Is a visiting probe a dirty snowball in another star? Radio astronomy now has one of the hardest reality checks on one of the viral ideas put by astronomers and the answer is nothing.
Comet 3I/ATLAS has a special place in the contemporary skywatching because it is even present in the catalog. It is the third object of the solar system to pass through the interstellar after 1I/Oumuamua in 2017 and 2I/Borisov in 2019. It is exactly this rarity that gave so much speculation such a rapid growth: when there are three specimens, every new arrival is a test case of all that has happened since the dawn of the universe as well as of science fiction.
In order to decouple the irresistible curiosity and wishful thinking, the scientists directed one of the most sensitive ears in the world at the comet, the 100-meter Green Bank Telescope, in West Virginia. The observations were conducted in the framework of the Breakthrough Listen program, which focuses on the search of such signs of technology as “technosignatures”, i.e. signals that can only be produced by technological equipment, scanning the sky.
“We all would have been thrilled to find technosignatures coming from 3I/ATLAS, but they’re just not there,” and they are not there, said Benjamin Jacobson-Bell of the University of California, Berkeley. “Finding no signals was the result we expected, due to the significant evidence for 3I/ATLAS being a comet with only natural features.”
The technical issue is straightforward: should a spaceship have wished to identify itself on the other side of the interstellar, a small band of radio signal would have been among the least energy-consuming methods of communication. Such signals can be relatively inexpensive to transmit and they travel well making them a popular target in SETI-like searches. The strength of the Green Bank Telescope is scale, location, and instrumentation its extensive frequency coverage coupled with the location in a radio-quiet area minimizes interference and increases the net.
Jacobson-Bell were only too happy to tell how deep the net descended. “Its sensitivity enables us to verify the absence of transmitters down to 0.1 watts, the strongest evidence against technology of any 3I/ATLAS observation to date.” That threshold in practice is lower than normal consumer hardware. This is to say that if there were any transmitters on 3I/ATLAS up to ten times weaker than a cell phone, we would have found them, he added.
But there were a few, tantalizing glimpses–the very kind of thing that helps to make online threads go round.“We did find nine ‘events,’ which is our term for signals that pass certain filters in our search strategy, but on closer inspection, we could readily attribute all nine of them to known radio transmitters here on Earth,” Jacobson-Bell said. The process of detecting, scrutinizing and discarding is not an appendix of a technosignature search in the present day, but the essence of the field. Radio leakage is so abundant on the earth that with great care even a filtering will give false alarms at a regular rate.
In the larger campaign of Breakthrough Listen other facilities had their turn, such as 1 indicate that 12 GHz observations as well as searches at sensitivities in the same neighborhood as “cell phone power”. All these attempts did not find any man-made radio emission associated with the comet. Even the simplest explanation is what the telescopes observe in visible light: 3I/ATLAS feels and appears like a comet.
Seizing that “just a comet” title is not an insult. Interstellar objects are a viable sample-return missions, which reach Earth without the rockets. Since comets and asteroids retain material remaining during the formation of planets, an interstellar comet can be a carrier of chemical and structural evidence of a planetary system otherwise too distant to remain reachable. As early as June 21, 2025, cometary activity was observed with the initial images of the commissioning of the Vera C. Rubin Observatory, and obtained a nucleus effective radius of the order of 5.6 ± 0.7 km (with the uncertainties introduced by a dusty coma). Other arrays in the radio have observed chemistry expected of comet physics such as the presence of hydroxyl signatures likely to be produced as sunlight breaks down various waters-related ices.
Simultaneously, the buzz surrounding the allegations of “alien probe” has pointed out to something true within the context of the mechanism of science: in the event that interstellar probes are any likelihood at all, it seems logical to conduct an efficient check and publicize constraints. “The evidence was against 3I/ATLAS being one such probe, but we would have been remiss not to check”. Another thing that he mentioned is that similar ideas have been taken seriously by humans, that is, the idea of sending lightweight probes to other stars, a vision related to such projects as Breakthrough Starshot.
The best thing about this moment besides it being a one-off debunk is that 3I/ATLAS can be an indicator of a more active decade. Jacobson-Bell predicts that discovery rates will increase dramatically as the Vera C. Rubin Observatory initiates the Legacy Survey of Space and Time (LSST), creating a population of interstellar objects so vast that it will be possible to see the difference between normal and actually strange. “Whereas each individual interstellar object is currently an anomaly, future surveys will amass such a population of interstellar objects that we’ll start to be able to tell which are typical and which are actually anomalous,” he said.
Currently, the best and most plausible conclusion happens to be the most plausible, “Past work has shown that 3I/ATLAS looks like a comet and behaves like a comet, and our observations show that, like a comet, 3I/ATLAS is not a source of technological signals. In the end, there were no surprises.”
