On July 1, 2025, the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Chile spotted something unusual, an object on a trajectory that could only have originated outside the Solar System. Designated 3I/ATLAS, it was the third confirmed interstellar object ever detected, following 1I/’Oumuamua (2017) and 2I/Borisov (2019). With an orbital eccentricity of 6.14, more extreme than either of its predecessors, and clear cometary activity, it immediately drew scientific interest.
Within 24 hours of the discovery announcement, the SETI Institute’s Allen Telescope Array (ATA) at Hat Creek Radio Observatory in Northern California began pointing its 28 active dishes at the object.
The results, published in The Astronomical Journal (DOI: 10.3847/1538-3881/ae6651), are a masterclass in null-result science, and a demonstration of how far SETI instrumentation has come.
The search
Over five observing sessions between July 3 and July 10, 2025, the ATA accumulated 7.25 hours of observation on 3I/ATLAS, covering frequencies from 1 to 9 GHz in three overlapping chunks. Each of the 28 dishes pointed simultaneously at the object while a second set of “off-beam” observations, offset by five synthesized beamwidths, recorded the same frequencies as a reference for distinguishing celestial signals from terrestrial interference.
The search pipeline, called `bliss`, the newest successor in a lineage that includes `turboSETI` and `seticore`, identified approximately 74 million narrowband signals (signal-to-noise threshold of 15) across the total data volume of approximately 22 terabytes. Each candidate was a brief, narrow-frequency spike that could, in principle, be a technosignature: an intentional radio transmission from an extraterrestrial source.
The filtering cascade
From 74 million initial hits, a multi-stage filtering process reduced the candidate list to zero.
First, frequency and drift-rate blanking removed 16% of the bandwidth (1.27 GHz) where persistent human interference saturated the receivers, including L-band and GPS downlinks. This narrowed the field to roughly 2 million hits.
Next, a drift-rate filter applied a range of 0.05 to 1.5 Hz/s, calculated from 3I/ATLAS’s known radial acceleration (3.2-3.5 x 10^-5 km/s^2 from JPL Horizons), with additional margin for possible rotational or tumbling contributions. Signals drifting faster or slower than physically possible for a transmitting object on this trajectory were discarded.
The most powerful filter was the spatial discrimination step. Each candidate’s signal strength was compared between the on-target beam and the off-target beam. A genuine extraterrestrial source should appear only in the on-target beam; terrestrial interference typically appears in both. The team applied three criteria, a signal-to-noise ratio between beams exceeding 5.29 (the square root of the number of antennas), a dot-product consistency metric between the two data arrays, and a point-source scoring algorithm, and retained only candidates that passed all three.
This reduced the list to 211 candidates, each examined by eye in time-frequency plots. All 211 traced to human-made sources, Earth-based transmitters, satellites, or other radio frequency interference.
The significance of the null result
The search established upper limits on any hypothetical transmitter aboard 3I/ATLAS. Across the surveyed frequencies, the team can now rule out signals stronger than approximately 10 to 110 watts of effective isotropic radiated power (EIRP), comparable to a household light bulb or a modest radio transmitter. If anything had been broadcasting at those power levels within 1 to 9 GHz, the ATA would have detected it.
The null result provides additional support for the conclusion that 3I/ATLAS is a natural comet-like object originating from another star system, consistent with its observed red color, cometary activity, and the separate discovery and characterization paper by Seligman et al. (2025, arXiv:2507.02757). A separate search using China’s Five-hundred-meter Aperture Spherical Radio Telescope (FAST) also returned null results (arXiv:2603.19023).
Lead author Sofia Z. Sheikh of the SETI Institute and UC Berkeley emphasized that the exercise demonstrated the ATA’s rapid-response capability. “The results from 3I/ATLAS show how realistic it is to detect a signal with the technology we have today,” co-author Valeria Garcia Lopez of Furman University told reporters.
The three interstellar visitors
The three known interstellar objects now form a small but instructive sample set. 1I/’Oumuamua (2017) was an elongated, tumbling object with no cometary activity, its origin remains debated. 2I/Borisov (2019) behaved like a typical comet from another star. 3I/ATLAS joins Borisov as a clearly cometary object, but with more extreme orbital parameters and the most intensive SETI search yet conducted on any interstellar visitor.
With next-generation observatories like the Vera C. Rubin Observatory now operational, the detection rate of interstellar objects is expected to increase dramatically, from one every few years to potentially several per year. Each will offer a new opportunity to ask whether any carry something more than rock and ice.
Source: The Astronomical Journal 172(1), DOI: 10.3847/1538-3881/ae6651, “A Search for Radio Technosignatures from Interstellar Object 3I/ATLAS with the Allen Telescope Array” by Sheikh et al., 2026.