3I/ATLAS Has Flipped Its Tail Toward the Sun, A First in Comet Physics

Interstellar visitors don’t drop by often, but when they do, they behave like messengers from a much colder, less civilised part of the galaxy. They cut across the Solar System on one-off trajectories, violate a few rules of polite celestial behaviour, and are gone again before the arguments about what they were have even settled.

3I/ATLAS is the latest of these uninvited guests.

And it wasted absolutely no time in causing trouble.

On 11 November, the Nordic Optical Telescope captured the object throwing out a plume apparently aimed toward the Sun, a bizarre echo of what had already been seen in July, when the Hubble Space Telescope recorded a pronounced sunward anti-tail in its early images. The observers behind the Nordic image, David Jewitt and Jane Luu, have built careers on small bodies and outer-system oddities. When their observations raise eyebrows, it isn’t theatre. It means something is genuinely off. In standard comet physics, jets and tails point away from the Sun as sublimating ices are pushed outward by radiation pressure. A jet pointing into the Sun is like smoke drifting back into the fire. It’s not how the diagrams in the textbooks go.

The universe refused to let this be a simple illusion

The first line of defence was geometric. Anti-tails are a known phenomenon: if large dust grains trail behind a comet in its orbit and the Earth happens to sit close to the orbital plane, that structure can appear to point toward the Sun. It’s the standard “nothing to see here” explanation.

But this time the data didn’t cooperate.

Detailed modelling of the July structure showed that the misalignment angle between the Sun–comet axis and our line of sight was small, only about ten degrees. Correcting for that geometry stretched the apparent anti-tail by a factor of nearly six, making it clear that this wasn’t a faint visual quirk but a real, physically extended feature that needed to be explained dynamically, not dismissed as perspective.

Then it changed.

By late August, the coma had grown more symmetric, and just days later, images from Gemini South reported that the sunward anti-tail had completely vanished, replaced by a standard tail pointing away from the Sun. Whatever was driving the sunward feature had flared, exhausted its particular volatile reservoir, and shut down.

By November, the Nordic Optical Telescope confirmed that 3I/ATLAS was still a single intact body and still showed a pronounced sunward elongation in its post-perihelion activity.

The illusion explanation was gone.

The object had physically vented material toward the Sun.

The recoil exposed a deeper contradiction

Gas goes one way; the nucleus recoils the other. Newton doesn’t negotiate on that.

So the orbit was examined in detail. When astronomers fitted 3I/ATLAS’s trajectory using Minor Planet Center and JPL data, a purely gravitational solution simply didn’t fit. Residuals remained that indicated a persistent non-gravitational acceleration of roughly 5 \times 10^{-7}\,\text{m s}^{-2}, comfortably within the range expected for active comets but unmistakable in the measurements. The strange part is that even though the visible jet appears sunward, the recoil acceleration points anti-solar, away from the Sun, exactly as you’d expect for ordinary outgassing. That means the venting isn’t being driven by direct surface heating on the sunlit side. The gas is coming from regions not simply facing the Sun at that moment.

In other words, this is rotational venting: heat seeping into an insulating crust, pressure accumulating beneath it, and jets turning on when fractures rotate into the right orientation. Monte Carlo jet simulations modelled this behaviour and reproduced the observed acceleration direction to within a degree, suggesting the jet geometry isn’t random at all but tightly constrained by the object’s spin and internal structure.

3I/ATLAS isn’t just active. It’s active in a way that doesn’t map neatly to the simple “sunny side bakes, tail points away” picture we’re used to from Solar System comets.

Its chemistry belongs to a colder, harsher neighbourhood

Another problem: 3I/ATLAS switched on early. Activity began when the object was around 4.5 AU from the Sun, well beyond the point where water ice can drive substantial sublimation. Something else was powering that early coma.

Thermal models and follow-up work pointed squarely at super-volatiles like CO₂ and CO, ices that sublimate at far lower temperatures than water. The sunward anti-tail observed in July is thought to have been dominated by water-ice grains blasted out by rapidly sublimating CO₂ beneath the surface, a short, violent episode that faded once that reservoir had been depleted. Those thermal profiles strongly suggest that 3I/ATLAS formed in the extreme cold, far outside the CO₂ ice line of its home system. That makes it a particularly pristine fragment of another star’s outskirts, chemistry from a region our own system simply doesn’t replicate. This object is not just dynamically foreign. It is chemically foreign too.

Its size, speed and orbit deepen the unease

On top of that, 3I/ATLAS is enormous. Depending on the assumed reflectivity, its diameter is estimated to lie between 5 and 20 kilometres, orders of magnitude larger than ʻOumuamua and significantly larger than Borisov. Its size and kinetic energy have already been discussed in the context of “what if” impact scenarios, because humans can’t see a big fast rock without mentally aiming it at themselves.

It also arrived fast, at around 60 km/s relative to the Sun, and yet, somehow, this interstellar intruder crosses the Solar System on a retrograde trajectory aligned to within a few degrees of the planetary ecliptic. That alignment has been noted as having only around a 0.2% chance of occurring by accident, if you assume random incoming orientations.

Put bluntly: it’s big, it’s fast, it’s chemically odd, and it has the social grace to align almost perfectly with the plane of our planets as it charges through.

At that point, people start entertaining less comfortable possibilities. Some commentary has suggested that an object venting in this way could, in principle, be configured to gain speed rather than slow down, raising the usual “what if it isn’t entirely natural?” question. More conservative voices have argued that, while 3I/ATLAS is indeed extreme, its behaviour remains consistent with a volatile-rich comet from a colder system, provided models are flexible enough to accommodate its thermal and structural quirks.

That’s how scientific caution manifests: acknowledge the anomaly, then carefully wall it off from implications you’re not ready to confront.

Three interstellar visitors, three different flavours of wrong

We now have three confirmed macroscopic interstellar objects.

ʻOumuamua: a small, elongated body with a measurable non-gravitational acceleration but no detectable dust or gas emission. Borisov: a more conventional-looking comet, but with an unusually high abundance of CO compared to typical Solar System objects. 3I/ATLAS: a massive, volatile-rich body with a physically real sunward anti-tail, a non-gravitational acceleration tied to its activity, and a formation temperature regime alien to our own.

The emerging pattern is that interstellar objects are not boring. They are chemically extreme, dynamically awkward, and structurally inconvenient. That’s why frameworks like the Interstellar Object Significance Scale have been proposed, a kind of Richter scale for weirdness. On that scale, both ʻOumuamua and 3I/ATLAS have been classified at Level 4: anomalous, worthy of heightened scrutiny, but not yet demanding a non-natural explanation. Borisov sits down at Level 0 as the “normal” one, which says a lot about the company it keeps. Two out of three interstellar visitors already sit in the anomaly band. That’s not a nice, symmetric curve. It’s a hint that our assumptions are wrong.

If 3I/ATLAS is in any way representative of what’s out there, then our Solar System is not the standard template. It’s the quiet suburb in a very loud galaxy, a place where icy bodies are relatively tame compared to the volatile-loaded, rotation-tormented things being thrown out of other systems.

3I/ATLAS doesn’t just misbehave. It suggests that the universe beyond our little bubble is colder, sharper and stranger than the “average Solar System” illusion we’ve been comforting ourselves with.

And every time something from out there passes through here, that illusion gets a little harder to maintain.