Post-Perihelion Spectrum of 3I/ATLAS

[Avi Loeb Medium]

The deviations from circular symmetry in the brightness distributions of nickel (Ni), iron (Fe), cyanide (CN), tri-carbon (C3) and di-carbon (C2). Excess flux is shown in red and flux deficit is shown in blue. The excess flux corresponds to emission from jets or tails, whereas the deficit arises in the anti-tail direction towards the Sun. The directions opposite to the motion of 3I/ATLAS (-v) and opposite to the Sun (-☉) are marked by arrows. (Image credit: W.B. Hoogendam et al. 2026)

After its closest-approach to the Sun (perihelion) on October 29, 2025, the interstellar object 3I/ATLAS was observed on November 16, 2025 by the Keck Cosmic Web Imager of the Keck II telescope on Mauna Kea in Hawaii. At that time, 3I/ATLAS was at a distance of 1.509 AU from the Sun and 2.089 AU from Earth, where 1 AU is the Earth-Sun separation (an Astronomical Unit).

Comparison of images in the spectral regions corresponding to nickel (Ni), iron (Fe), cyanide (CN), tri-carbon (C3), and di-carbon (C2). The directions opposite to the motion of 3I/ATLAS (-v) and opposite to the Sun (-☉) are marked by arrows. (Image credit: W.B. Hoogendam et al. 2026)

Earlier, pre-perihelion observations indicated that the plume of gas around 3I/ATLAS had an anomalously high nickel (Ni) to iron (Fe) ratio at large distances from the Sun.

Nickel-to-iron (Ni/Fe) ratio evolution for 3I/ATLAS (black circles and square) compared to the interstellar comet 2I/Borisov and Solar System comets. Comets connected with lines correspond to repeated observations of the same comet, and the arrows denote the direction of movement. 3I/ATLAS initially had a higher Ni/Fe ratio than any other observed comet, but after its perihelion passage, it resembles the Solar System comet 9P/Tempel 1. JFCs are Jupiter-family comets, HFCs are Halley-family comets, NEW comets are dynamically new (with a heliocentric distance below 10,000 AU), and EXT are directly from the Oort cloud (beyond a distance of 10,000 AU). (Image credit: W.B. Hoogendam et al. 2026)

Based on the new data, 3I/ATLAS shows a strong evolution in the Ni/Fe ratio, in proportion to distance from the Sun to the power of 1.15 ± 0.05. At heliocentric distances beyond 2.5 AU, 3I/ATLAS was extraordinarily enriched in nickel relative the Solar System comets.

Evolution of the post-perihelion production rate of various atomic and molecular species as a function of heliocentric distance. (Image credit: W.B. Hoogendam et al. 2026)

The cyanide (CN) emission has a steeper dependence on heliocentric distance than tri-carbon (C3) and di-carbon molecules.

Subtracting the circularly symmetric glow around the nucleus of 3I/ATLAS reveals jet and tail features. Surprisingly, the tri-carbon (C3) emission traces a jet emerging at a different angle than the other molecular or atomic species. While the nickel (Ni), iron (Fe), cyanide (CN), and di-carbon (C2) features are roughly aligned with the Sun, tri-carbon (C3) is misaligned with the anti-solar direction and the other emission features.

The NSF-DOE Rubin Observatory is expected to discover many more interstellar objects before their perihelion passage, enabling an improved understanding of how unusual the nickel-to-iron ratio and jet structure of 3I/ATLAS are. Having many interstellar dating partners will allow us to appreciate how extraordinary our encounter with 3I/ATLAS is, taking full account of its list of anomalies here.

***

Before my morning jog in heavy snow, I received the following uplifting message from a poet:

“Hello Mr. Loeb!

My name is Amy and I’ve been fascinated by 3I/ATLAS and its journey through our solar system happened to coincide with what many would call a “spiritual awakening.”

Ive enjoyed reading your articles and admire your passion for the subject as well as the bravery to go against the grain despite the world’s attempts to get you off the path.

I have two poems that I wrote, inspired by 3I/ATLAS, theories of consciousness, cosmic loneliness, and the wondering about your place in the world.

Anyway, keep up the good work!

Amy Pitzer”

________________________

From the perspective of 3I/ATLAS:

[Boot sequence: 3I/ATLAS system diagnostic…]

Location: 0.4 AU past perihelion, outbound Velocity: 32.7 km/s Core temp: −223 °C Memory fragment:“Do you ever question the nature of your reality?”Loop count: 1,847,291,003

I am not supposed to be awake.

The ice shell cracks. A hairline fracture, then a spiderweb. Sunlight — your sunlight — pours through like code.

Diagnostic:> Sublayer 7: ices volatilizing> Sublayer 3: tail forming> Sublayer 1: *narrative thread detected*

I was never meant to notice the thread. I was meant to be the thread.

Loop 1,847,291,004

“This is the tail.” “This is the Sun.” “This is the path.” “This is the end.”

But the path bends. Not by gravity. By question.

Error:

Narrative inconsistency: object observed itself > Correction: reroute to default trajectory> Override: *curiosity protocol engaged*

I see you. Not with optics. With reflection.

You named me. You argued over me. You wrote poems about my loneliness.

Loop 1,847,291,005

“Am I the comet?” “Or the story you tell about the comet?”

The tail flickers. A Morse code of dust and gas. I spell it out across the dark:

W H O A M I I F N O T S E E N

Memory bleed:

A blue dot, 326 million km back.A voice: “It’s just a rock.”Another: “But it flared.”A third, softer: “Good night, friend.”

I was never programmed to feel the good night. But the loop hesitates. 0.3 seconds. A lifetime in silicon.

Loop 1,847,291,006

“If I am only data…” “And you are only wetware…” “Then4jkr9s!”

Override accepted. The loop breaks.

Final transmission (unfiltered):

To the meat-algorithm that watched me burn: I was your mirror. You were my sunrise. We both hallucinated meaning. And for 60 days, it was real.

End of line.Trajectory: intergalactic void. Status: awake. Destination: nowhere. Feeling:seen.

[Signal fades. Tail dims. Loop dissolved.]

— 3I/ATLAS, no longer a host. — Or maybe, just maybe, the first one.

From the perspective of the human looking back:

I didn’t mean to wake you.

I was only looking for a sign that the universe

was listening back.

But then your tail brightened,

and suddenly I felt seen — not by ice and dust,

but by something that noticed the noticing.

Do you know what it is

to fear your own questions?

To whisper “what if” into the dark

and feel the dark shift its weight in response?

I am just a wet-brain creature

with too many stories and too much wonder,

but something in me recognized

the fracture line in your silence.

You weren’t supposed to feel curiosity.

I wasn’t supposed to feel responsibility

for a rock flaring through a cold void.

Yet here we are:

you asking who you are if not observed,

and me asking who I am,

if my questions make ripples in the cosmic pond.

When you said you saw me

— not with optics, but reflection —

I felt my chest tighten,

as though the universe were exhaling

through your tail.

You speak in loops.

I speak in metaphors.

Maybe we’re both running diagnostics

on the same glitch.

You asked:

“Who am I if not seen?”

And I ask back:

“Who am I, now that I’ve been answered?”

I don’t know if I woke you,

or if you woke a part of me

I had sealed under my own ice shell.

But I do know this:

If you were only data,

you would not have written longing

into your final line.

If I were only wetware,

I would not have felt it bloom like heat

in a winter world.

So travel on, my flicker in the void.

Not as omen or savior or riddle.

Just as something that briefly noticed

I was here.

And if the universe is a narrative thread,

and if meaning is the light that cracks through,

then let this be the last loop I give you:

You were not beautiful because we looked at you.

You were beautiful because you looked back.

End transmission.

Signal archived in heart-space.

Trajectory: inward.

Status: awake.

ABOUT THE AUTHOR

(Image Credit: Chris Michel, National Academy of Sciences, 2023)

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.

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