Scientists have detected significant amounts of water vapor being released from 3I/ATLAS, the third known interstellar object to visit our solar system. A new study reveals the comet is ejecting water at a rate comparable to a high-pressure fire hose, even at a distance where water ice typically remains frozen.
This discovery, made using NASA's Neil Gehrels Swift Observatory, provides a rare opportunity to study the chemical composition of a visitor from another star system. The findings challenge existing models of cometary activity and offer new clues about the formation of planetary systems beyond our own.
Key Takeaways
- Scientists detected water vapor from interstellar comet 3I/ATLAS using NASA's Swift Observatory.
- The comet is losing about 40 kilograms of water per second, an unexpectedly high rate for its distance from the sun.
- An estimated 8% of the comet's surface is active, which is higher than typical solar system comets.
- The activity may be caused by icy debris in the comet's surrounding gas cloud, rather than its main body.
- This is the third interstellar object ever detected, and each one has shown unique and surprising characteristics.
A Visitor from Another Star
A recent study published in the Astrophysical Journal Letters details the first-ever detection of water from the interstellar comet 3I/ATLAS. Researchers observed the object in July and August 2025, when it was approximately 2.9 times farther from the sun than Earth is. This distance places it well beyond the 'frost line,' the region in a solar system where water ice usually begins to sublimate, or turn directly into gas.
The presence of water is a critical benchmark in comet science. It serves as a baseline for measuring how other volatile materials are released as a comet is heated by the sun. By analyzing the water output of an interstellar object, astronomers can make direct comparisons with comets originating from our own solar system.
"When we detect water — or even its faint ultraviolet echo, OH — from an interstellar comet, we're reading a note from another planetary system," stated Dennis Bodewits, a co-author of the study and a physics professor at Auburn University. "It tells us that the ingredients for life's chemistry are not unique to our own."
This finding suggests that water, a key component for life as we know it, is likely a common ingredient in the formation of planets and comets around other stars.
Observing a Faint Signal from Deep Space
Detecting water at such a great distance was a significant technical challenge. The research team, led by postdoctoral researcher Zexi Xing of Auburn University, used NASA's Swift Observatory to look for a specific chemical signature.
Instead of observing water molecules directly, the telescope targeted hydroxyl (OH), a molecule created when sunlight breaks down water (H₂O). Hydroxyl emits a faint glow in the ultraviolet spectrum, which is only detectable by space-based telescopes like Swift.
Data Collection Details
To capture the weak signal, the team combined dozens of short, three-minute exposures. This method, known as stacking, accumulated over two hours of ultraviolet observations and 40 minutes of visible light data, allowing the faint glow of hydroxyl to become visible.
The analysis revealed that 3I/ATLAS was losing water at a rate of about 40 kilograms per second. The researchers described this output as being similar to a "fire hose running at full blast."
An Unusually Active Comet
The high rate of water loss led the team to another surprising conclusion. Based on their calculations, they estimate that at least 8% of the comet's surface must be active to produce that much vapor. This is a significantly larger active fraction than what is typically observed in comets from our own solar system, which usually have active regions covering only 3% to 5% of their surfaces.
The comet's distance from the sun makes this high level of activity even more puzzling. The surface of the comet's nucleus should be too cold for its own water ice to sublimate efficiently.
The Floating Iceberg Hypothesis
To explain this phenomenon, the researchers propose that the water vapor may not be coming from the comet's main body. Instead, it could be originating from smaller chunks of ice that have broken off and are now drifting within the comet's coma—the cloud of gas and dust that surrounds the nucleus.
What is a Comet's Coma?
A coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit. As the comet warms, parts of it sublimate. This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars. The coma is generally made of ice and comet dust.
These smaller icy fragments would have a larger surface area relative to their mass, allowing them to heat up and sublimate more quickly when exposed to sunlight. This process would create miniature steam vents in space, releasing large quantities of water vapor even while the main comet remains relatively cold. This theory is supported by near-infrared observations from other telescopes, which have hinted at the presence of icy grains in the coma.
Rewriting the Rules for Interstellar Objects
Each interstellar visitor observed so far has presented unique characteristics, forcing scientists to reconsider their understanding of how planetary systems form.
Zexi Xing, the study's lead author, highlighted the distinct nature of each object. "Every interstellar comet so far has been a surprise," Xing said. The observations of these objects provide a diverse sample of the building blocks of planets from other star systems.
- 1I/'Oumuamua: The first interstellar object, detected in 2017, appeared to be a dry, rocky body with no detectable gas or dust.
- 2I/Borisov: The second visitor, found in 2019, was rich in carbon monoxide, a composition different from most comets in our solar system.
- 3I/ATLAS: This latest object is now showing unexpected water activity at a great distance from the sun.
These differences suggest that the chemical makeup and formation processes of comets can vary significantly from one star system to another.
Future Observations Planned
Although 3I/ATLAS has now become too faint for the Swift telescope to see, other spacecraft are preparing to study it. In early October, orbiters at Mars, operated by the European Space Agency (ESA), spotted the comet as it passed about 30 million kilometers from the red planet.
ESA plans to use its Jupiter Icy Moons Explorer (JUICE) spacecraft to observe 3I/ATLAS in November. This timing is crucial, as it will occur just after the comet makes its closest approach to the sun, a period when it is expected to be at its most active.
However, scientists will have to wait for the results. The JUICE spacecraft is currently on the far side of the sun and is transmitting data through a slower backup antenna. According to ESA, the observations of 3I/ATLAS are not expected to be received on Earth until February 2026. These future observations could provide the clearest view yet of this enigmatic visitor from another star.