3I/ATLAS Blue Glow Shock
Astronomers using South Africa’s MeerKAT telescope detected a narrow-band signal at 1665 and 1667 MHz from interstellar comet 3I/ATLAS on October 24, 2025. Found in the so-called ‘water-hole’ range, the unprecedented absorption signature marks the first time a comet-like object has produced such a radio signal. Atlastellar Instagram Account Photo

On 24 October 2025, South Africa's MeerKAT radio telescope detected a signal that has left astronomers stunned. The interstellar comet 3I/ATLAS produced a narrow-band hydroxyl absorption at 1665 and 1667 MHz, a range known as the 'water hole.' This quiet region of the spectrum has long been considered significant in the search for extraterrestrial intelligence. Yet no comet-like object has ever produced such a signature before.

A Rare Record of Radio-Active Comets

Radio detections of comets are historically rare. Over the past fifty years, only about twenty comets have been confirmed through radio observation, including Kohoutek, Hale–Bopp, Hyakutake, NEOWISE, and Halley's Comet. These detections typically involve natural processes: microwave emissions from hydroxyl radicals, thermal noise from cometary comae, or radar reflections revealing nucleus size and rotation. The Rosetta mission later expanded this record by directly sensing comet 67P/Churyumov–Gerasimenko. None of these cases, however, ever displayed a narrow-band absorption signature like 3I/ATLAS.

Why the Signal Is So Unusual

The hydroxyl radical normally produces emission lines when comets approach the Sun, as heating maximises activity. Instead, 3I/ATLAS registered absorption at precisely those frequencies, just days before its perihelion. This reversal violates decades of cometary physics. The specificity of the signal, combined with its location in the water-hole range, has raised questions about whether the phenomenon is purely natural or something more.

Comparing 3I/ATLAS to Other Interstellar Visitors

Previous interstellar objects, such as 1I/ʻOumuamua and 2I/Borisov, produced no radio anomalies. 3I/ATLAS stands apart by delivering a narrow-band absorption pattern at frequencies historically associated with potential communication. The contrast with traditional comets, which generate broad and noisy signals, is absolute. For scientists, this anomaly represents either the strangest natural comet ever recorded or a possible hint of interstellar technology.

Calls for Transparency

Researchers and commentators have urged agencies like NASA, JPL, and the Planetary Defense Coordination Office to release all available data, including spectra and high-resolution images. The public, they argue, deserves transparency at a moment when the scientific community is grappling with whether humanity has witnessed a new kind of cosmic behaviour or something unprecedented in the search for signals beyond Earth.

Waiting for 19 December

As 3I/ATLAS continues its journey, scientists are watching closely. December 19 has become a focal point, as researchers wait to see whether the water hole frequencies will remain silent or speak again. If the signal repeats, it could mark one of the most significant discoveries in planetary science. If not, 3I/ATLAS will still stand as a comet that defied expectations and expanded the boundaries of what astronomers thought possible.

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