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A Search for Radio Technosignatures from Interstellar Object 3I/ATLAS with the Allen Telescope Array

Using the Allen Telescope Array to observe the interstellar object 3I/ATLAS across 1–9 GHz, researchers found no evidence of artificial radio signals after analyzing millions of candidates, thereby establishing an effective isotropic radiated power upper limit of 10–110 W for potential technosignatures from the object.

Original authors: Sofia Z. Sheikh, Valeria Garcia Lopez, Isabel Gerrard, James R. A. Davenport, Wael Farah, Blayne Griffin, Steve Croft, Luigi F. Cruz, Imke de Pater, Ben Jacobson-Bell, Mark Masters, Karen I. Perez, Al

Published 2026-05-22

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sofia Z. Sheikh, Valeria Garcia Lopez, Isabel Gerrard, James R. A. Davenport, Wael Farah, Blayne Griffin, Steve Croft, Luigi F. Cruz, Imke de Pater, Ben Jacobson-Bell, Mark Masters, Karen I. Perez, Alexander W. Pollak, Carol Shumaker, Andrew Siemion

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Listening for a "Hello" from a Cosmic Visitor

Imagine a cosmic traveler, a rock from another star system, just arrived in our neighborhood. Astronomers call it 3I/ATLAS. It's the third interstellar object ever found (after 'Oumuamua and Borisov), and it's currently zooming through our Solar System.

The scientists in this paper asked a fascinating question: Could this visitor be an alien probe?

Just like the Voyager probes we sent out in the 1970s to explore our own neighborhood, it's possible that an advanced alien civilization sent a similar "spacecraft" to explore ours. If they did, that spacecraft might be trying to talk to us, or at least broadcasting a signal.

This paper is the report of a team of astronomers who pointed a giant radio telescope at 3I/ATLAS to listen for that signal.

The Tool: The Allen Telescope Array (ATA)

To do this listening, they used the Allen Telescope Array (ATA) in California. Think of the ATA not as one giant ear, but as a choir of 42 smaller dishes working together in perfect harmony.

The Range: They listened across a massive range of radio frequencies, from 1 GHz to 9 GHz. Imagine tuning a radio dial from the bottom of the FM band all the way up to the top of the TV channels, and then some.
The Time: They spent about 7.25 hours listening over several days in July 2025.

The Challenge: Finding a Needle in a Haystack

The problem with listening to the universe is that the "haystack" is incredibly noisy. Our own Earth is full of radio chatter: GPS satellites, cell phones, Wi-Fi, and radar. This is called Radio Frequency Interference (RFI).

If you try to listen for an alien signal, you have to filter out all the human noise first. It's like trying to hear a whisper in a crowded stadium during a rock concert.

The Process: How They Searched

The team used a three-step filtering process, which they describe in the paper:

The Initial Sweep (The "Bliss" Filter):
They ran their data through a computer program called bliss. This program is like a super-fast metal detector. It scanned the 7.25 hours of data and found 74 million "hits."
- Analogy: Imagine the metal detector beeping 74 million times. Most of those beeps were just buried soda cans (human interference), not gold (aliens).
The Noise Cancellation (RFI Blanking):
They knew exactly which radio frequencies are "dirty" on Earth (like the frequencies used by GPS or Iridium satellites). They threw out all the data from those specific frequencies.
- Result: This knocked the number of hits down from 74 million to about 11 million.
The Physics Check (Drift Rate & Location):
- Drift Rate: Because 3I/ATLAS is moving fast relative to Earth, any signal coming from it would "drift" in pitch (like a siren passing by). The team calculated exactly how much the pitch should change. They threw out any signal that didn't match that specific movement.
- Location: They used a special technique to check if the signal was coming from the sky (the target) or from the ground (local interference). They compared a "beam" pointing at the object with a "beam" pointing at empty space next to it. If the signal was loud in both, it was likely just local noise.
- Result: This whittled the list down to 211 suspicious signals.

The Verdict: Silence

The team then looked at these final 211 signals with human eyes. They looked at the "waterfall plots" (which look like colorful rainbows showing signal strength over time).

The Result: Every single one of the 211 signals turned out to be Radio Frequency Interference (RFI).
- Some were from Iridium satellites (like a cell phone in the sky).
- Some were from local ground-based transmitters.
- None of them were from 3I/ATLAS.

Conclusion: They found zero evidence of alien technology.

What Does This Tell Us? (The "Upper Limit")

Even though they didn't find aliens, the paper is still very important. It tells us what would have been needed to find them.

The scientists calculated that if 3I/ATLAS was broadcasting a signal, it would have to be incredibly weak for them to miss it. Specifically, they set a limit:

If the object was sending a signal, it would have to be weaker than 10 to 110 Watts (roughly the power of a bright lightbulb or a small radio transmitter).

The Takeaway:
If 3I/ATLAS is an alien probe, it's either:

Not broadcasting at all (maybe it's a silent, passive probe like a rock).
Broadcasting with a signal so weak it's like trying to hear a whisper from a mile away with a broken ear.

Summary

This paper is a "negative result," but in science, knowing what isn't there is just as valuable as knowing what is. The team successfully demonstrated a new, highly sensitive way to listen for alien probes on interstellar objects. They proved that 3I/ATLAS is not currently shouting at us with a radio beacon, and they set the rules for how quiet such a signal would have to be to go unnoticed.

For now, 3I/ATLAS remains a mysterious cosmic rock, not a cosmic messenger.

Technical Summary: A Search for Radio Technosignatures from Interstellar Object 3I/ATLAS with the Allen Telescope Array

Problem and Motivation
The discovery of 3I/ATLAS, the third known interstellar object (ISO), presents a unique opportunity to search for "artifact technosignatures"—physical probes or spacecraft potentially deployed by extraterrestrial intelligence (ETI). While historical technosignature searches have focused on directed electromagnetic radiation, the existence of the Voyager probes provides a proof-of-existence that technological objects can become ISOs. Unlike active transmitters, passive artifacts do not require constant power to remain detectable via unique physical properties (shape, composition). However, distinguishing a natural ISO from an artificial one requires significant evidence of artificiality. The most unambiguous observable for such a distinction is narrowband radio emission, which is difficult to produce naturally. This study aims to constrain the prevalence of radio transmitters among ISOs by searching for narrowband signals from 3I/ATLAS, thereby setting limits on the fraction of such objects that host technological emitters.

Methodology
The study utilized the Allen Telescope Array (ATA), a 42-element radio interferometer (with 28 active elements at the time of observation) located in Hat Creek, California. Observations were conducted between July 3 and July 10, 2025, totaling 7.25 hours of on-source integration across a frequency range of 1–9 GHz. The data were collected in three frequency chunks ("low": 1–3.7 GHz, "mid": 3.7–6.4 GHz, "high": 6.4–9.1 GHz) using the BLADE beamformer backend in high-frequency resolution mode (1.9 Hz resolution, 16.8 s sampling time).

The data analysis pipeline proceeded through several stages:

Signal Detection: The bliss software pipeline was used to identify Doppler-drifting narrowband signals with a Signal-to-Noise Ratio (SNR) threshold of 15 and drift rates between -4 Hz/s and +4 Hz/s. This initial pass yielded approximately 74 million hits.
RFI Mitigation: To address Radio Frequency Interference (RFI), specific frequency bands known to be contaminated (e.g., GPS downlinks, satellite services) were "blanked," removing 16% of the total bandwidth. This reduced the dataset to ~11 million hits.
Dynamic Filtering: Drift rates were filtered based on the expected kinematic acceleration of 3I/ATLAS relative to Earth. Using JPL Horizons ephemerides, the expected drift rate range was calculated to be 0.05–1.5 Hz/s across the survey frequencies, accounting for both orbital acceleration and potential rotational/tumbling effects. This step reduced the dataset to ~2 million hits.
Spatial Localization: The NBeamAnalysis pipeline was applied to the remaining hits. This code compared "on-beam" (targeted at 3I/ATLAS) and "off-beam" (5 beamwidths away) data. Hits were retained for visual inspection only if they exhibited an SNR ratio between the two beams greater than $\sqrt{28} \approx 5.29$ (indicating a localized point source) and a low "DOT" score (indicating distinct beam characteristics).
Visual Inspection: The final 211 candidates were visually inspected in the time-frequency domain.

Key Results

Non-Detection: After the full filtering pipeline and visual inspection, no signals were identified as worthy of follow-up as technosignatures. All 211 candidates were attributed to terrestrial RFI. Examples included signals consistent with Iridium satellite downlinks and other local interference, which often mimic point-source characteristics but fail narrowband or spectral criteria upon closer inspection.
Upper Limits: Based on the non-detection, the authors calculated upper limits on the Effective Isotropic Radiated Power (EIRP) for potential transmitters on 3I/ATLAS. Across the 1–9 GHz band, the survey sets EIRP upper limits ranging from $10^{-110}$ W, depending on the frequency and the drift rate (dechirping efficiency).
Sensitivity Comparison: The study notes that while the MeerKAT telescope (Pisano et al. 2025) achieved a lower EIRP limit (~0.17 W) in the 900–1670 MHz range, the ATA survey significantly expands the frequency coverage for 3I/ATLAS by a factor of 10, probing the previously less-explored high-frequency regions (up to 9 GHz).

Significance and Claims
The paper claims that this work represents the first technosignature search of a Solar System object that utilizes expected kinematic drift rates as a specific filtering criterion. By covering 1–9 GHz, the study addresses a parameter space largely unexplored in previous SETI campaigns for ISOs. The authors emphasize that while no technosignatures were found, the result is scientifically valuable for setting population-level constraints on the fraction of ISOs that host radio transmitters. The study adheres to standard practice in the field by performing such searches on all detected ISOs, regardless of initial natural characterization. The authors conclude that future work should focus on refining RFI removal strategies in high-interference bands and applying these drift-rate filtering techniques to future ATA observations to further push into unexplored parameter space.