When a faint, fast-moving speck named 3I/ATLAS slipped into view earlier this year, it carried more than a whiff of cosmic intrigue. Only two other confirmed interstellar objects have ever wandered through our Solar System, and both left scientists with more questions than answers. The internet promptly filled the gap with theories ranging from the plausible to the eccentric. However, while the speculation churned, a quieter, methodical effort began to take shape in South Africa.

At the centre of it: MeerKAT, one of the world’s most sensitive radio telescopes and a research team drawn from the University of Cape Town, SARAO, Rhodes University, Sweden’s KTH Royal Institute of Technology, and the Breakthrough Listen initiative. Their goal was straightforward: to establish whether 3I/ATLAS could be seen in radio waves. And their findings now confirm: it behaves like a comet, and not a technological artefact broadcasting radio signals.

For Rhodes University, the project reflects something larger than a single discovery. It is a demonstration of how fundamental science, rigorous technology development and local expertise intersect to produce research with global relevance. The institution’s Centre for Radio Astronomy Techniques and Technologies (RATT), headed by Distinguished Professor Oleg Smirnov, plays a central role in that effort.

Prof Smirnov notes that the project almost never happened, were it not for an unexpected nudge. “This entire thing began because the coach of the Rhodes University Rifle Club asked me, quite innocently, whether MeerKAT was looking at this interstellar ‘thing’ he’d heard about in the news,” he says. “I was ready to answer  ‘no, radio telescopes don’t really see comets’ and then remembered a very specific exception. So yes, you can thank Geoff for the plot twist.”

That “exception” is a particular radio signature produced when sunlight heats a comet’s icy surface. As the ice vaporises, water molecules split into hydrogen and hydroxyl. Under the right conditions, those hydroxyl molecules start behaving a bit like microscopic radio beacons, absorbing and emitting signals at precise frequencies. MeerKAT happens to be able to observe those frequencies.

Because these signals only peak at certain moments in a comet’s journey, the team had only narrow observational windows to work with, but the SARAO team did wonders to free up the telescope at the right time. Early attempts yielded nothing useful aside from a reassuringly bright hydroxyl signal from an unrelated star that happened to drift through the telescope’s field of view. 

Then, on 24 October 2025, MeerKAT finally caught the first whisper of hydroxyl absorption from 3I/ATLAS. A brief disappearance followed on 4 November, followed by more detections on 6 and 11 November, and finally, with absorption turning into emission on 12 November, exactly as predicted by cometary models. 

At the same time, scientists from the Breakthrough Listen initiative analysed data from MeerKAT’s BLUSE backend for narrowband signals that would hint at technological rather than natural origins. Nothing appeared – while, notably, MeerKAT’s exquisite sensitivity meant that it should be able to pick up something as faint as a cell phone at the distance to 3I/ATLAS. The interstellar visitor was silent in all the ways a technological artefact – at least, one that used radio transmissions – shouldn’t be.

For Prof Smirnov, the scientific payoff is clear, but so is the broader value. “We’re entering an era where interstellar objects won’t be exotic anomalies anymore,” he says. “Our telescopes are becoming sensitive enough to catch them regularly. Understanding them isn’t only about astronomy; it touches on how planetary systems form, what material gets exchanged between them, and ultimately, how life’s building blocks travel.”

He also notes the uniquely South African contribution. “MeerKAT’s world-beating sensitivity is what made this detection possible in the first place. Add the BLUSE system riding shotgun, and you get an instrument combination you won’t find anywhere else. It’s a reminder that cutting-edge research doesn’t only happen in the places people traditionally expect.”

The work reflects Rhodes University’s longstanding commitment to research that pushes scientific boundaries while building capacity for national and global benefit. From advanced signal-processing techniques to collaborations stretching across continents, the institution’s focus remains on inquiry that tangibly deepens understanding of our world – and, through the work being done by RATT, of worlds way beyond those belonging to own Solar System.

Prof Smirnov puts it more simply: “There’s something wonderful about using serious scientific machinery to follow up on a casual query from the shooting range, which suddenly opens up lofty questions about the Universe. If that’s not the spirit of research, I don’t know what is.”

 

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The study was led by Professor DJ Pisano (UCT), Distinguished Professor Oleg Smirnov (Rhodes University & SARAO), Sarah Buchner (SARAO), Prof Mykola Ivchenko and Dr Lorenz Roth (KTH Royal Institute of Technology, Sweden), Dr Chenoa Tremblay (SETI Institute & Breakthrough Listen) and Dr Andrew Siemion (Breakthrough Listen & SETI Institute & Oxford Astrophysics), and was made possible by the entire SARAO and Breakthrough Listen team.