3I/ATLAS Radio Signals: What Telescopes Actually Detected
Few topics generated more confusion during 3I/ATLAS's passage through the solar system than the subject of radio signals. Sensationalist headlines screamed about "geometric radio emissions" and "mysterious signals" from the interstellar comet. Social media amplified claims of alien transmissions. Deepfake videos put fabricated words in the mouths of prominent physicists.
The reality is both simpler and more scientifically interesting. Radio telescopes did detect signals from 3I/ATLAS — natural ones. And four of the world's most powerful SETI facilities conducted the most comprehensive technosignature search ever performed on an interstellar object. Here is what actually happened.
MeerKAT Makes the First Radio Detection
The first confirmed radio detection of 3I/ATLAS came from South Africa's MeerKAT array — 64 radio dishes spread across the Karoo semi-desert, operated by SARAO (the South African Radio Astronomy Observatory).
The initial observations on September 20 and 28, 2025 found nothing — the comet was still too distant and not active enough for radio detection. But on October 24, 2025, the team detected what they were looking for: hydroxyl (OH) absorption at 1665 and 1667 MHz — the classic 18-cm radio lines that comets are known to produce.
Doppler velocity measurements showed shifts of -15.59 and -15.65 km/s, precisely matching the comet's predicted radial velocity. The comet was over 350 million km from Earth at the time.
Follow-up observations on November 4, 6, and 11-12 tracked the signal as it transitioned from absorption to emission — a behavior predicted by standard cometary physics and confirming that the detection was genuine. Professor Mykola Ivchenko of KTH Royal Institute of Technology stated plainly: "Detecting the hydroxyl signal is an important confirmation that 3I/ATLAS is behaving like a comet."
The research team was led by Professor DJ Pisano (University of Cape Town) and Professor Oleg Smirnov (Rhodes University and SARAO), with results published via Astronomer's Telegrams #17473 and #17499.
How Comets Produce Radio Signals
The radio emissions from 3I/ATLAS are entirely natural and well understood. The process works as follows:
As the comet approaches the Sun, solar heating causes water ice in the nucleus to sublimate — transitioning directly from solid ice to water vapor. This water vapor streams away from the nucleus into the coma, the cloud of gas and dust surrounding the comet's solid core.
Solar ultraviolet light then photodissociates the water molecules, breaking H₂O into its components: hydrogen atoms (H) and hydroxyl radicals (OH). These OH molecules have four hyperfine transitions at radio wavelengths near 18 cm (1612, 1665, 1667, and 1720 MHz).
The OH molecules undergo resonance fluorescence — they absorb solar radiation at infrared wavelengths and re-emit at radio wavelengths. Whether this produces absorption or emission depends on the geometry between the Sun, the comet, and the observer, as well as the comet's heliocentric distance. The transition from absorption to emission that MeerKAT observed in November 2025 is a textbook prediction of this physics.
This process has been used to monitor water production rates in solar system comets for over 40 years. Detecting it in 3I/ATLAS confirmed that this interstellar comet produces water in the same way as comets born in our own solar system.
ALMA Maps Methanol and Hydrogen Cyanide
While MeerKAT worked at centimeter wavelengths, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile probed 3I/ATLAS at much shorter wavelengths — around 0.85 mm (335-355 GHz) — where individual molecular species can be identified.
Over six observing sessions from August 28 to October 1, 2025, ALMA's Atacama Compact Array detected two key molecules:
Methanol (CH₃OH) was detected on four dates, with production rates of approximately 6.6-8.2 × 10²⁶ molecules per second. The methanol showed an enhancement on the sunward side of the coma with evidence of production from sources in the coma itself — not just from the nucleus surface — at distances greater than 258 km from the nucleus center.
Hydrogen cyanide (HCN) was detected on September 12 and 15. Unlike methanol, HCN was depleted on the sunward side, consistent with direct sublimation from the nucleus.
The most striking finding was the CH₃OH/HCN production rate ratio: values of 124 and 79 are among the highest ever measured in any comet, surpassed only by the anomalous solar system comet C/2016 R2 (PanSTARRS). This extreme methanol enrichment provides a chemical fingerprint of the conditions in the protoplanetary disk where 3I/ATLAS originally formed — likely a colder, more methanol-rich environment than where most solar system comets were born.
The results were published by Nathan X. Roth, Martin A. Cordiner, Dominique Bockelée-Morvan and colleagues in November 2025.
The SETI Search: Four Telescopes, Zero Technosignatures
In parallel with the natural radio observations, the Breakthrough Listen project and SETI Institute launched the most comprehensive technosignature search ever conducted on an interstellar object. Four major radio telescopes participated:
Allen Telescope Array (July 2, 2025) — Within 24 hours of 3I/ATLAS's discovery, the SETI Institute's 42-dish array in Northern California began observing. Over 7.25 hours, it collected 22 TB of data across 1-9 GHz. The analysis pipeline processed nearly 74 million narrowband "hits," whittled them down through RFI filtering and sky-localization checks to 211 candidate events — all of which were ruled out upon visual inspection. Zero confirmed technosignatures. Lead author: Sofia Z. Sheikh.
Murriyang / Parkes (July 31, September 12, October 5, 2025) — Australia's 64-meter dish observed across 704-4032 MHz using its Ultra-Wideband Low receiver. At closest approach on October 5, it achieved sensitivity to signals as weak as approximately 5 watts EIRP (equivalent isotropic radiated power). No artificial emission detected. Data is publicly available through the Berkeley SETI Research Center.
MeerKAT / BLUSE (November 5, 2025) — While MeerKAT's primary receivers detected natural OH, the Breakthrough Listen User-Supplied Equipment (BLUSE) backend simultaneously searched 900-1670 MHz for narrowband technosignatures. Sensitivity reached 0.17 watts EIRP — approximately the power output of a mobile phone handset at 3I/ATLAS's distance. No narrowband signals of technological origin detected.
Green Bank Telescope (December 18, 2025) — The crown jewel of the campaign. Less than 24 hours before 3I/ATLAS's closest Earth approach, the 100-meter GBT — the world's largest fully steerable radio dish — observed from 1-12 GHz across four receivers. The search achieved approximately 0.1 watts EIRP sensitivity — the most sensitive radio observation of any interstellar object ever conducted. Of 471,198 initial candidate hits, 9 survived automated filtering, and all 9 were ruled out as radio frequency interference. Zero confirmed technosignatures.
The Green Bank Result in Context
The Green Bank observation deserves special attention for what its sensitivity means in practical terms.
At 0.1 watts EIRP, the GBT could have detected a signal with one-tenth the power of a typical cell phone emanating from 3I/ATLAS at a distance of 1.8 AU (approximately 269 million km). If 3I/ATLAS were broadcasting with even a modest directional antenna — say, equivalent to a Wi-Fi router — the GBT would have detected it easily.
The observation used an ABACAD cadence — alternating between on-target and off-target pointings in 5-minute blocks over 30 minutes. This technique is specifically designed to distinguish genuine extraterrestrial signals from terrestrial radio frequency interference (RFI). A real signal from 3I/ATLAS would appear in the on-target scans and disappear in the off-target scans.
The research team — Ben Jacobson-Bell, Steve Croft, Ellie White, Andrew Siemion, and colleagues — stated the scientific rationale clearly: "There is currently no evidence to suggest that [interstellar objects] are anything other than natural astrophysical objects. However, given the small number of such objects known (only three to date), and the plausibility of interstellar probes as a technosignature, thorough study is warranted."
In other words: the search was worth doing even though the expected result was a non-detection. That is how science works — you test the hypothesis regardless.
The Misinformation Tsunami
Unfortunately, the gap between what scientists actually found and what the public believed widened dramatically throughout 2025 and into 2026.
The OH detection became "alien signals." When MeerKAT's natural hydroxyl detection was announced, social media accounts stripped away the context. The fact that a radio telescope detected a "radio signal" from 3I/ATLAS was amplified without the critical qualifier: it was a natural emission that all active comets produce. The result spread across at least five countries as evidence of alien communication.
"Geometric radio signals" — pure fabrication. In late January 2026, sensationalist outlets published claims that 3I/ATLAS was emitting "geometric radio signals" and "mysterious emissions" that had mobilized space agencies worldwide. These articles cited no scientific sources. They directly contradicted every published paper and preprint, all of which found only natural emissions and zero technosignatures.
Deepfake videos targeted physicists. AI-generated videos made Michio Kaku appear to confirm that 3I/ATLAS was an alien spacecraft being hidden by NASA. Brian Cox was similarly targeted. Both physicists publicly denounced the videos as fabrications. TikTok and YouTube removed them for misinformation policy violations.
The CIA "Glomar response." On December 31, 2025, the CIA replied to a FOIA request about 3I/ATLAS with a standard "Glomar response" — neither confirming nor denying the existence of records. Avi Loeb called this suspicious. Experts noted this is a routine response the CIA uses for virtually all FOIA requests involving space-related topics, classified satellite data, or national security-adjacent subjects.
How 3I/ATLAS Compares to Previous SETI Searches
All three known interstellar objects have now been searched for technosignatures at radio wavelengths. The results are consistent:
1I/'Oumuamua (2017) — The Green Bank Telescope observed for 2 hours in December 2017, accumulating 90 TB of data across 1.1-11.6 GHz. Sensitivity reached approximately 0.08 watts EIRP — slightly better than the 3I/ATLAS observation due to 'Oumuamua's closer distance. No technosignatures detected. Notably, 'Oumuamua showed no cometary activity at all — no coma, no tail, no natural radio emission — making it a pure technosignature search with no complicating natural signals.
2I/Borisov (2019-2020) — Both the Green Bank Telescope and Murriyang observed 2I/Borisov, along with optical observations from the Automated Planet Finder at Lick Observatory. No technosignatures were detected in any dataset. Borisov was clearly a natural comet with a visible coma and tail, chemically similar to solar system comets.
3I/ATLAS (2025) — The most thoroughly searched of the three, with four facilities (ATA, Murriyang, MeerKAT, GBT) conducting systematic observations from July through December 2025. Coverage spanned 700 MHz to 12 GHz. The most sensitive search achieved 0.1 W EIRP. Zero technosignatures detected — but the campaign also produced the first natural radio detection (MeerKAT OH) and millimeter molecular spectroscopy (ALMA CH₃OH/HCN) of an interstellar comet.
The combined result across all three objects is unambiguous: no credible evidence of artificial radio transmission has been detected from any interstellar object.
What the Radio Data Actually Tells Us
Strip away the misinformation and the genuine radio observations of 3I/ATLAS are among its most scientifically valuable datasets.
MeerKAT's OH detection confirmed that 3I/ATLAS actively outgasses water — the same fundamental volatile that drives activity in solar system comets. The absorption-to-emission transition tracked in real time provided a textbook demonstration of cometary OH maser physics at interstellar-comet scales.
ALMA's molecular mapping revealed extreme methanol enrichment relative to hydrogen cyanide — a ratio exceeded by only one known comet. This chemical fingerprint points to formation in a cold, methanol-rich region of another star's protoplanetary disk, distinct from the conditions that produced most comets in our own solar system.
The SETI non-detections are scientifically meaningful in their own right. They establish the most stringent constraints ever placed on artificial radio emission from an interstellar object, ruling out transmitters as weak as a cell phone. Future interstellar objects can be compared against this baseline.
As Penn State astronomer Jason Wright summarized in his systematic rebuttal of the alien hypothesis: "Broadcasting signals is not something that comets do." The radio data from 3I/ATLAS tells us exactly what it is — a natural interstellar comet, carrying water, methanol, and hydrogen cyanide from another star system, behaving precisely as cometary physics predicts.
Track 3I/ATLAS's trajectory on our Orbit page, explore the full discovery timeline, and learn about the science of interstellar comets.