A bold truth first: the interstellar visitor 3I/ATLAS wasn’t meant to be part of our solar system at all, and its discovery reveals how modern astronomy runs on a delicate edge between software precision and human judgment. Here’s how it happened, in plain terms, with enough detail to understand what makes this finding so remarkable.
On the night of July 1, 2025, Larry Denneau, a senior software engineer and astronomer at the University of Hawaii’s Institute for Astronomy, started his shift much like countless others. Data flowed in from a network of wide‑field telescopes designed to scan vast swaths of the sky. Denneau works with ATLAS, the Asteroid Terrestrial‑impact Last Alert System, whose job is to image large portions of the heavens repeatedly to catch anything that moves—especially near‑Earth objects. The system captures four quick images of the same sky patch to form a short motion path, a “tracklet,” which helps identify moving objects. It then subtracts out stars and galaxies using reference images, leaving moving points that could be asteroids, comets, or something else. After automated filters, promising candidates are handed to a human reviewer to confirm they’re real and ready to be cataloged by the Minor Planet Center.
That night, one of the surviving candidates appeared before Denneau in the ATLAS software. At first glance, it looked unremarkable. “I was the reviewer when 3I popped out of the pipeline,” Denneau later told Space.com. “And at the time, it looked like a completely garden‑variety new Near Earth Object.” He followed the software’s instruction and clicked submit.
But the moment the discovery notice reached astronomers worldwide, Denneau wasn’t near his inbox. He was on Mauna Loa, high in Hawaii, servicing a telescope. An entire day passed with him effectively offline, while excitement quietly built about a peculiar object moving through the solar system.
When he finally returned, the reality hit all at once. “I was oblivious to them until we got back that night,” he recalled. “And my inbox was completely exploded with all of this stuff… Within a day, hundreds of observations from different telescopes were confirming the orbit.”
The Minor Planet Center assigned the object the designation 3I/ATLAS. It became only the third confirmed interstellar visitor seen crossing our solar system, following 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. Interstellar objects, unlike typical comets or asteroids, aren’t bound by the sun’s gravity and originate around other stars, briefly becoming visible as they transit our solar system.
ATLAS detects moving objects by looking for lights that shift against a starry background. “What comes out of our pipeline are really positions,” Denneau explained. “Things that look like stars but are moving.” Yet a human eye must still decide whether the signal is real.
So, Denneau did what he does best: he submitted the initial observations. It wasn’t until follow‑up observations poured in from other telescopes and orbital models were computed that the unusual nature of 3I/ATLAS became clear. The orbit didn’t behave like something bound to the Sun; instead, all fits suggested a trajectory that indicated an origin from outside the solar system.
JPL and others started asking for previous observations to see if earlier clues existed. In the end, the team tracked the object’s path back, uncovering earlier data that helped confirm its interstellar origin.
Denneau’s path to this discovery is as telling as the object itself. He isn’t a traditional astronomer in the sense of decades spent studying stars and planets. His background lies in engineering and computer programming (his degree is in electrical engineering). He earned a Ph.D. in astrophysics later, but his specialty has always been software. After moving to Hawaii, he built the software backbone for major sky surveys, first with Pan‑STARRS and then with ATLAS. He views astronomy as a blend of hardware and software, with the latter often driving what we can see and understand.
ATLAS itself is a massive software‑centric enterprise. The telescopes take thousands of images each night; the system images areas larger than 100 full moons at once, covering nearly the entire visible sky every day. All of this data is transferred, processed, compared, and filtered by custom software designed to detect anything that moves, with a strong emphasis on avoiding false positives that would waste precious telescope time.
Denneau emphasizes reliability: ATLAS aims for near‑perfect confidence before a detection is sent out as an alert. “We want to be 99‑plus percent reliable on that front,” he notes.
Remarkably, Denneau had already been on duty for another notable discovery a few months earlier: near‑Earth asteroid YR4. Like 3I/ATLAS, YR4 appeared as a faint moving point and was confirmed before sending data to the Minor Planet Center. While initial concerns suggested a potential impact with Earth in 2032, NASA later concluded YR4 posed no significant threat.
Why was 3I/ATLAS hard to detect initially? It’s partly a matter of sky clutter. On its first sighting, the object was moving through a densely populated region of the Milky Way, full of bright stars. ATLAS requires four clean detections to flag a new object. In a crowded field, the moving object can blend with stars, leaving only three usable detections. It was only once 3I/ATLAS moved into a less crowded patch of sky that the software could reliably pick it up and “precover” earlier observations that helped reconstruct its unusual path.
Since its discovery, 3I/ATLAS has captured public imagination. Interstellar visitors are incredibly rare, and each one opens a fleeting window into material formed around another star. In this case, the glimpse began not with a dramatic telescope image but with software, data, and a single button press at the right moment.
To Denneau, the thrill remains: he still loves coming to work and working in astronomy. It’s “super fun” to him, even as his software work reveals the most extraordinary denizens of the galaxy.
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