On July 1, 2025, astronomers detected something unusual: a comet that didn’t originate in our solar system. Designated 3I/ATLAS, it became only the third confirmed interstellar object ever observed passing through our cosmic neighborhood, after ‘Oumuamua in 2017 and Comet Borisov in 2019.
Within days, some of the world’s most powerful radio telescopes turned toward this visitor from another star system. The search wasn’t random. It was coordinated by Breakthrough Listen, the scientific initiative founded by Yuri Milner that represents humanity’s most comprehensive effort to answer one of our oldest questions: Are we alone?
A Rare Opportunity
Interstellar objects are extraordinarily rare. Only three have been confirmed in human history, and each represents a fragment of another solar system: material formed around a distant star, ejected into the void, and by cosmic coincidence passing close enough to Earth for us to observe.
For scientists searching for evidence of extraterrestrial technology, such objects present a unique opportunity. While the probability that any given interstellar visitor carries signs of alien engineering is vanishingly small, the scientific community broadly agrees that thorough investigation is warranted. As researchers noted in a 2025 paper, “given the small number of such objects known, and the plausibility of interstellar probes as a technosignature, thorough study is warranted.”
Yuri Milner’s Breakthrough Listen was positioned to conduct exactly that study. The initiative maintains partnerships with radio telescopes across five continents, creating a global network capable of scanning objects like 3I/ATLAS across a wide range of frequencies.
The Search Protocol
The investigation of 3I/ATLAS demonstrated the scientific rigor that Yuri Milner has built into the Breakthrough Initiatives. Within days of the comet’s discovery, the Allen Telescope Array in northern California—a facility specifically designed for SETI research—began observations. The Parkes Observatory in Australia, the MeerKAT array in South Africa, and the Green Bank Telescope in West Virginia soon followed.
On December 18, 2025, less than 24 hours before 3I/ATLAS made its closest approach to Earth at 167 million miles, the Green Bank Telescope conducted its most sensitive observations yet. Using four receivers spanning frequencies from 1 to 12 GHz, researchers scanned for any signals that might indicate artificial origin.
The methodology was precise. Telescopes alternated between pointing directly at 3I/ATLAS and pointing away, creating an on-off pattern that helps distinguish potential signals from local interference. Any signal that appeared only when the telescope pointed at the target would merit closer examination.
MeerKAT’s observations yielded additional scientific value beyond the SETI search. The telescope detected signatures of hydroxyl, a chemical expected when sunlight breaks down water ice in a comet’s nucleus. This confirmed that 3I/ATLAS was behaving like a natural comet, its ice sublimating as it approached the Sun.
What They Found
After processing data from multiple facilities, Breakthrough Listen reported its findings: no artificial radio emission was detected from 3I/ATLAS. The Green Bank observations were sensitive enough to detect a transmitter with the power output of a typical cell phone at the comet’s distance. None was found.
Nine signals initially flagged for closer examination were ultimately determined to be human-made radio noise, interference from terrestrial sources that appeared in the data even when telescopes weren’t pointing at the target. This kind of careful elimination is central to SETI methodology: extraordinary claims require extraordinary evidence, and the burden of proof remains high.
“In summary, 3I/ATLAS continues to behave as expected from natural astrophysical processes,” the Breakthrough Listen team reported. The comet appears to be exactly what it seems: a frozen remnant of another star system, remarkable in its rarity but showing no signs of technological intervention.
Why the Search Matters to Yuri Milner
For Yuri Milner, the investigation of 3I/ATLAS represents exactly the kind of work the Breakthrough Initiatives were designed to accomplish. In his Eureka Manifesto, Milner argues that the search for extraterrestrial intelligence addresses one of the most profound questions humanity can ask.
“Ever since I was a boy, and read Shklovsky and Sagan’s Intelligent Life in the Universe, I have seen the existence of other civilizations as one of the central questions in science,” Milner writes. That childhood fascination eventually led him to partner with Stephen Hawking in 2015 to launch Breakthrough Listen, now headquartered at the University of Oxford.
As a Giving Pledge signatory, Yuri Milner has committed substantial resources to this search. Breakthrough Listen surveys the million stars closest to Earth, scans the entire galactic plane, and monitors the hundred nearest galaxies. The instruments are 50 times more sensitive than any previous SETI program.
The 3I/ATLAS investigation also showcased recent technological advances. In November 2025, Breakthrough Listen announced a partnership with NVIDIA that produced an AI system capable of detecting signals 600 times faster than existing methods. “This technology doesn’t just make us faster at finding known types of signals—it enables us to discover completely unexpected signal morphologies,” explained Dr. Andrew Siemion, Principal Investigator for Breakthrough Listen.
The Larger Mission
The fact that 3I/ATLAS showed no signs of alien technology doesn’t diminish the value of looking. Each null result refines our understanding of what’s possible, what’s likely, and how we should direct future searches. Science advances through careful observation whether or not findings match our hopes.
For Yuri Milner, this patient accumulation of evidence reflects the philosophy outlined in his manifesto. Humanity’s mission, he argues, is to explore and understand our universe. That mission requires sustained investment in fundamental research even when definitive answers remain elusive.
The Breakthrough Prize, which Milner co-founded in 2012, celebrates this kind of persistent scientific inquiry. The Breakthrough Junior Challenge inspires the next generation to pursue it. And Breakthrough Listen continues the search, telescope by telescope, signal by signal, waiting for the detection that might one day change everything we know about our place in the cosmos.
Until then, the search continues. As Yuri Milner’s initiative demonstrated with 3I/ATLAS, when the next interstellar visitor arrives, or when any promising signal emerges, humanity will be ready.
