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

In 2025 July, the third-ever interstellar object, 3I/ATLAS, was discovered on its ingress into the Solar System. Similar to the NASA Voyager missions sent in 1977, science probes by extraterrestrial life (artifact ``technosignatures’’) could be sent to explore other stellar systems like our own. In this campaign, we used the SETI Institute’s Allen Telescope Array to observe 3I/ATLAS from 1–9GHz. We detected nearly 74 million narrowband hits in 7.25hr of data using the newly-developed search pipeline \texttt{bliss}. We then applied blanking in frequency and drift rate to mitigate Radio Frequency Interference (RFI) in our dataset, narrowing the dataset down to $\sim$2 million hits. These hits were further filtered by the localization code \texttt{NBeamAnalysis}, and the remaining 211 hits were visually inspected in the time-frequency domain. We did not find any signals worthy of additional follow-up. Accounting for the Doppler drift correction and given the non-detection, we are able to set an Effective Isotropic Radiated Power (EIRP) upper limit of $10-110$~W on radio technosignatures from 3I/ATLAS across the frequency and drift rate ranges covered by our survey.

💡 Research Summary

In July 2025 the third interstellar object, 3I/ATLAS, entered the Solar System, prompting a rapid SETI follow‑up with the Allen Telescope Array (ATA). The authors conducted a dedicated campaign covering the full 1–9 GHz band over five observing sessions totaling 7.25 hours. Using the BLADE beamformer they recorded two simultaneous synthesized beams (on‑target and off‑target) with a high‑resolution spectrometer (1.9 Hz channels, 16.8 s integrations) and saved Stokes‑I filterbank data amounting to ~22 TB.

Signal detection was performed with the newly‑developed pipeline bliss, a successor to turboSETI and seticore, optimized for large drift‑rate searches and fine channelization. The search parameters were a drift‑rate range of –4 to +4 Hz s⁻¹ and a signal‑to‑noise ratio (SNR) threshold of 15, deliberately low to capture faint candidates in a radio‑frequency‑interference (RFI) rich environment. This yielded ~74 million narrow‑band “hits”. To tame the overwhelming RFI, the authors manually identified and blanked 0.5 MHz intervals that showed persistent high hit densities across all drift rates, removing 16 % of the total band and reducing the candidate list to ~2 million.

The remaining candidates were processed with the localization code NBeamAnalysis, which compares the on‑ and off‑beam detections to isolate signals that are spatially consistent with the target. After this step only 211 hits survived. Each was inspected visually in the time‑frequency plane; none displayed the hallmark of an artificial narrow‑band transmitter (a compact, line‑like feature with a consistent drift). All were attributed to terrestrial RFI or statistical noise.

Given the non‑detection, the authors derived an upper limit on the Effective Isotropic Radiated Power (EIRP) of any transmitter aboard 3I/ATLAS. Accounting for system temperature, antenna gain, integration time, and the 1–9 GHz coverage, the 5‑σ EIRP limit ranges from roughly 10 W at the low‑frequency end to about 110 W at the high‑frequency end. This sensitivity surpasses previous ISO radio searches (e.g., the 1I/‘Oumuamua campaign) by one to two orders of magnitude, implying that if 3I/ATLAS carried a radio beacon it would have to be extraordinarily low‑power or highly directional.

The paper also discusses the broader context of “Solar System technosignatures” and the rationale for searching interstellar objects for artificial radio emission. It emphasizes that while no compelling evidence for an artifact was found, the methodology—rapid target‑of‑opportunity scheduling, wide‑band simultaneous coverage, high‑resolution beamforming, and the use of the modern bliss pipeline—sets a new standard for future ISO technosignature searches. The authors suggest that longer integration times, expanded frequency coverage, and more sophisticated real‑time RFI mitigation could push EIRP limits down into the sub‑watt regime, further constraining the prevalence of artificial transmitters on interstellar visitors.