On a quiet summer night in July 2025, a telescope in Chile spotted something extraordinary racing through our cosmic neighborhood. But this wasn’t just any space rock passing by. This visitor had traveled for billions of years from a distant corner of the Milky Way, carrying secrets from an era when the universe looked completely different than it does today.
The object, now designated 3I/ATLAS, represents only the third confirmed interstellar visitor ever detected entering our solar system, following the mysterious ‘Oumuamua in 2017 and Comet Borisov in 2019. What astronomers have discovered about this ancient traveler has fundamentally challenged our understanding of how planetary systems form across the galaxy, and it’s doing something that has scientists absolutely baffled.
A Cosmic Bottle Cast Into the Ocean of Space
When the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope at Río Hurtado, Chile, first captured images of 3I/ATLAS on July 1, 2025, the object was already 4.5 astronomical units away from the Sun, barreling inward at an astonishing 58 kilometers per second. That’s nearly twice the velocity of the previous interstellar visitors, making it one of the fastest objects ever observed passing through our solar system.
The comet follows what astronomers call a hyperbolic trajectory, meaning it’s not gravitationally bound to our Sun and will eventually slingshot back into interstellar space, never to return. Its hyperbolic excess velocity of 58 kilometers per second relative to the Sun tells scientists this object originated from somewhere far beyond our solar neighborhood.
Initial observations raised questions about whether this was even a comet at all. Several astronomers, including veteran comet hunter Alan Hale, reported seeing no obvious cometary features at first. But by July 2, multiple observatories including the Lowell Discovery Telescope in Arizona and the Canada-France-Hawaii Telescope at Mauna Kea confirmed they were seeing a marginal coma with a potential tail-like elongation, definitively identifying it as an active comet.
The Water Mystery That Has Scientists Scratching Their Heads
Here’s where things get weird. When astronomers pointed NASA’s Neil Gehrels Swift Observatory at 3I/ATLAS, they detected strong ultraviolet emissions that could only mean one thing: hydroxyl gas, the unmistakable chemical signature of water breaking apart in space. The emissions could only be spotted from orbit because Earth’s atmosphere absorbs ultraviolet light before it reaches ground-based telescopes.
What they discovered left them stunned. This ancient comet was ejecting water vapor at a torrential rate of about 40 kilograms per second, roughly 88 pounds every single second. As one research team put it, that’s equivalent to a fire hose running at full blast, constantly.
But the truly extraordinary part? This water activity was happening at a distance of nearly three astronomical units from the Sun, about three times farther than Earth orbits our star. Typically, comets need to venture much closer to the Sun before the ice in their nucleus heats up enough to sublimate, instantly transforming from solid to gas. Something else entirely must be driving the water release in 3I/ATLAS.
“When we detect water, or even its faint ultraviolet echo, OH, from an interstellar comet, we’re reading a note from another planetary system,” explained Dennis Bodewits, a professor of physics at Auburn University who led the Swift observations. “It tells us that the ingredients for life’s chemistry are not unique to our own.”
The research team estimates that at least 8% of the comet’s surface must be actively releasing material, a surprisingly large fraction compared to the 3% to 5% typically seen in comets from our own solar system. Scientists hypothesize that sunlight might be heating up ice grains released from the nucleus, which then vaporize into the surrounding coma rather than the surface ice sublimating directly.
A Snapshot From the Ancient Universe
Perhaps the most mind-bending aspect of 3I/ATLAS is just how old it might be. By analyzing its velocity components in the galactic coordinate system, astronomers have determined the comet follows a tilted orbit around the Milky Way, belonging to either the thin disk or thick disk populations of our galaxy.
A study led by Matthew Hopkins estimated with 68% confidence that 3I/ATLAS is between 7.6 and 14 billion years old. An independent analysis by Aster Taylor and Darryl Seligman came to a similar conclusion, estimating the comet should be 3 to 11 billion years old. The most striking possibility? This comet could be approximately 10 billion years old, making it roughly 3 billion years older than our entire solar system.
“What makes 3I/ATLAS unique is that it allows us to study the evolution of objects originating from other stellar systems, something we had only theorised about until now,” said Pérez Couto, Centre for Research in Information and Communication Technologies researcher and team leader of one study. “Each observation is like opening a window into the Universe’s past.”
Researchers from the University of A Coruña attempted to trace the comet’s path backward through time, running simulations involving over 13 million stars from the European Space Agency’s Gaia mission data. They traced the object’s trajectory back 10 million years, identifying 93 potential stellar encounters, 62 of which were statistically significant.
The results were surprising. “None of these encounters produced any meaningful perturbation,” the team wrote in their paper submitted to the Astrophysical Journal. Even the strongest gravitational influence from a nearby star barely altered 3I/ATLAS’s trajectory, imparting only a minuscule velocity change. The comet’s origin remains fundamentally mysterious.
A Chemistry Unlike Anything We’ve Seen
What makes each interstellar visitor special is how radically different they are from one another and from objects native to our solar system. ‘Oumuamua appeared completely dry with no detectable outgassing. Comet Borisov was rich in carbon monoxide. Now 3I/ATLAS is rewriting the rulebook again.
Observations by the James Webb Space Telescope revealed that 3I/ATLAS has an unusually high ratio of carbon dioxide to water, a chemical composition markedly different from solar system comets. Additional observations by the Very Large Telescope showed the comet is emitting cyanide gas and atomic nickel vapor at concentrations similar to those in our local comets, suggesting some chemical processes may be universal.
The Hubble Space Telescope captured remarkably sharp images of 3I/ATLAS on July 21, 2025, when the comet was 277 million miles from Earth. These observations suggest the diameter of the comet’s nucleus (the solid rocky center, excluding the coma) is somewhere between 0.32 and 5.6 kilometers, with the most likely diameter being less than 1 kilometer. The comet’s total estimated mass? Over 33 billion tons.
“Every interstellar comet so far has been a surprise,” said Zexi Xing, a postdoctoral researcher at Auburn University and lead author of the water detection study. “‘Oumuamua was dry, Borisov was rich in carbon monoxide, and now ATLAS is giving up water at a distance where we didn’t expect it. Each one is rewriting what we thought we knew about how planets and comets form around stars.”
The Great Disappearing Act
As 3I/ATLAS approached its closest point to the Sun, called perihelion, something inconvenient happened from an Earth-based observer’s perspective: the comet swung around behind the Sun. On October 21, 2025, the comet reached solar conjunction, appearing on the opposite side of the Sun from Earth. This means during its October 29 perihelion passage at a distance of 1.36 astronomical units (about 126 million miles from the Sun), the comet was essentially invisible to telescopes on Earth.
But all was not lost. The comet’s trajectory brought it remarkably close to Mars, passing just 0.19 astronomical units (about 18 million miles) from the Red Planet on October 3, 2025. NASA’s Perseverance rover and other spacecraft stationed near Mars had a front-row seat to observe this ancient visitor during its perihelion when it was hidden from Earth’s view.
After swinging around the Sun at its maximum speed of 68 kilometers per second, 3I/ATLAS will emerge from behind our star and become visible again from Earth in late November 2025. The comet will then continue its journey outward, passing 0.65 astronomical units from Venus on November 3, coming no closer than 1.80 astronomical units to Earth on December 19, and then passing 0.36 astronomical units from Jupiter on March 16, 2026.
Because 3I/ATLAS cannot come close to Earth, it poses absolutely no threat to our planet. After its brief tour through our solar neighborhood, the comet will continue on its hyperbolic trajectory, eventually leaving the solar system and returning to the vast emptiness of interstellar space.
What This Ancient Traveler Tells Us
The discovery of water in 3I/ATLAS carries profound implications for our understanding of planetary system formation and the distribution of life’s building blocks throughout the galaxy. Water is essential for life as we know it, and finding it in an object that potentially formed 10 billion years ago in a completely different stellar system tells us something remarkable: the chemistry necessary for life may be truly universal.
The comet likely formed in the primordial disk of an early planetary system, possibly around a star in the transition region between the thin and thick disks of the Milky Way. At some point, perhaps during a close encounter with another star or a gravitational disturbance within its home system, 3I/ATLAS was ejected into interstellar space. There it drifted alone for billions of years, a cosmic time capsule sealed and preserved in the deep freeze of the void between stars.
This summer, that sealed bottle finally washed ashore in our solar system, giving astronomers a precious glimpse into how the universe looked when it was less than a quarter of its current age. The fact that such an ancient object still contains water ice tells us that whatever processes formed it must have been efficient at incorporating volatiles, even in the early, chaotic days of the Milky Way’s formation.
“Together, all data indicate that while 3I/ATLAS follows a thin-disk orbit in the solar neighborhood, it may nonetheless be an old object, consistent with ejection from a primordial planetesimal disk in an early-formed system, or from an exo-Oort cloud,” the University of A Coruña team concluded in their study. “Its origin remains undisclosed.”
The Future of Interstellar Object Studies
As 3I/ATLAS continues its journey through and eventually out of our solar system, scientists will keep gathering data from every available instrument. The comet will grow progressively dimmer as it moves away from the Sun, but telescopes on Earth, Mars, and Jupiter will continue tracking it for as long as possible.
Each interstellar visitor teaches us something completely unexpected about the incredible diversity of objects that exist beyond our solar system. With improved detection methods and more sensitive instruments, astronomers expect to discover additional interstellar objects in the coming years. The Vera C. Rubin Observatory, currently under construction in Chile, will dramatically increase the rate at which we detect these rare visitors.
For now, 3I/ATLAS stands as a reminder of just how much we still don’t understand about the cosmos. An object older than Earth, older than the Sun, older than every planet in our solar system combined, has graced us with a brief visit before disappearing back into the darkness between stars. It carries with it secrets from an era of the universe we can only glimpse through fossils like this, ancient wanderers that somehow survived billions of years of cosmic evolution intact.
As Xing noted, each interstellar object is rewriting our understanding of planetary formation. And with every surprising discovery, from ‘Oumuamua’s bizarre tumbling motion to Borisov’s carbon monoxide atmosphere to 3I/ATLAS’s unexpected water release, we’re learning that the universe is far stranger and more diverse than we ever imagined. The cosmos is full of surprises, and sometimes, those surprises come knocking on our door at 58 kilometers per second.
