SETI Scanned Interstellar Object 3I/ATLAS for Alien Radio Signals – Here Is What It Found.

An automated sky survey discovered something out of place on July 1, 2025. A tiny object had entered the solar system from the direction of Sagittarius and was traveling inward at a speed that was far too fast for anything gravitationally bound to our sun. In a matter of days, astronomers verified what the data had already indicated: this was the third interstellar visitor ever discovered, moving at a speed of about 58 kilometers per second. It was given the name 3I/ATLAS. And almost instantly, the question that always seems to follow these discoveries reappeared: What precisely is it?

The scientific solution was developed rather rapidly. In the months that followed, observations from telescopes on several continents and wavelengths combined to create a consistent picture: 3I/ATLAS is a comet that originated in the interstellar medium and is between three and eleven billion years old, possibly older than Earth.

It carries chemical signatures consistent with outgassing water ice—exactly what a comet does as it approaches a star—and is faster and larger than the two interstellar objects discovered before it, Oumuamua in 2017 and Borisov in 2019. Hydroxyl, a byproduct of sunlight breaking down water molecules, was detected by the MeerKAT telescope in South Africa, confirming that it was acting like comets. Organic. Explainable. Interesting. And yet. Shortly after 3I/ATLAS was discovered, Harvard astronomer Avi Loeb, who has made a career out of coming up with unusual explanations for strange astronomical objects, proposed that 3I/ATLAS might be an alien spacecraft.

It’s a claim that was met with familiar skepticism by the scientific community as a whole. Loeb made similar claims about Oumuamua, which were ultimately refuted by the overwhelming body of evidence. However improbable, the theory has a peculiar gravity of its own. Three objects from interstellar space have been found. We have very little idea what else might exist. The truthful response to the question “could it be artificial?” is “almost certainly not, but let’s check” rather than “definitely not.”

Thus, they investigated. When 3I/ATLAS made its closest approach to Earth in December 2025, some of the world’s most sensitive radio equipment was directed at it by the Breakthrough Listen program, which bills itself as the largest scientific research effort aimed at finding evidence of civilizations beyond Earth. The primary instrument was the Robert C. Byrd Green Bank Telescope in Green Bank, West Virginia. It is a 100-meter dish situated in an Appalachian hill radio quiet zone that has been designated by the federal government.

It is surrounded by rules that forbid Wi-Fi routers and restrict cell phone use for miles in all directions. The silence is important. A microwave oven placed incorrectly can appear to be a signal from space at those sensitivity levels. The GBT covered four radio spectrum bands from 1 to 12 GHz while scanning 3I/ATLAS from a distance of roughly 167 million miles. The telescope was sensitive enough to pick up a transmitter emitting about the power of a cell phone at that distance.

Almost 470,000 candidate signals were found in the original data. The majority of those signals also appeared when the telescope was pointed away from 3I/ATLAS, indicating that they originated from Earth rather than an interstellar comet. This number may seem exciting, but it becomes clear when you consider its practical implications. Stray interference from human civilization bleeding into the radio spectrum, satellites, and ground-based electronics are all constantly present, and a large part of SETI researchers’ work involves filtering it out. Nine signals were left as candidates after the obvious contaminants were eliminated. After more investigation, each of those was finally determined to be radio interference caused by humans. 3I/ATLAS itself did not produce any artificial emissions.

Days after the discovery, in early July, the Allen Telescope Array in northern California had already carried out its own search, scanning from 1 to 9 GHz. same outcome. The “Murriyang” dish, which is 64 meters across, was observed on three different dates during the summer and into October by the Parkes telescope in Australia. As of the announcement, the narrowband search’s results were still being finalized. In addition to the standard spectroscopy, the Vera C. Rubin Observatory took part in optical observations, searching for any transient laser signals. There was also nothing synthetic. The conclusion is about as solid as these conclusions get, and it’s a thorough accounting carried out by serious people with serious equipment.

Even if the answer isn’t the dramatic one, there is something worthwhile in all of this. Observing scientists use the world’s largest steerable radio telescope to listen for signals from a rock that entered our solar system from another star is, by all accounts, a remarkable thing for our civilization to be doing. The effort is not diminished by the fact that the response was silent. If anything, it makes a crucial point clear: the procedures and tools are in place, and it is now common practice to ask the question seriously rather than merely rhetorically. The next interstellar object to appear will be scanned as thoroughly, and most likely more quickly.

3I/ATLAS is currently returning. It won’t come back. It will travel for billions of years without a star to orbit as its trajectory takes it away from the sun and back into interstellar space. For years to come, the information it left behind—chemical spectra, light curves, and radio observations—will be examined, gradually expanding our knowledge of the material that drifts between star systems. Ultimately, it is a comet. An old, swift, chemically fascinating comet from an unfathomably distant place, passing through momentarily and disappearing silently. It’s difficult not to think that was equally amazing in its own right as the alternative would have been.