It came from another star, and it will never pass this way again. Since astronomers spotted it in July 2025, the interstellar object known as 3I/ATLAS has been tracked by more NASA hardware than any comet in history. It has also spent months at the center of a fiery debate, with Harvard astronomer Avi Loeb arguing it could be an ancient alien relic and most of the scientific community insisting it is a perfectly natural comet.
As of September 2026, two independent teams of scientists have pointed powerful radio telescopes directly at 3I/ATLAS for over 11 hours of combined listening time. They analyzed more than 74 million candidate signals between them. The question everyone was waiting for has an answer, and it is a firm one.
In this guide, we explain what the signal scans actually measured, what the newest chemical analyses reveal about where 3I/ATLAS formed, why the researchers say the results are meaningful even though they found nothing, where the object is right now, and whether the “spaceship” hypothesis has finally been put to rest.
Last updated September 29, 2026 with new chemistry results from ALMA, the William Herschel Telescope and the James Webb Space Telescope.
What Is 3I/ATLAS? The Third Confirmed Interstellar Visitor
3I/ATLAS was discovered on July 1, 2025, by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Hawaii. The “I” in its name stands for “interstellar,” and the 3 marks it as only the third object ever confirmed to have entered our solar system from another star, following 1I/’Oumuamua in 2017 and 2I/Borisov in 2019.
Unlike its two predecessors, 3I/ATLAS turned out to be big. Estimates put its nucleus at roughly 10 kilometers wide, possibly much larger than its cousin ‘Oumuamua, and large enough that ground-based telescopes picked it up while it was still more than 4 astronomical units (370 million miles) from the Sun. Its orbit is steeply inclined and wildly hyperbolic, meaning it will slingshot back into interstellar space on a path it will never retrace.
As the object dove toward the Sun, it began behaving exactly like a comet: venting gas and dust to produce a glowing coma and tail, glowing a distinctive green from carbon emissions, and brightening steadily. Astronomers catalogued jets and a curious symmetric jet structure that caught the eye of both SETI researchers and Avi Loeb. When the James Webb Space Telescope turned its mid-infrared eye toward the comet in late 2025, it produced the first chemical fingerprint ever recorded of an interstellar object, building on techniques similar to those used in advanced asteroid detection algorithms.
The Latest News: Two Big Scans, Zero Signals
Despite the scientific consensus that 3I/ATLAS is a comet, the object’s unusual size and trajectory kept the “alien probe” question alive long enough that two separate teams decided to test it directly. Both have now published their results, and both came back empty.
The SETI Institute / Breakthrough Listen Survey
The most comprehensive scan was conducted by a team led by SETI Institute researcher Sofia Sheikh, working with colleagues from the Breakthrough Listen project. Published in The Astronomical Journal in June 2026, the study used the Allen Telescope Array (ATA) at the Hat Creek Radio Observatory in Northern California to observe 3I/ATLAS for more than seven hours across a wide frequency range of 1 to 9 gigahertz.
The numbers behind the analysis are staggering. Using a newly developed search pipeline called Bliss, the team identified nearly 74 million narrowband hits in 7.25 hours of data. Narrowband signals are radio transmissions confined to a tiny sliver of frequency, something natural processes do not produce. After filtering out radio-frequency interference from human sources on Earth and in orbit and cross-referencing any signal that matched the comet’s motion relative to the telescope, the list shrank to roughly 200 candidates. When the 211 strongest hits were manually inspected in the time-frequency domain, every single one turned out to be human-made interference.
The FAST Telescope Survey
Across the Pacific, a Chinese-led team used the Five-hundred-meter Aperture Spherical Telescope (FAST), the largest filled-aperture radio telescope on Earth, to perform its own search published as a preprint. Their campaign ran from October 2025 to January 2026, deliberately timing observations to four key moments in the comet’s passage: its closest approach to Mars, its perihelion (closest approach to the Sun), its closest approach to Earth, and a final look as it flew away.
Scanning the 1.05 to 1.45 gigahertz band for drifting signals, the FAST team likewise found “no credible narrowband radio technosignature.” Their work went a step further by adding a Bayesian inference framework, a statistical method for constraining how powerful any hypothetical transmitter would have to be to escape detection, rather than simply reporting a hard cutoff.
What the Signal Scans Actually Measured
A “null result” in SETI research sounds disappointing, but the constraints these two surveys placed are scientifically precise and genuinely useful. Here is how they compare:
| Detail | SETI Institute / ATA Survey | FAST Survey |
|---|---|---|
| Telescope | Allen Telescope Array, Hat Creek, California | Five-hundred-meter Aperture Spherical Telescope, Guizhou, China |
| Frequency range | 1 to 9 GHz | 1.05 to 1.45 GHz |
| Total observation time | 7.25 hours | ~4 hours across 4 dates |
| Raw signal hits | ~74 million | Filtered at SNR threshold above 10 |
| Final candidates inspected | 211, all identified as RFI | None passed visual inspection |
| Transmitter power upper limit | 10 to 110 W (EIRP) | 2.862 x 10-3 W (EIRP, narrowband) |
| Publication | The Astronomical Journal, June 2026 | arXiv:2603.19023, 2026 |
The difference in the two power limits deserves a note. The ATA survey cast a much wider frequency net, making it sensitive to stronger transmitters over a broad swath of the radio spectrum, while FAST’s narrower band search reached extraordinary sensitivity to weak, narrowband emitters within its frequency window. In plain terms: neither team found anything broadcasting from 3I/ATLAS with more power than a household appliance, and in FAST’s narrow window, nothing above a few milliwatts.
What NASA and the Rest of the Fleet Have Observed
The radio scans are just one line of evidence. Since its discovery, 3I/ATLAS has been photographed and measured by an unprecedented fleet of more than a dozen spacecraft and observatories, creating what NASA calls its most extensive observation campaign for a comet ever.
- Hubble Space Telescope: Reobserved the comet in late November 2025, capturing the symmetric jet structure that sparked so much debate.
- James Webb Space Telescope: Collected the first mid-infrared chemical fingerprint of an interstellar object.
- ESA’s JUICE spacecraft: While en route to Jupiter, its JANUS camera captured 120 images of 3I/ATLAS from 63 million kilometers away between November 5 and 25, 2025, a period when the comet was hidden from Earth’s view behind the Sun.
- STEREO, SOHO, PUNCH, Psyche, Lucy, MAVEN, Parker Solar Probe, TESS: Each contributed observations tracking the comet’s tail, hydrogen envelope, trajectory, and rotation at different stages of its solar passage, a fleet approach reminiscent of proposals for NASA’s Moon-based radio telescope concept.
- ALMA and ground-based spectroscopy: Detected methanol and hydrogen cyanide in the coma, ingredients consistent with natural cometary chemistry.
3I/ATLAS reached perihelion on October 29 to 30, 2025, at roughly 1.36 AU from the Sun, just inside the orbit of Mars. At every stage, the physical and chemical behavior has matched what planetary scientists expect of a natural, icy interstellar body.
What Astronomers Have Learned Since the Scans
The radio scans answered the loudest question, but they were never the only line of evidence. Through 2026, and especially in September, three separate chemistry results landed. All three point the same way: 3I/ATLAS looks like an object that formed extremely cold, far from its home star, in a chemically primitive environment.
It is bursting with methanol
Using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a team led by Nathan Roth of American University mapped the comet’s coma in unusual detail. They found methanol at levels exceeding almost every comet known in our own solar system, with methanol-to-hydrogen-cyanide ratios of roughly 70 and 120 on two observation dates. The imaging also showed methanol escaping not only from the nucleus but from icy grains drifting in the coma, which behave like miniature comets as they warm up. It is the first time this outgassing behaviour has been traced in an object from another star system. The results were published in The Astrophysical Journal Letters.
It formed in the coldest conditions ever measured for a comet
UK astronomers turned the William Herschel Telescope’s new WEAVE instrument on the comet’s plasma tail in late 2025, after perihelion, and simultaneously detected five different ions streaming away from it. The team, led by Lea Ferellec of Northumbria University, measured a nitrogen-to-carbon-monoxide ratio above 0.023, placing 3I/ATLAS in a far more nitrogen-rich regime than solar system comets. Because nitrogen is only trapped efficiently in ice at very low temperatures, that ratio implies the comet formed colder than about -240°C (-400°F), roughly 33 degrees above absolute zero. The Royal Astronomical Society announced the findings, which were published in Monthly Notices of the Royal Astronomical Society.
Its “heavy water” now has a natural explanation
One of the strangest things about 3I/ATLAS is its water. The deuterium-to-hydrogen ratio in its water is around 1 percent, compared with 0.015 to 0.03 percent for a typical comet in our solar system, meaning this visitor carries far more “heavy water” than anything local. In September 2026, a modelling study led by Kenji Furuya of the RIKEN Pioneering Research Institute showed that the anomaly falls out naturally if the comet formed in a low-metallicity environment, at less than about half the Sun’s metallicity. Under those conditions, chemical models readily build deuterated water ice, a result that also fits the James Webb Space Telescope’s measurement of unusually high carbon isotope ratios in the comet’s carbon-bearing molecules.
Together, the three results describe an object that condensed in extreme cold, far from its star, out of primitive material. That is precisely the kind of body cometary physics predicts for an interstellar visitor, and it is the main reason the newest data have made the natural explanation stronger rather than weaker.
Is 3I/ATLAS a Spaceship? The Avi Loeb Debate
No single person has shaped the public conversation around 3I/ATLAS more than Harvard University astronomer Avi Loeb, who also led the controversial scientific reanalysis of ‘Oumuamua. Loeb published a detailed catalog of the object’s “anomalies” that he argues are difficult to explain through conventional cometary physics: its large size relative to other interstellar objects, its highly inclined orbit, the unusual deuterium concentration reported in some spectroscopic studies, and the puzzling symmetric jet structure imaged by Hubble. The deuterium point has since been addressed directly: a September 2026 modelling study showed that the comet’s anomalously high heavy-water ratio follows naturally from formation in a low-metallicity environment.
Loeb stopped short of declaring it artificial, but argued openly that the possibility should not be dismissed without direct evidence, writing publicly that it is “too early to call it.” That stance has drawn sharp criticism from other astronomers, who counter that every additional observation, from the CO2-to-water ratios measured by the Subaru Telescope to the SPHEREx water-gas data, reinforces the natural comet explanation.
The new radio scans land squarely on the side of the skeptics. A truly technological object, the reasoning goes, would have had to either shut down entirely or run its equipment at power levels below that of a household appliance, which is exactly what the SETI upper limits now constrain. For his part, Loeb has responded to critics by noting that a dead or dormant alien relic would be expected to fall silent and that absence of a signal is not proof of absence.
Where Is 3I/ATLAS Now?
For anyone searching for 3I/ATLAS “current location,” the answer depends on when you read this. As of late September 2026, the comet is approximately 11 AU from the Sun (roughly 1 billion miles), a little beyond the orbit of Jupiter, and fading fast at magnitude 22.7, far too faint for amateur telescopes and reachable only by the world’s largest professional observatories.
A brief northern-hemisphere observing window reopened in September 2026, about 15 months after discovery, giving astronomers one final season to study the object before it climbs toward the orbital distance of Saturn and recedes beyond practical reach for ground-based characterization. A spatial chemistry map published on September 3, 2026, by a team led by Anita Cochran of the University of Texas at Austin, using data from the Hobby-Eberly Telescope’s VIRUS spectrograph, provides one of the last detailed looks at the comet’s inner coma structure.
To answer the most frequently asked question plainly: 3I/ATLAS poses no impact risk to Earth whatsoever. Its trajectory was locked in long before discovery, its closest approach to Earth is now firmly in the past, and it is currently moving away from the solar system at tremendous speed on a hyperbolic orbit that will carry it back into interstellar space. It will never return.
What 3I/ATLAS Means for Finding Life
The SETI researchers themselves are quick to note that a null result is still a valuable result. As Valeria Garcia Lopez, a Breakthrough Listen member and Furman University physics professor, put it, “The results from 3I/ATLAS show how realistic it is to detect a signal with the technology we have today. That is why it is important to keep searching for technosignatures, even from objects we might not expect to have signals.”
Sheikh, the lead author, framed the campaign as a rehearsal for a future that is statistically likely. “Eventually, our own Voyager spacecraft will be extraterrestrial artifacts in other stellar systems,” she said. “Given that, it is important that we understand the natural distribution of interstellar objects so that we will be able to identify any anomalies that could one day be signs of an artificial interstellar object.”
In other words, each interstellar visitor, whether it turns out to be a rock, a comet, or something stranger, adds a data point to the baseline of what “normal” looks like. When an object finally shows up that deviates from that baseline in ways natural physics cannot explain, humanity will need exactly the kind of rapid-response radio search that was just performed on 3I/ATLAS to know what it is looking at. The same frontier thinking is driving research into wetware computing using living cells and next-generation space communications like China’s lunar laser communication test.
Frequently Asked Questions
What is the current situation of 3I/ATLAS?
As of September 2026, 3I/ATLAS is roughly 11.1 AU from the Sun and receding, visible only to the largest professional telescopes. Two independent radio surveys have published their results, both finding no artificial signals, and the object continues to be studied during a final observing window before it fades from reach entirely.
What is NASA saying about the 3I/ATLAS?
NASA’s official position, backed by its unprecedented multi-spacecraft observation campaign, is that 3I/ATLAS behaves exactly as a natural comet does. The agency’s fleet of observatories has documented its gas emissions, tail structure, rotation, and chemical composition, all consistent with a natural interstellar body.
Did the signal scans find anything unusual at all?
They found nothing artificial, but the sensitivity achieved was itself significant. The ATA survey ruled out any transmitter stronger than roughly 10 to 110 watts across 1 to 9 GHz, while China’s FAST telescope constrained narrowband transmitters in its band to below 2.862 milliwatts. These are the tightest limits ever placed on technosignatures from an interstellar object.
Will 3I/ATLAS come back or hit Earth?
No. 3I/ATLAS is on a hyperbolic orbit, meaning it is not gravitationally bound to our Sun. It has already made its closest pass to Earth, is now moving away at interstellar speeds, and will never return to the inner solar system.
Why do some scientists still call it an anomaly?
A minority of researchers, most prominently Avi Loeb, point to its large size, unusual orbit, and reported deuterium levels as features that warrant continued scrutiny. The broader astronomical community regards each of these features as within the range of natural variation, especially given the small sample size of known interstellar objects.
