In the vastness of space, extraordinary discoveries sometimes hide in ordinary data. That was the case for 3I/ATLAS, our newest interstellar visitor. Officially discovered in July 2025, this object was already leaving faint traces in telescope archives months earlier—just waiting for someone to notice. Thanks to the global network of sky-surveying instruments, astronomers are now able to retrace its path, uncovering details about its strange behavior and composition that challenge our understanding of how comets form beyond our solar system.
The story of 3I/ATLAS is not just about another comet-like body entering our celestial neighborhood. It is about how science thrives on curiosity, persistence, and the clever use of tools designed for entirely different purposes.
TESS: A Telescope with a Surprise Role
The Transiting Exoplanet Survey Satellite (TESS) was not built to study wandering interstellar objects. Its mission is to search for planets orbiting distant stars by recording the subtle dimming that occurs when a planet crosses in front of its star. But when researchers decided to examine TESS’s archives, they found that it had been silently capturing snapshots of 3I/ATLAS as early as May 7, 2025—two months before the object was officially recognized.
This remarkable find came from the work of Adina Feinstein, Darryl Seligman, and John Noonan, who employed a technique known as “shift-stacking.” Because 3I/ATLAS was moving rapidly across the sky compared to background stars, a single TESS image did not reveal much. By predicting the object’s position in each frame, shifting the images so that 3I/ATLAS aligned in every shot, and stacking them together, the researchers revealed the faint streak of light that betrayed its presence. What had been invisible in isolation became clear through accumulation—a beautiful metaphor for how science itself works.
A Stranger from Another Star
3I/ATLAS is only the third interstellar object (ISO) ever detected passing through our solar system, after the enigmatic ‘Oumuamua in 2017 and comet Borisov in 2019. Unlike the countless comets and asteroids native to our solar system, these objects are cosmic refugees, ejected from their home stars and set adrift for millions—perhaps billions—of years before crossing paths with us. Each one carries with it the secrets of another planetary system, offering a fleeting glimpse into the diversity of worlds beyond our own.
During TESS’s observations, 3I/ATLAS moved from about 6.35 astronomical units (AU) from the Sun to 5.47 AU—still farther than Jupiter’s orbit, but close enough to begin feeling the Sun’s warmth. What happened next caught astronomers by surprise.
A Sudden Brightening in the Darkness
Over just a few weeks, the brightness of 3I/ATLAS increased by a factor of five—far more than expected from its gradual approach toward the Sun. This surge of light hinted at something active happening on the object’s surface.
The simplest explanation was outgassing, a process common to comets in which sunlight warms frozen material, turning it directly into gas and releasing dust into space. But the twist here was that 3I/ATLAS was still too far away for water ice, the usual driver of cometary activity, to sublimate significantly. Instead, astronomers concluded that the interstellar traveler was venting “hypervolatile” substances such as carbon monoxide and carbon dioxide—chemicals that evaporate at much lower temperatures than water ice.
This discovery suggested that 3I/ATLAS, unlike most comets in our solar system, had preserved these fragile materials despite its long journey. It is as if the object carried an untouched sample of the chemical inventory of its home system, giving scientists a rare opportunity to study the diversity of planetary building blocks across the galaxy.
Searching for a Spin
One question researchers hoped to answer was whether 3I/ATLAS’s nucleus—the solid heart of the object—was rotating. In principle, TESS data could reveal subtle variations in brightness as the nucleus spun. Yet, no clear signal emerged. Most likely, the object’s coma—a hazy cloud of gas and dust surrounding it—was too thick, masking any rotational features. While disappointing, this finding underscored just how dynamic and active 3I/ATLAS had already become at such a great distance from the Sun.
Lessons from Interstellar Wanderers
Each interstellar visitor is precious, because they are so rare. ‘Oumuamua sparked controversy with its cigar-like shape and unusual trajectory, leading some to speculate about alien technology before natural explanations prevailed. Borisov behaved more like a typical comet, reinforcing the idea that cometary formation is a common process across planetary systems.
Now, 3I/ATLAS adds another layer to this growing narrative. Its strong outgassing of hypervolatile compounds suggests that not all planetary systems process their comets in the same way ours does. Some may preserve exotic ices that our own Sun has long since burned away. Every ISO we study expands the boundaries of our cosmic imagination, reminding us that the chemistry of planets and comets is not universal but shaped by the unique history of each star system.
Old Data, New Discoveries
One of the most remarkable aspects of this discovery is how it emerged from re-examining existing archives. TESS was never intended to hunt for ISOs, yet its data has become invaluable for this purpose. The practice of “mining the archives” highlights one of astronomy’s greatest strengths: nothing is ever wasted. Even if a telescope was aimed at stars for one mission, its data can later reveal unexpected treasures—asteroids, comets, or even rare events like interstellar visitors.
This strategy ensures that as technology improves, older observations gain new value. Every image stored in a digital vault may contain mysteries waiting for the right question. In the case of 3I/ATLAS, the answers came months after the images were first taken, showing that discovery is often about perspective as much as it is about timing.
Looking Ahead
3I/ATLAS is just beginning its journey through our solar system, and astronomers will continue to track it with every instrument available. Each new observation will refine our understanding of its chemistry, activity, and trajectory. As more interstellar objects are discovered, scientists will build a comparative catalog, allowing us to see patterns across these galactic messengers.
The hope is that, in time, we may even intercept one of these objects with a dedicated mission, capturing a sample to study directly. Until then, telescopes and ingenuity remain our only tools, and stories like that of 3I/ATLAS remind us of how much can be achieved with both.
A Window into the Unknown
What makes 3I/ATLAS so captivating is not only that it is an interstellar visitor, but that it embodies the essence of science itself: the ability to uncover the extraordinary within the ordinary. Hidden in the quiet records of a satellite designed for another task, this wandering comet whispered of its existence months before anyone recognized it. Through patience, creativity, and careful analysis, astronomers brought it into the light.
Each interstellar object we encounter is a messenger from another corner of the galaxy, bearing secrets of its origins. Though they pass quickly, their lessons linger. 3I/ATLAS is one more reminder that the universe is alive with stories, and that with the right tools—and a persistent spirit of curiosity—we can hear them.
More information: Adina D. Feinstein et al, Precovery Observations of 3I/ATLAS from TESS Suggests Possible Distant Activity, arXiv (2025). DOI: 10.48550/arxiv.2507.21967
