FAST Telescope Searches Interstellar Object 3I/ATLAS for Alien Radio Signals
China’s Five-hundred-meter Aperture Spherical Telescope (FAST), the world’s largest single-dish radio telescope, has conducted the first search for periodic radio signals from 3I/ATLAS — the third confirmed interstellar object to visit our solar system. No artificial signals were found, but the search itself represents a methodological leap for the field of SETI.
The study, submitted to The Astronomical Journal by a team led by Jian-Kang Li of Shanghai Dianji University, targeted 3I/ATLAS for periodic modulated transmissions — a class of signals distinct from the narrowband drifting signals that most previous SETI searches have looked for. The object was observed with FAST’s L-band 19-beam receiver on three separate dates between October 2025 and January 2026.
A visitor from another star
3I/ATLAS was discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System in Chile, and immediately recognized as the third confirmed interstellar object after 1I/’Oumuamua (2017) and 2I/Borisov (2019). Unlike ‘Oumuamua, 3I/ATLAS is clearly an active comet, displaying a coma and tail as it approached the Sun. JWST detected water ice and vapor, carbon dioxide, carbon monoxide, and methane in its composition — a pristine sample from another planetary system.
The object reached perihelion on October 29, 2025, at a distance of 1.36 AU from the Sun. Its hyperbolic trajectory, with an eccentricity of 6.137, confirmed its interstellar origin beyond any doubt.
A novel search methodology
The team employed a technique called Canonical Polyadic Decomposition (CPD) to analyze FAST’s multibeam data. Traditional SETI searches typically require separate ON-source and OFF-source observations to distinguish genuine signals from Earth-based radio-frequency interference (RFI). CPD instead factorizes the full multibeam dataset into a set of separable components, each with characteristic time, frequency, and beam signatures. This allows the algorithm to identify signals concentrated in the center beam (pointing at 3I/ATLAS) while rejecting those that appear across multiple beams (characteristic of local RFI) — all in a single operation.
From the three observation sessions, the team analyzed nearly 2,000 components from each data tensor. After applying a series of stringent filtering criteria, three candidate signals survived initial scrutiny. All three were traced back to FAST’s own calibration diode, which injects a periodic signal at 300-second intervals to calibrate the instrument.
No credible artificial radio transmissions were detected. The search established an upper limit of 0.146 watts equivalent isotropic radiated power (EIRP) at the distance of 3I/ATLAS — far more sensitive than previous efforts, including the Allen Telescope Array and Green Bank Telescope searches.
Consistent with a natural origin
The null result is consistent with every other SETI search of 3I/ATLAS conducted to date. Breakthrough Listen’s observations with the Allen Telescope Array (covering 1 to 9 GHz across 7.25 hours) detected 74 million narrowband hits, all rejected as RFI, with EIRP upper limits of 10 to 110 watts. The MeerKAT telescope in South Africa searched for intelligent signals and detected none. The Green Bank Telescope covered a broad frequency range with similar results.
Taken together, the cumulative SETI coverage strongly supports the conclusion that 3I/ATLAS is a naturally occurring comet from another star system, not an artificial object.
What the methodology means for the future
The paper’s significance extends beyond its null result. The CPD-based approach for periodic signal detection is the first application of tensor decomposition to multibeam SETI data, and it could prove valuable for future searches with FAST and other large radio telescopes. The technique naturally handles multi-beam RFI rejection, reducing false positives and streamlining the analysis pipeline.
The team’s companion paper (Li et al. 2026b, arXiv:2603.19023) covers a narrowband Doppler-drift search using the same FAST observations, providing the most comprehensive SETI coverage of any interstellar object to date with the world’s most sensitive telescope.
The paper, “Periodic Radio Technosignature Search toward 3I/ATLAS with FAST,” is available as a preprint on arXiv (2607.01666) and has been accepted for publication in The Astronomical Journal.