Radio Signal from 3I/ATLAS: What Telescopes Actually Found and What It Means

In late 2025, headlines around the world announced that a radio signal had been detected from 3I/ATLAS — the third confirmed interstellar object to pass through our solar system. Social media lit up with speculation. Was this proof of alien technology? Had we finally detected a transmission from another star system?

The truth is more nuanced — and, for scientists, arguably more exciting. Telescopes did detect radio emissions from 3I/ATLAS, but they weren't alien broadcasts. They were the chemical fingerprints of water being destroyed by sunlight — the same process that creates the glowing tails of every comet we've ever observed, except this time, the water came from another star system entirely. Here's the complete story of what was actually detected, what it means, and why the most powerful alien signal search in history came up empty.

The First Radio Detection: MeerKAT and the Hydroxyl Signal

On October 24, 2025, South Africa's MeerKAT radio telescope — an array of 64 dishes in the Karoo desert — became the first instrument to detect a radio signal from 3I/ATLAS. The comet was just 3.76 degrees from the Sun at the time, making optical observation nearly impossible. But radio telescopes don't care about sunlight — they listen in wavelengths invisible to our eyes.

What MeerKAT detected were absorption lines from hydroxyl radicals (OH molecules) at frequencies of 1.665 and 1.667 gigahertz — the so-called "hydroxyl maser" lines that radio astronomers have studied in comets for decades.

Here's the chemistry: as a comet approaches the Sun, solar heat vaporizes its water ice. Solar ultraviolet radiation then breaks the water molecules apart into hydrogen (H) and hydroxyl (OH). These hydroxyl radicals absorb and re-emit radio waves at very specific frequencies — frequencies that act like a chemical barcode, unmistakably identifying the presence of water.

Previous attempts to detect the hydroxyl signal from 3I/ATLAS on September 20 and 28 had failed. The October 24 detection came as the comet drew closer to the Sun and its water production rate surged. Follow-up observations on November 4 and 6 confirmed more absorption, and by November 11–12, the signal had transitioned from absorption to emission — exactly as cometary physics predicted.

Avi Loeb, the Harvard astrophysicist who has been outspoken about the possibility of interstellar technology, wrote about the detection on Medium, analyzing the Doppler velocity shifts and thermal properties of the OH molecules. Even Loeb acknowledged the signal was entirely consistent with natural cometary processes — water ice from an alien star system behaving exactly as water ice from our own solar system does.

Why the Hydroxyl Detection Matters

The MeerKAT hydroxyl detection might not be alien technology, but it's scientifically profound for a different reason: it proved that water ice from another stellar nursery undergoes the same photodissociation process as water in our solar system.

This might sound obvious, but it's not. Water ice that formed around a different star, in a different chemical environment, billions of years ago, could theoretically behave differently — different isotopic ratios, different molecular structures, different outgassing patterns. The fact that 3I/ATLAS's water breaks down and produces hydroxyl masers in exactly the same way as our local comets is a powerful confirmation that the basic chemistry of water is universal.

For astrobiologists, this is significant. Water is the foundation of life as we know it. Confirming that interstellar water ice follows the same physical rules everywhere strengthens the case that the ingredients for life are not unique to our solar system — they're scattered across the galaxy.

The Breakthrough Listen Search: 471,000 Signals Reduced to Zero

While MeerKAT was detecting natural hydroxyl emissions, the Breakthrough Listen initiative — the world's most ambitious SETI program — was conducting the most sensitive technosignature search ever attempted on an interstellar object.

The campaign was massive, spanning multiple telescopes and months of observation:

Allen Telescope Array (ATA)

Observations began within days of 3I/ATLAS's discovery on July 1, 2025. The ATA, a dedicated SETI instrument in northern California, conducted early scans during the comet's inbound approach.

Parkes Observatory (Murriyang)

Australia's iconic 64-meter dish recorded data across three sessions: July 31, September 12, and October 5, 2025. At the October 5 distance, the telescope was sensitive to transmitters with an effective isotropic radiated power (EIRP) of approximately 5 watts — roughly the output of a walkie-talkie.

Green Bank Telescope (GBT): The Main Event

The crescendo came on December 18, 2025 — less than 24 hours before 3I/ATLAS's closest approach to Earth at 1.7 AU (167 million miles / 269 million kilometers).

The 100-meter Green Bank Telescope in West Virginia — the largest fully steerable radio telescope on Earth — trained its enormous dish on the interstellar visitor. The team used four receivers (L, S, C, and X bands) spanning frequencies from 1 to 12 GHz, covering the range where artificial narrowband transmissions would most likely be detected.

At closest approach, the GBT achieved sensitivity to transmitters with an EIRP of approximately 0.1 watts — one-tenth the power of a typical smartphone. This was, by a significant margin, the most sensitive radio search ever conducted on an interstellar object.

The Results: 471,000 Candidates, Zero Confirmed

The raw data processing initially flagged 471,000 candidate signals — a number that briefly accelerated heartbeats in the research team. But SETI scientists are experienced in the painstaking art of signal verification.

The vast majority of candidates were immediately eliminated because they also appeared when the telescope pointed away from 3I/ATLAS — a telltale sign of terrestrial radio frequency interference (RFI). Our planet is drenched in radio noise: Wi-Fi, cellular networks, satellite communications, radar systems, and even microwave ovens all produce signals that can contaminate astronomical observations.

After filtering, only nine signals remained that appeared exclusively when the telescope was pointed at 3I/ATLAS. The team visually inspected each one. All nine were ultimately identified as RFI or known contaminants — signals that coincidentally appeared during on-target scans but matched the signatures of human-made interference.

The final verdict, published on arXiv as "Breakthrough Listen Observations of 3I/ATLAS with the Green Bank Telescope at 1–12 GHz": no credible detections of narrowband radio technosignatures originating from 3I/ATLAS.

What "No Signal" Actually Means

It's tempting to interpret "no signal found" as "there's nothing there." But scientists are careful to note what the search actually ruled out — and what it didn't.

What was ruled out:

• Any continuously broadcasting narrowband radio transmitter on or near 3I/ATLAS with a power output above 0.1 watts in the 1–12 GHz range
• Any device equivalent to a smartphone, walkie-talkie, or stronger radio beacon within the comet or its immediate vicinity

What was NOT ruled out:

• Transmitters broadcasting at frequencies outside 1–12 GHz (the search didn't cover all possible radio frequencies)
• Pulsed or broadband signals (the search focused on narrowband, continuous transmissions)
• Non-radio technologies — optical, gravitational, neutrino, or other exotic communication methods
• Passive technology — a dormant probe, an inactive relay, or technology that simply isn't transmitting
• Technology too weak to detect — anything broadcasting below 0.1 watts at 1.7 AU would have been missed

In other words, the search was incredibly powerful but not omniscient. As the Breakthrough Listen team noted, 3I/ATLAS could theoretically carry technology that simply doesn't match our current detection capabilities. The absence of evidence is not evidence of absence — but the evidence we do have is entirely consistent with a natural object.

The Misinformation Problem

The 3I/ATLAS radio signal story became a case study in how scientific findings get distorted as they travel through the media ecosystem.

What actually happened: MeerKAT detected natural hydroxyl emissions — the chemical signature of water being destroyed by sunlight. This is routine cometary science, observed in hundreds of comets.

What headlines said: "Radio Signal Detected from Interstellar Object" — technically true but wildly misleading to a general audience primed by decades of science fiction to interpret "radio signal" as "alien communication."

What social media amplified: Claims that alien signals had been found, that NASA was "on global alert," and that the hydroxyl frequency was suspiciously close to the famous 1977 Wow! Signal — a comparison that, while factually noting both involve the 1.6–1.7 GHz range, ignored that this is simply where hydroxyl naturally emits. It's like noting that two different fires both produced smoke.

The episode highlights a growing challenge: as scientific instruments become more sensitive and interstellar objects become a focus of public attention, the gap between what researchers actually find and what the public believes they found is widening.

What Comes Next

The 3I/ATLAS radio campaign set a new standard for how the scientific community responds to interstellar visitors. Future interstellar objects — and astronomers believe we'll detect them more frequently as survey telescopes improve — will face an even more rigorous battery of observations.

Key developments to watch:

• The Vera C. Rubin Observatory, expected to begin full operations in 2026, will dramatically increase the discovery rate of interstellar objects, potentially finding several per year
• Next-generation radio arrays like the Square Kilometre Array (SKA), partially under construction in South Africa and Australia, will achieve sensitivities orders of magnitude beyond current capabilities
• The Galileo Project at Harvard is developing a dedicated multi-sensor observatory specifically designed to characterize anomalous objects — whether interstellar comets or unexplained aerial phenomena

The search for a radio signal from an interstellar object came up empty in 2025. But the search itself — the speed of response, the international coordination, the extraordinary sensitivity achieved — demonstrated that humanity is ready. When the next interstellar visitor arrives, we'll be listening even more carefully.

---

Track 3I/ATLAS in real time: explore the orbit visualization, view the observation timeline, or read about UFO sightings in 2025.