase shift. The data showed that ice grains were replaced by heavier dust particles at the exact predicted heliocentric distance, as though someone had read the physics and decided to demonstrate it.

Still, the weirdness didn’t stop there. Spectral analysis from the Keck II telescope revealed a chemical fingerprint that shouldn’t exist outside a refinery: nickel tetracarbonyl (Ni(CO)₄), an organometallic compound commonly associated with industrial processing on Earth. Scientists from the IFLScience report noted that nickel was venting in large quantities, while iron barely registered. The outgassing ratio was completely inverted from that of normal comets.

Mainstream astrophysicists rushed to explain it. Some suggested the carbonyls formed naturally when nickel dust met carbon monoxide in ultra-cold environments. Others admitted this chemistry requires very specific conditions, ones that would be difficult to sustain even in laboratory simulations. Yet the official consensus solidified: natural, albeit rare. The alternative, that an object from another star system might carry the byproducts of engineered material, was too radioactive a concept for professional careers to entertain.

Still, there were patterns. Every time an interstellar visitor has entered our system, it has done so with a touch of theatre. ʻOumuamua rotated like a solar sail and accelerated without outgassing. Borisov broke apart in a way that defied standard fragmentation models. And now ATLAS, with its dry-ice engine, backward tail, and industrial chemistry, seemed to continue the trend: nothing about these things behaves like they should.

Even its trajectory raised eyebrows. According to NASA data, ATLAS entered the Solar System on a hyperbolic path, an eccentricity of 6.1374, moving at 58 kilometres per second, faster than any previous interstellar object. More curious still, its approach was almost perfectly aligned with the Solar System’s ecliptic plane. For an object supposedly ejected at random from the galactic disk, the odds of that alignment are microscopic. Some models suggest it could only happen if the object originated from a system with orbital dynamics similar to our own. Others, less politely, call it navigation.

Around the same time, NASA quietly announced a “training exercise” to refine planetary defence tracking for “high-speed interstellar bodies,” scheduled for November 2025 to January 2026. Officially, it was just practice. Unofficially, the timing, immediately after ATLAS’s perihelion, didn’t go unnoticed. When governments start running drills right after denying concern, it usually means something’s keeping them awake.

Then came the light. Polarimetric readings from the ESA Mars orbiters and ground-based observatories showed that ATLAS reflected sunlight in a way unlike any known comet, an extremely deep, narrow negative polarisation branch, consistent with surfaces that are porous, irregular… or manufactured. The scattering pattern resembled that of composite materials designed to absorb or diffuse radiation. Natural? Possibly. Convenient? Certainly.

NASA and ESA doubled down on their explanation: nothing artificial, just unusual physics. The energy spike? CO₂ sublimation. The tail flip? Thermal evolution. The nickel gas? Exotic chemistry. And technically, they’re right, each of those explanations can exist independently. But when they all happen in the same object, within months, on a hyperbolic trajectory that brushes the plane of the planets? At some point, “coincidence” stops being a theory and starts sounding like a cover story.

It’s worth remembering that Loeb, the same astrophysicist who suggested ʻOumuamua might be a probe, isn’t alone anymore. A growing minority of researchers believe these anomalies could be deliberate, that what we call interstellar comets might, in fact, be camouflaged probes drifting between systems, studying the signatures of intelligent life. It’s not that they have propulsion; it’s that they don’t need it. When you can move at interstellar speed for billions of years, patience becomes the ultimate technology.

And if you wanted to hide something advanced, you’d make it look boring, a dusty rock, a frozen relic. The universe is full of those. Nobody looks twice.

ATLAS reached perihelion on 29 October 2025, just inside Mars’s orbit, before slipping behind the Sun. When it re-emerges in December, NASA plans to resume monitoring, mainly to check whether it fragmented. But certain teams are quietly studying another dataset: residual thermal emissions that don’t fit the expected cooling curve. If the readings persist, it would imply internal heat retention, dense, possibly metallic. Not proof of a power source, but proof of something more than ice.

Whether that “something” is natural or designed, no one knows. But the pattern across interstellar visitors is getting harder to dismiss. Each one bends the rules just enough to stay plausible, but not enough to feel accidental. Each one leaves behind a trail of unanswered questions that look increasingly deliberate.

The official consensus is still firm: 3I/ATLAS is a comet. A strange one, yes, but natural. The scientific mind doesn’t like to reach for extraordinary explanations when ordinary ones suffice. Yet the universe doesn’t seem to share that restraint. Every time we label something “unexplainable but natural,” it smiles and drops another riddle into our orbit.

Maybe ATLAS really is a frozen relic from a dead solar system. Or maybe it’s something older, something that’s been watching.

Because when an object slices through the Solar System at interstellar speed, burns brighter than it should, flips its tail like a signal flag, and bleeds industrial gas into the vacuum, it stops being “just a comet.” It becomes a question.

And as of today, that question remains unanswered.

Inline Sources: arXiv 2509.08792 | SETI Institute | ESA FAQ | Space.com | Avi Loeb – Medium | IFLScience | NASA Science | Universe Today