Interstellar Visitor 3I/ATLAS: The Unlikely Discoverer Behind a New Kind of Comet
What looked like a routine July evening on 1, 2025, for Larry Denneau—a senior software engineer and astronomer at the University of Hawaii’s Institute for Astronomy—began the same way many nights do: data continuing to stream in from telescopes sweeping the night sky.
Denneau contributes to ATLAS, the Asteroid Terrestrial-impact Last Alert System. This network of wide-field telescopes scans vast swaths of sky each night to catch anything that moves, with a focus on near-Earth asteroids. The system photographs the same region four times in quick succession to form a short motion path, or “tracklet,” indicating potential movement. It then uses reference images of the sky in a still state to subtract stars and galaxies, leaving only moving points that could be asteroids, comets, or other objects. Automated filters prune the candidates, and the survivors are reviewed by a human observer before being forwarded to the Minor Planet Center for cataloging.
That July evening, one of the surviving candidates appeared before Denneau. When 3I/ATLAS first appeared in ATLAS’s software, it looked ordinary. “I was the person reviewing when 3I popped out of the pipeline,” Denneau told Space.com. “And at the time, it looked like a completely garden-variety new Near Earth Object.” So he followed protocol and clicked “submit.”
Weeks later, the awe-inspiring discovery would reach astronomers worldwide, though Denneau was far from his inbox at the moment. He was up on Mauna Loa, Hawaii, servicing a telescope. For an entire day, he was effectively offline, while the rest of the world buzzed about a mysterious object zipping through the solar system.
When he returned that night, the reality sank in in an instant.
“I was oblivious to them until we got back that night,” he recalled. “And my inbox was completely exploded with all of this stuff… At that point, we were asking, where is it, how fast is it going? Within a day, there were hundreds of observations from different telescopes confirming the orbit.”
In the latest space news, rocket launches, skywatching events, and more, the object was classified by the Minor Planet Center as 3I/ATLAS—the third confirmed interstellar visitor observed passing through our solar system, following 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. Unlike ordinary asteroids or comets, interstellar objects are not bound by the Sun’s gravity; they originate around other stars and only briefly appear to us as they traverse our planetary system.
To detect such travelers, software like ATLAS hunts for anything that moves—points of light shifting against a field of stars.
“What comes out of our pipeline are really positions,” Denneau explained. “Things that look like stars that are moving across the background.”
A human eye must still confirm the finding. Someone reviews the data and decides whether it’s real.
“So, yeah,” Denneau said, “I’m literally the person who clicked the button and submitted the discovery observations for this object.”
Only later did the oddities become clear, especially when models attempted to determine where 3I/ATLAS might have originated. Follow-up observers scanned the sky where the object was predicted to move, and reports converged to reveal a trajectory unlike any bound to the Sun. “All of the orbit fits turned out to be really poor,” Denneau noted. “They didn’t look like the solar system—they had a funny trajectory that suggested it was moving very fast and wasn’t bound to the Sun.” That’s when the reality set in: this was an object from outside our solar system.
“I was being asked by JPL, do we have any earlier possible observations to confirm the trajectory?” he said. “So we scrambled to find more observations from previous nights.”
From engineer to astronomer: a non-traditional path
Denneau doesn’t fit the archetype of a classic astronomer who studied stars or planets from the start. He began in engineering and coding, earning a degree in electrical engineering rather than physics or astronomy. He later earned a Ph.D. in astrophysics from Queen’s University Belfast, but his software skills continued to shape his career. After moving to Hawaii, he served as the software architect for the asteroid-detection pipeline on the Pan-STARRS project, and eventually joined ATLAS, a NASA-funded effort designed to scan the sky nightly for near-Earth objects. For Denneau, astronomy is as much a software endeavor as a hardware one: the field relies on systems that ingest, clean, subtract, detect, match, and archive vast data streams while weeding out false positives that could waste valuable telescope time.
“We built some telescopes,” he said, “but after the telescopes are built, it’s really a software project.” The very software he helped develop would eventually capture images of an interstellar comet.
Each night, ATLAS telescopes capture thousands of images of the sky. Because the system uses wide-field lenses, it can monitor an area larger than 100 full moons at once, essentially covering a sky-wide swath every 24 hours and repeatedly revisiting the same regions to spot movement. These images are automatically transferred, processed, compared, and filtered by custom software designed to identify anything that moves—especially near-Earth asteroids.
“We have automated software that controls the telescopes, copies the data to Honolulu, and then searches these images for moving objects,” Denneau explained. The data volume is immense: four or five telescopes can generate a substantial portion of a terabyte of data each night. The project has grown to multi-petabytes, which keeps him awake at night given the challenges of data security and backup.
For Denneau, astronomy is as much about software as hardware: the pipeline must ingest, clean, subtract, detect, match, and archive data while filtering out false positives that would waste other astronomers’ time chasing non-existent objects.
“We’re really careful not to publish false discoveries on the confirmation page,” he noted, since other telescopes often spend precious time following up on misleading signals. ATLAS aims for near-perfect reliability before issuing alerts—ideally 99-point-something percent accurate.
Tracking other moving objects
Not only did Denneau initiate the 3I/ATLAS discovery, but just months earlier he was also on duty for another notable find: near-Earth asteroid YR4. As with most ATLAS detections, YR4 appeared as a faint moving point extracted from the background. After confirming its reality, Denneau sent the data to the Minor Planet Center. Initially, YR4 was thought to have only a slim chance of hitting Earth in 2032. Yet subsequent studies by astronomers led NASA to conclude that YR4 poses no significant impact threat.
Why 3I/ATLAS was harder to spot
Unlike YR4, uncovering earlier observations of 3I/ATLAS to trace its origins proved more challenging. In July 2025, the interstellar visitor moved through a densely populated region of the Milky Way’s star field, complicating detection. ATLAS requires four clean detections to formally flag a new object, and 3I/ATLAS remained hidden in the data until it drifted into a less crowded portion of the sky. “When there are so many stars in the background, sometimes an asteroid goes right on top of a star,” Denneau explained. “So you only get three detections. We had to wait for it to reach a less dense region for our pipeline to admit it automatically.” Once it did, the software not only confirmed the discovery but also retrieved earlier, “precovery” observations that helped verify its unusual, interstellar orbit.
Public fascination followed the scientific community’s excitement. Interstellar visitors are incredibly rare, and each one offers a fleeting chance to glimpse material formed around another star.
In this case, that glimpse began with software, data, and a single button press at just the right moment.
“I still love coming to work and doing astronomy every day,” Denneau said. “It’s just incredibly fun.”
Kenna Hughes-Castleberry is Space.com’s Content Manager. Formerly the Science Communicator at JILA, she is a freelance science journalist covering topics from quantum technology to animal intelligence and beyond.
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