An ancient interstellar comet has emitted the first confirmed radio signal ever detected from such an object, as astronomers track 3I/ATLAS during its close passage through the inner Solar System and use the landmark detection to probe both its origins and its natural, cometary behavior.
Ancient Interstellar Comet Emits First-Ever Radio Signal
Astronomers have picked up the first “radio signal” from an ancient interstellar comet, a faint but decisive signature coming from 3I/ATLAS as it races through the Solar System on a one‑time visit from another star.
The detection, made in late October 2025 with South Africa’s MeerKAT radio telescope, reveals natural radio absorption from hydroxyl molecules in the comet’s coma and strongly supports the view that 3I/ATLAS is a normal, if highly unusual, comet rather than alien technology.
The observation marks the first time radio waves linked to water chemistry have been recorded from an interstellar comet, giving scientists a new way to study how ice, gas, and dust behave in objects born around other stars.
What Scientists Detected And When
MeerKAT, a 64‑antenna radio array operated by the South African Radio Astronomy Observatory (SARAO), observed 3I/ATLAS for several hours on October 24, 2025, shortly after the comet swung around the Sun at perihelion on October 29.
During that session, a team led by astronomers D.J. Pisano and Oleg Smirnov detected absorption at radio frequencies of 1665 and 1667 megahertz, the classic “fingerprint” of hydroxyl (OH) molecules created when sunlight breaks apart water streaming off the comet.
Earlier MeerKAT observations in September showed no such signal, indicating that the radio absorption only appeared once solar heating intensified and outgassing increased near perihelion.
The result was reported through The Astronomer’s Telegram and confirmed by SARAO, which emphasized that no artificial, modulated transmission was found and that 3I/ATLAS behaves like a typical comet at radio wavelengths.
Key Radio Detection Facts
| Parameter | Detail |
| Telescope | MeerKAT (South African Radio Astronomy Observatory) |
| Observation date | October 24, 2025 |
| Frequencies detected | 1665 MHz and 1667 MHz OH lines |
| Physical process traced | Solar‑driven photodissociation of water (outgassing) |
| Artificial/technological signal | None detected |
Why This Radio Signal Matters
This is the first time astronomers have captured radio emission linked to gas chemistry from an interstellar object, adding a new layer to optical and infrared data from observatories such as Hubble and the James Webb Space Telescope (JWST).
Because the OH lines arise when water molecules are destroyed by sunlight, the signal confirms that 3I/ATLAS is actively shedding volatile material and supports its classification as a water‑bearing comet rather than an asteroid or solid metallic object.
The measurement also lets researchers estimate how quickly the comet is losing mass and how its activity changes with distance from the Sun, providing clues to the composition and thermal history of an object that likely formed 5–7 billion years ago in the outer regions of another star system.
By comparing these radio characteristics with those of comets in the Solar System, scientists can test how similar or different interstellar ices and dust grains are from local material.
An Ancient Visitor From Another Star
3I/ATLAS is only the third known interstellar object, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019, and the first such visitor to show this kind of detailed cometary activity across the spectrum.
Discovered on July 1, 2025, by a telescope in the NASA‑funded ATLAS (Asteroid Terrestrial‑impact Last Alert System) network in Chile, it was quickly recognized as interstellar because its orbit is hyperbolic, with an eccentricity of about 6.14, far higher than any bound Solar System comet.
Orbital modeling suggests that 3I/ATLAS was likely ejected from a distant “frontier” region of the Milky Way, perhaps up to 7 billion years ago, and has wandered between stars ever since.
Polarization measurements and early infrared data show a dark, dust‑rich coma with a mix of ices, including an unusually high carbon‑dioxide‑to‑water ratio compared with many Solar System comets, hinting at a cold, chemically distinct birthplace.
Basic Properties Of 3I/ATLAS
| Property | Approximate Value / Status |
| Object type | Interstellar comet (3rd known interstellar object) |
| Discovery date | July 1, 2025 (ATLAS survey, Chile) |
| Orbit | Hyperbolic, eccentricity ~6.14 |
| Likely age | Up to ~7 billion years |
| Origin region | Outer “frontier” of the Milky Way |
Close Approach To Earth And Multi‑Mission Campaign
As the radio detection was made, 3I/ATLAS was on course for a rare flyby that brought it into the inner Solar System and, eventually, relatively close to Earth.
The comet reached perihelion on October 29, 2025, and then headed outward, making its closest approach to Earth in mid‑December at a distance of roughly 93 million kilometers (about 1 astronomical unit).
NASA, ESA, and other agencies organized a broad observing campaign using Hubble, JWST, and several planetary spacecraft to monitor changes in the comet’s coma, tail, and interaction with the solar wind as it passed.
X‑ray space telescopes detected a high‑energy glow stretching hundreds of thousands of miles behind the comet, produced as the solar wind slammed into its gas cloud and stripped away charged particles.
By mid‑December, as 3I/ATLAS moved through the constellations Virgo and Leo, its brightness faded to about magnitude 13–15, leaving it accessible mainly to larger amateur telescopes under dark skies.
Trajectory calculations show that after it leaves the inner Solar System in early 2026, the comet will not return, continuing into deep space on a one‑way path out of the Sun’s gravitational reach.
Addressing Alien Speculation And Misconceptions
Even before the radio detection, 3I/ATLAS attracted speculation that it might be an artificial probe, echoing earlier debates around ʻOumuamua.
Some commentators linked the phrase “radio signal” to the idea of an intentional transmission, and pointed to the comet’s changing brightness, unusual color, and non‑gravitational forces as possible signs of technology.
However, the MeerKAT team and other observatories report that all measurements—including the OH absorption at 1.665–1.667 GHz, the X‑ray emission, and the comet’s activity pattern—are fully consistent with natural cometary processes.
Researchers explain that apparent anomalies, such as an “anti‑tail” pointing toward the Sun or transient color shifts, can arise from viewing geometry, variations in dust size, and changing gas mixtures, and that similar behavior has been documented in Solar System comets.
How The Radio Signal Helps Decode Comet Chemistry
The newly detected radio signal acts as a chemical probe of the comet’s atmosphere, giving astronomers a more precise picture of its composition and outgassing rate.
Hydroxyl molecules that absorb radio waves at 1665 and 1667 MHz form when ultraviolet sunlight breaks apart water molecules released from the nucleus, so the depth and width of these lines trace how much water is present and how fast it is being destroyed.
By comparing observations from September, when no OH absorption was visible, with the October detection, scientists can track how quickly the comet “switches on” as it nears the Sun, and how its activity depends on solar energy rather than on internal heat or any unusual mechanism.
These data, combined with infrared spectra from JWST and ultraviolet measurements from Hubble, help reconstruct the balance of water, carbon dioxide, and other gases, which in turn reveal how and where the comet’s ices originally froze.
What Comes Next For Interstellar Comet Science
Although 3I/ATLAS is already fading and will soon disappear from view, the radio detection and associated multi‑wavelength campaign are expected to shape interstellar comet research for years.
Astronomers are using the new data to refine models of how interstellar comets form, how often they enter the Solar System, and how their chemistry compares with local objects such as Kuiper Belt and Oort Cloud comets.
Future surveys like the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) are anticipated to find many more interstellar visitors, which could each be followed up with radio telescopes to search for similar OH signatures.
Space agencies are also evaluating mission concepts that could one day send a dedicated spacecraft to intercept a future interstellar comet, using the 3I/ATLAS experience as a template for what instruments and observing strategies would be most informative.
Final Thoughts
The first radio signal from 3I/ATLAS is not a message from extraterrestrials but a subtle chemical whisper from water molecules breaking apart under sunlight around a comet born in another star system.
This detection confirms that at least some interstellar objects behave very much like ordinary comets, even as they carry exotic ices and dust that reflect conditions in distant regions of the galaxy.
For scientists, 3I/ATLAS is both a scientific windfall and a fleeting opportunity: a natural probe that briefly passes through the Solar System and then vanishes forever into deep space, leaving behind only data.
As telescopes and surveys improve, more such visitors will likely be discovered, and each will offer another chance to listen in on the quiet radio signatures of alien ices and to better understand how common the building blocks of planets and life may be across the Milky Way.
