- 3I/ATLAS is only the third interstellar object confirmed in our solar system.
- Its CO2-to-water ratio is 60 times higher than any solar system comet.
- The unusual chemistry may reflect cosmic ray processing during interstellar travel.
On the night of July 1, 2025, the ATLAS telescope in Chile's Atacama foothills was doing what it always does: scanning for rocks that might hit Earth.
What it found instead was 3I/ATLAS, the third confirmed interstellar comet ever observed passing through our solar system, and one carrying chemistry that no one had anticipated.
The object was moving too fast. Its trajectory described a curve that no solar gravity could explain. Within hours, archival searches turned up pre-discovery images stretching back to May, when NASA's planet-hunting TESS satellite had unknowingly captured the comet months before anyone was looking for it.
The first image of comet 3I/ATLAS, taken when it was discovered on July 1, 2025, by the ATLAS survey telescope in Chile. Image credit: NASA
A day after the initial report, David Jewitt and Jane Luu, observing with the Nordic Optical Telescope in the Canary Islands, confirmed the object was "clearly active," with a diffuse coma already forming around its nucleus.
The designation 3I/ATLAS followed quickly. The "I" stands for interstellar. The "3" means we have now seen three of them.
A Brief History of Interstellar Comets
Two interstellar visitors preceded 3I/ATLAS. The first, 'Oumuamua, arrived in 2017 as a puzzling elongated rock with no visible coma or tail, tumbling in a way that generated years of debate about its nature and origin.
The second, 2I/Borisov, reached us in 2019 and behaved rather more like a conventional comet: icy, active, its coma containing the volatiles astronomers expected.
Between them, they suggested interstellar space might routinely scatter material between planetary systems. Field estimates put the crossing rate at roughly one object per year, most of them passing undetected.
With three confirmed objects, astronomers could at last begin asking whether interstellar comets share common traits or whether each carries the idiosyncratic chemistry of a different birthplace.
The answer, for 3I/ATLAS, turned out to be unexpectedly complicated.
The interstellar comet 3I/ATLAS observed by the Hubble Space Telescope on November 30, 2025, using the Wide Field Camera 3 instrument. Image Credit: NASA
A Trajectory Nothing Else Could Explain
The numbers describing 3I/ATLAS's path are genuinely unusual. Orbital eccentricity measures how curved a trajectory is: anything below 1 is a closed orbit, bound to return. Solar system comets cluster well below that threshold.
'Oumuamua registered 1.2. 2I/Borisov reached 3.4. The eccentricity of 3I/ATLAS came in at 6.14, more than double its predecessor's, producing a trajectory so close to a straight line that the comet appeared barely to notice the Sun's gravity.
It passed perihelion on October 29, 2025, reaching its closest approach at 1.36 astronomical units from the Sun, just inside the orbit of Mars. At that moment it was moving at 68 kilometres per second. It reached its closest point to Earth on December 19, 2025, still some 270 million kilometres away, before beginning its outbound journey.
By March 2026, it had passed Jupiter. It will not return.
What made the trajectory notable beyond its raw numbers was the early activity. TESS data showed 3I/ATLAS already outgassing in May 2025, when the comet was still beyond Jupiter's orbit at roughly 6.4 astronomical units from the Sun.
At that distance, water ice in a solar system comet would be nearly inert. Something more volatile was driving the activity, and that something would become the central puzzle of the entire encounter.
What is orbital eccentricity?
Eccentricity describes how curved an orbit is. Values below 1 mean the object eventually returns. Above 1 means a hyperbolic path: a one-way trip. 3I/ATLAS registered 6.14, the highest ever recorded for any observed object passing through our solar system.
Not the Visitor We Imagined
When the James Webb Space Telescope trained its infrared instruments on 3I/ATLAS in August 2025, the comet was at roughly 3.3 astronomical units, the distance of the inner asteroid belt. The spectrum was not what anyone expected.
Martin Cordiner, an astronomer at the Catholic University of America who led the JWST observations, described the CO2-to-water ratio as "among the highest ever seen in any comet." The number his team measured came in at approximately 8 to 1. In solar system comets, that ratio typically sits around 0.12, and has rarely been observed above 0.3. The gap placed 3I/ATLAS roughly 4.5 standard deviations from the cometary norm.
Carbon dioxide was not a trace gas. It was the dominant volatile by a wide margin.
Key figure
8:1
CO2-to-water ratio in 3I/ATLAS's coma, versus 0.12 in typical solar system comets
NASA's SPHEREx observatory confirmed the picture. Observing across early August, it detected a vast CO2 cloud extending at least 348,000 kilometres from the nucleus, and found no detectable water vapor at all. The carbon dioxide was venting at roughly 70 kilograms per second. Water remained below the detection limit.
Stefanie Milam, an astrochemist at NASA's Goddard Space Flight Center who worked on the JWST observations, offered a hypothesis. Floating through the near-absolute-zero void for potentially billions of years, bombarded continuously by galactic cosmic rays, the outer layers of 3I/ATLAS may have undergone sustained chemical conversion.
CO would have been progressively transformed into CO2 within a processed crust extending roughly 15 to 20 metres deep. That outer shell was now heating in sunlight and sublimating, filling the coma with altered chemistry rather than anything pristine from the comet's birthplace.
The implication was quietly striking. We might not be reading the chemistry of another solar system at all. We might be reading the chemistry of interstellar space itself.
Nickel Without Iron
While the JWST team mapped carbon dioxide, a separate group working with the Very Large Telescope in Chile encountered a different anomaly.
Thomas Puzia, an astronomer at the Pontifical Catholic University of Chile, announced the detection of atomic nickel in the coma. Solar system comets do occasionally show nickel, but always accompanied by iron, the two metals sublimating together from iron-nickel dust grains.
In 3I/ATLAS, the iron signal was absent.
The VLT team proposed an explanation: nickel tetracarbonyl, a highly volatile organometallic compound that breaks down under ultraviolet light to release nickel and carbon monoxide while leaving iron behind. If confirmed, this would represent a chemical process never previously observed in any comet.
We just cracked open the door to a whole new world of chemistry that we never had access to before.
Thomas Puzia, Pontifical Catholic University of Chile
Post-perihelion observations deepened the puzzle further.
When JWST's MIRI spectrometer examined 3I/ATLAS in December 2025, as the comet was retreating at distances between 2.2 and 2.5 astronomical units, it detected something entirely absent from earlier observations: methane. The molecule appeared for the first time after perihelion, suggesting it lay beneath the CO2-dominated outer shell and was only exposed when solar heating finally penetrated deep enough.
The timing raised a question that remains unresolved. Carbon monoxide, a gas more volatile than methane, had been tentatively detected before perihelion. Methane, which should have sublimated first if surface temperature were the only factor, appeared only afterward.
The sequence implies a layered interior with a complex thermal history, not a simple onion of progressively volatile ices.
Reading the Scars of a Long Journey
The picture that emerges from 3I/ATLAS carries a particular irony for the study of interstellar objects.
The instinctive assumption was that a comet from another star system would function as a chemical messenger from its point of formation, carrying material that could reveal the conditions of a distant protoplanetary disk. 3I/ATLAS appears to complicate this idea considerably.
What outgassing has revealed so far is primarily the record of the object's journey through interstellar space, the processed outer layer rather than the pristine interior.
Research modelling galactic cosmic ray irradiation suggests the outer 15 to 20 metres of a comet with a long interstellar residence time would be substantially reworked. CO converts to CO2. Organic crusts accumulate. The material that formed in another solar system lies deeper, possibly beyond what solar heating during a single perihelion passage can reach.
Whether 3I/ATLAS's perihelion excavated any deeper material is something the data may eventually show, but the chemical signal so far is dominated by the surface.
This is not necessarily a disappointment. A comet that records the effects of billions of years of cosmic ray bombardment tells us something real about what happens to matter in interstellar space, which is where most matter in the galaxy spends most of its time.
The processed chemistry is itself a finding. It simply may not be the window into alien planetary chemistry that the arrival of a third interstellar comet first seemed to promise.
What JUICE May Yet Add
ESA's Jupiter Icy Moons Explorer made observations of 3I/ATLAS around the March 2026 Jupiter flyby using cameras, spectrometers, and a particle sensor. Most of that data was still being downloaded in early 2026, slowed by the spacecraft's distance and communication constraints. Instrument teams were in the early stages of analysis.
The ALMA interferometer has also been mapping the comet's molecular outgassing at high resolution, and those data may yet reveal details about the spatial distribution of different volatiles that single-dish or space telescope observations could not resolve.
The broader question raised by 3I/ATLAS is one that will sharpen as the sample grows. With next-generation sky surveys, interstellar crossings that currently go undetected may soon become routine.
Within a decade, the comparison sample could expand from three objects to dozens.
The central question shifts from what any single comet reveals to what the population as a whole tells us about the spread of chemical environments across the galaxy, and whether the cosmic ray processing seen in 3I/ATLAS is universal or merely one trajectory among many.
3I/ATLAS is leaving. But the questions it has opened are just beginning to be posed.
Sources
- Primary: Comet 3I/ATLAS (NASA Science)
- JWST observations: JWST Detection of a Carbon Dioxide Dominated Gas Coma (Astrobiology / Cordiner et al.)
- Chemistry findings: Telescopes Reveal Surprising Chemistry of a Rare Interstellar Object (Smithsonian Magazine)
- SPHEREx data: SPHEREx Pre-Perihelion Mapping (Astrobiology)
- Cosmic ray processing: Evidence for Galactic Cosmic Ray Irradiation (arXiv)
- NASA FAQ: 3I/ATLAS Facts and FAQs (NASA Science)
- ESA observations: Comet 3I/ATLAS: Frequently Asked Questions (ESA)
- NASA open data: How Open NASA Data on Comet 3I/ATLAS Will Power Tomorrow's Discoveries (NASA Science)
Fact Check: Claim-by-Claim Verification Verified
All major claims verified across orbital parameters, JWST chemistry findings, SPHEREx observations, VLT nickel detection, and MIRI methane data. Hypotheses correctly framed as such.
Commentary
- The cosmic ray processing hypothesis is an active area of research; the 15-20 metre crust depth is modelled, not directly measured.
- CO2 production rates differ between instruments and epochs (70 kg/s from SPHEREx vs ~129 kg/s from JWST), reflecting different observation conditions rather than contradictions.
- The nickel tetracarbonyl explanation remains one of several hypotheses for the iron-free nickel signature.
- The "one object per year" crossing rate is at the lower end of published estimates but reasonable.
Sources used for verification
Academic/Peer-reviewed:
- JWST Detection of CO2-Dominated Coma (Cordiner et al.) - arxiv.org
- TESS Pre-Discovery Observations - arxiv.org
- Evidence for Galactic Cosmic Ray Irradiation - arxiv.org
- Nordic Optical Telescope Observations - arxiv.org
Other reliable sources:
- Comet 3I/ATLAS Overview - science.nasa.gov
- 3I/ATLAS Facts and FAQs - science.nasa.gov
- ESA JUICE Observations FAQ - esa.int
- Surprising Chemistry of 3I/ATLAS - smithsonianmag.com
- JWST CO2 Coma Detection - astrobiology.com
- SPHEREx Pre-Perihelion Mapping - astrobiology.com
Fact-checked by Perplexity Sonar Pro on 2026-04-08
