Physicists have detected water activity during observations of the mysterious comet 3I/ATLAS, marking the first time hydroxyl gas, a chemical signature of water, has been detected from the interstellar visitor.
Using NASA’s Neil Gehrels Swift Observatory, the Auburn University team determined 3I/ATLAS is releasing water at a rate of roughly 40 kilograms per second, comparable to a fire hose running at full blast, while positioned nearly three times farther from the Sun than Earth.
What makes this finding particularly remarkable is the distance at which this activity was observed: 2.90 astronomical units, well beyond the region where most solar system comets show any significant signs of such activity.
“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, professor of physics at Auburn University and principal investigator of the study, in a press release. “It tells us that the ingredients for life’s chemistry are not unique to our own.”
According to their paper, the scientists used ultraviolet light to detect water molecules coming off the comet. The team found that there is a specific signal water gives off as it breaks apart in space, and the water the comet was releasing grew significantly between late July and mid-August 2025.
To make this discovery, they relied on NASA’s Swift telescope, which orbits high above Earth. Even though Swift only has a 12-inch mirror, it can see ultraviolet light much better than telescopes on the ground because it’s beyond our atmosphere, which normally blocks most ultraviolet light from reaching telescopes on the surface. From its location in space, Swift has the same power as a much larger 13-foot telescope would have on Earth for this type of observation.
Since its discovery on July 1, 2025, by the NASA-funded ATLAS survey telescope in Chile, 3I/ATLAS has consistently defied expectations. Previous research reported by The Debrief has revealed that this interstellar visitor is “anomalously massive” compared to its predecessors, with recent studies suggesting a minimum nucleus diameter of roughly five kilometers and a mass of at least 33 billion tons.
This extraordinary size places 3I/ATLAS several orders of magnitude more massive than the first two known interstellar objects, 1I/’Oumuamua and 2I/Borisov. As Harvard astronomer Avi Loeb noted in previous correspondence with The Debrief, “We should have detected an order of 100,000 ‘Oumuamuas before discovering an object as big as 3I/ATLAS”.
The comet’s massive nature helps explain its ability to maintain course despite significant outgassing activity. Unlike smaller objects that would be deflected by gas jets from their sun-facing surfaces, 3I/ATLAS has shown remarkable orbital stability, with non-gravitational acceleration measured at less than 15 meters per day squared.
The discovery of water emanating from the comet only adds to its complexity and mystique.
Rather than simple nuclear sublimation, the research suggests water production originates from extended sources, most likely large icy grains in the comet’s coma that sublimate as they’re heated by sunlight. This mechanism, observed in only a handful of distant comets, points to complex, layered ice structures that preserve their ancient formation clues.
The water detection adds another chapter to the evolving story of interstellar objects in our solar system. As lead researcher Zexi Xing noted, “Every interstellar comet so far has been a surprise. ‘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,” Xing said.
Moreover, recent observations by ESA’s ExoMars Trace Gas Orbiter during 3I/ATLAS’s close approach to Mars provided additional imaging opportunities. While the challenging observation conditions highlighted the technical difficulties of studying these rare visitors, it was still considered a unique opportunity to use the orbiter for something outside of its mission parameters.
“Though our Mars orbiters continue to make impressive contributions to Mars science,” ESA project scientist Colin Wilson said at the time. “It’s always extra exciting to see them responding to unexpected situations like this one.”
3I/ATLAS will become observable again after mid-November 2025, offering additional opportunities to track its evolution as it approaches perihelion on October 30. The comet’s trajectory will take it within 1.4 astronomical units of the Sun, just inside Mars’s orbit, before it exits our solar system forever, traveling at a speedy 130,000 miles per hour.
The discovery has profound implications for our understanding of planetary system formation across the galaxy. The presence of water activity in an interstellar comet demonstrates that the basic chemical ingredients for life are not unique to our solar system. Moreover, the complex ice structures indicated by extended water sources preserve information about formation conditions in distant stellar environments billions of years ago.
As 3I/ATLAS continues on its journey through our solar system, astronomers worldwide are seizing this rare opportunity to study material that has drifted through interstellar space for potentially billions of years. Each observation brings new insights into the diversity of planetary systems and the cosmic processes that shape the building blocks of life throughout our galaxy.
MJ Banias covers space, security, and technology with The Debrief. You can email him at mj@thedebrief.org or follow him on Twitter @mjbanias.