Radio Signals

• MeerKAT’s 64-antenna array in South Africa detected hydroxyl absorption lines at 1665 and 1667 MHz—the same “water-hole” frequencies long proposed for interstellar communication.
• Harvard astrophysicist Avi Loeb says 3I/ATLAS’s signal appeared just five days before its perihelion, when solar radiation should have maximized OH emission, not absorption.
• Next checkpoint: December 19 — Earth’s closest approach, when researchers will test whether the object repeats its signal or stays silent.

By Samuel Lopez

CAPE TOWN (USA HERALD) — A radio telescope on South Africa’s Karoo plain has detected what may be the first microwave whisper from an interstellar object. The MeerKAT Observatory recorded narrow-band hydroxyl (OH) absorption lines at 1665.4018 and 1667.3590 megahertz—frequencies that scientists since the 1970s have nicknamed the “water hole,” a naturally quiet slice of the spectrum between hydrogen and hydroxyl emissions where intelligent civilizations might choose to broadcast.

The detection occurred on October 24, five days before 3I/ATLAS reached its closest pass to the Sun. According to the South African Radio Astronomy Observatory, the signal showed OH absorption rather than emission—possibly because the object’s near-solar geometry favored shadowed absorption.

“Five weeks ago, I encouraged radio observatories like MeerKAT to search for radio emission from 3I/ATLAS,”Prof. Avi Loeb wrote in a Medium post. “In response, I was assured that 3I/ATLAS will be monitored.”

Loeb has noted in the past, that the arrival direction of 3I/ATLAS matches the 1977 Wow! Signal within 9 degrees at 1.4204556 GHz — another coincidence inside the hydrogen-OH band long theorized as an interstellar “greeting channel.”

So far, no other radio detections have been confirmed beyond the OH absorption lines, but the team plans continuous monitoring to see if the molecule’s production fluctuates or if the signal shows any non-thermal modulation that might hint at engineering.

Why OH Is Elegant for Interstellar Communication

• It’s chemically universal (wherever there’s water chemistry, there’s OH).
• It sits in the quietest natural frequency window of the cosmos.
• It offers quantum-level control mechanisms (hyperfine transitions, spin coupling).
• And crucially, its presence symbolically represents water, which could be interpreted as an intentional “language of life.”

Thus, if an advanced civilization or even a trans-dimensional intelligence wanted to send a message that says “we know you, water-based lifeforms,” it might literally whisper through the hydroxyl molecule.

Why a Comet Is a Perfect Natural Transmitter

A comet is not just a dirty snowball — it’s a complex electromagnetic system containing:

• Water ice and organics (which release OH radicals as sunlight breaks them down);
• Dust grains and metallic ions that interact with solar radiation and magnetic fields;
• Plasma tails that act as dynamic antennas for radio and plasma-wave propagation.

That means a comet already broadcasts across multiple electromagnetic bands — infrared, visible, UV, and radio — every time it approaches a star.
An advanced species could exploit those natural emissions as a carrier wave.

Why Use a Comet at All?

Strategically, a comet offers several advantages to a civilization that wants to communicate without revealing its location:

• Stealth and deniability: Appears as a natural body.
• Durability: Can travel for eons through interstellar space.
• Universality: Any intelligent species with astronomy will study comets.
• Self-powering: Solar energy triggers emissions automatically near stars.
• Symbolism: A comet is a literal messenger — its name derives from “long-haired star,” and its periodic return makes it ideal for time-coded communication.

It’s the ultimate low-energy, high-longevity interstellar post card.

Meanwhile, back here on Earth, the SETI Institute and other facilities are calibrating receivers around the 1420–1720 MHz window to search for repeatable patterns or mathematical sequences.

Researchers expect to collect the next round of spectra in mid-December as 3I/ATLAS makes its closest approach to Earth on December 19, 2025. Any repetition of narrow-band activity at the same frequencies could confirm a non-random source. Juno’s March 2026 flyby will then offer an independent space-based check. Results from both campaigns are expected by mid-2026.

Sources

• MeerKAT Observatory Radio Spectral Report (Oct. 24, 2025) — South African Radio Astronomy Observatory
• Avi Loeb,Medium Post, “Why the Wow! Signal Still Matters,” (Oct. 2025)
• SA RAO Observation Log (Sept. 20–28, Oct. 24, 2025)
• SETI Institute Briefing on Hydroxyl Band Monitoring (2025)