Astronomers Detect First ‘Radio Signal’ from Interstellar Comet 3I/ATLAS
London-UK, 14, November, 2025
Astronomers Detect First ‘Radio Signal’ from Interstellar Comet 3I/ATLAS
In a major astronomical first, researchers utilising the Arecibo Observatory’s vast radio telescope array have successfully detected a distinct, unambiguous “radio signal” emanating from Interstellar Comet 3I/ATLAS.
The signal, which is not artificial but rather the electromagnetic signature of highly energized hydroxyl (OH) radicals in the comet’s atmosphere, confirms that the interstellar visitor is actively outgassing water.
This is the first time that a radio signature of a chemical element has been confirmed from a comet originating outside our solar system, providing astronomers with an unprecedented opportunity to study the chemical composition of an object that formed in an entirely different star system.
The finding validates theories about the ubiquity of water-bearing bodies across the galaxy and opens a crucial new window into the diversity of planetary nurseries beyond our own.
Key Headlines
Interstellar Confirmed:
The comet, 3I/ATLAS, is only the third interstellar object ever detected passing through our solar system, following Oumuamua and Borisov.
Hydroxyl Emission:
The detected signal is a telltale emission line at 1667 MHz produced by hydroxyl (OH), a breakdown product of water ice as it vaporizes in the sun’s warmth.
Water Abundant:
The strength of the radio signal confirms the presence of substantial quantities of water ice within the comet’s nucleus, suggesting that water is a common component of comets regardless of their origin.
Arecibo’s Legacy:
The detection is a crucial, high-profile success for the reconstructed Arecibo Observatory in Puerto Rico, showcasing its continuing, unique capabilities in deep space radio astronomy.
Interstellar Comet 3I/ATLAS (the ‘I’ denotes Interstellar) was first discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey and was subsequently confirmed to be travelling on a highly hyperbolic trajectory, a telltale sign that it did not originate from the Sun’s gravitational influence.
Its path indicates it is merely passing through our solar system before continuing its journey back into the vast reaches of the Milky Way galaxy.
The current breakthrough was achieved by aiming the powerful, targeted radio instruments of the reconstructed Arecibo Observatory at the comet as it made its closest approach to the Sun (perihelion) and Earth.
Radio astronomers focused their observations on key molecular frequencies, particularly the 1667 MHz line which is the specific radio fingerprint of the hydroxyl (OH) radical.
This radical is formed when the abundant water ice within a comet’s nucleus is vaporized by the Sun’s heat and then broken apart by solar ultraviolet radiation.
The clear and powerful detection of the hydroxyl signal confirms two crucial facts.
Firstly, it provides definitive proof that 3I/ATLAS is an actively outgassing water-bearing body. Previously, astronomers relied on spectroscopic analysis of the comet’s visible coma (the fuzzy envelope around the nucleus) to infer the presence of water, but this radio signal provides direct chemical evidence.
Secondly, and more profoundly, it shows that the physical and chemical processes that led to the formation of water-rich comets in our own Kuiper Belt and Oort Cloud are also operating in the diverse, distant planetary nurseries across the galaxy.
The team reported that the strength of the hydroxyl signal suggests a significant water production rate, estimated to be in the order of millions of kilograms of water per second as the comet heated up.
This water abundance is remarkably similar to that observed in typical comets originating from the fringes of our own solar system. This similarity implies a degree of uniformity in the chemical makeup of planet-forming discs across the galaxy, which is highly significant for the overall search for extraterrestrial life.
The detection is also a tremendous triumph for the scientific community, which successfully advocated for the reconstruction and modernisation of the Arecibo Observatory after its dramatic 2020 collapse. The ability of the vast radio dish to capture such a weak, unique signal from a fast-moving, distant object validates its continued importance as a singular tool for deep space observation.
Future observations using this and other next-generation radio telescopes will now focus on detecting the subtle chemical signatures of other trace elements in 3I/ATLAS, potentially revealing an even more detailed portrait of a solar system far, far away.
