Interstellar Comet 3I/ATLAS Water Plumes

Humanity's third interstellar visitor is spewing water like a fire hose far from the Sun, challenging our ideas about comets and planetary ingredients.

Hubble Space Telescope image of Comet 3I/ATLAS showing a bright core and a teardrop-shaped coma of gas and dust. — This Hubble Space Telescope image from July 21, 2025, shows Comet 3I/ATLAS with its distinct coma (atmosphere) of gas and dust, indicating its activity even while far from the Sun. Credit: NASA, ESA, STScI.

Key Findings

3I/ATLAS is the third confirmed object from another star system to visit ours.
Its extreme speed and open-ended trajectory prove its interstellar origin.
It ejects ~40 kg of water vapor per second, detected via its byproduct (OH) using the Swift space telescope.
This water activity occurs much farther from the Sun (2.9 AU) than expected for water ice sublimation.
The likely cause is a two-step process: ejecting icy dust grains, which then sublimate efficiently in sunlight.
It has a unique chemical mix: very high CO2, but low CN, differing greatly from previous visitors.

Primary Source

Source: Water Production Rates of the Interstellar Object 3I/ATLAS

Author(s): Zexi Xing, Shawn Oset, John Noonan, and Dennis Bodewits

Link: https://iopscience.iop.org/article/10.3847/2041-8213/ae08ab

Interstellar Visitor 3I/ATLAS Surprises Scientists

Imagine a piece of another solar system, traveling for billions of years across the vast emptiness between stars. This isn’t science fiction. On July 1, 2025, astronomers spotted exactly that: a faint light moving through our cosmic neighborhood.¹ Named 3I/ATLAS, it’s only the third visitor from interstellar space humanity has ever confirmed. This cosmic messenger offers a rare chance to study material forged around a long-dead star. But ATLAS isn’t just a quiet traveler. It’s behaving strangely, spewing huge amounts of water vapor far from the Sun, forcing scientists to rethink how comets work.

How Do We Know 3I/ATLAS Isn’t From Around Here?

Confirming an object comes from another star system requires rock-solid proof. Scientists look for two key clues in its path, and 3I/ATLAS showed both in extreme measure.

First, its path is a hyperbolic trajectory. Objects belonging to our solar system, even comets from its farthest edges, orbit the Sun in closed loops called ellipses. ATLAS follows an open-ended path, like a slingshot trajectory that will take it back out into deep space. We measure this with orbital eccentricity. A value below 1 means a closed orbit; above 1 means an open, hyperbolic one. ATLAS’s eccentricity is about 6.1,² vastly higher than its predecessors ‘Oumuamua (~1.2) and Borisov (~3.4). This proves it’s not tied to our Sun’s gravity.

Second, its speed is incredible. Even far from the Sun’s pull, ATLAS travels at about 58 kilometers per second (around 130,000 mph). That’s much faster than ‘Oumuamua (~26 km/s) or Borisov (~32 km/s). Nothing originating within our solar system could naturally reach such a speed;³ it had to fall into our system from interstellar space.

What Was So Surprising About Its Behavior?

While its origin was amazing, the comet’s activity caused the biggest stir. A team led by Zexi Xing at Auburn University used NASA’s Neil Gehrels Swift Observatory to study it. Swift looked for the chemical signature of hydroxyl (OH) using its ultraviolet telescope. Hydroxyl is a fragment produced when sunlight breaks apart water molecules (H₂O). Because Earth’s atmosphere blocks this UV light, a space telescope was essential.

They found a lot of hydroxyl. The team calculated that 3I/ATLAS was ejecting water at a rate of roughly 40 kilograms (88 pounds) every single second.⁴ That’s like a fire hose running at full blast, spraying water into space.

Why Is Releasing Water a Big Deal?

The surprise wasn’t just the amount of water, but where it was happening. These observations occurred when ATLAS was 2.9 Astronomical Units (AU) from the Sun. That’s almost three times farther from the Sun than Earth is. At this distance, sunlight is usually too weak to turn water ice on a comet’s surface directly into gas (a process called sublimation). Comets from our solar system usually only “turn on” their water jets when they get much closer, inside about 2.5 AU (the “snow line”). Seeing so much water activity so far out was unprecedented for an interstellar visitor and defied standard comet models.

How Can It Release Water So Far From the Sun?

Scientists think a clever two-step process is happening. It suggests a different way comets can become active.

Ejecting Icy Grains: Instead of the main body’s ice sublimating, the comet likely throws off tiny dust grains coated in ice. Observations from other telescopes hinted at these icy grains existing in the comet’s fuzzy atmosphere (its coma).⁵
Grain Sublimation: Once these tiny grains are floating in the coma, their combined surface area is huge. They can absorb the faint sunlight much more efficiently than the comet’s large, solid nucleus. This gentle heating is enough to sublimate the ice off the grains, turning the whole coma into a massive source of water vapor.

This explains the “cosmic fire hydrant” in the cold outer solar system. It means a comet’s internal structure and ability to shed dust might be just as crucial as solar heating for its activity. Calculations suggest at least 8% of ATLAS’s surface must be active to produce this much water. This is much higher than the 3-5% typical for our own comets. This marks ATLAS as exceptionally volatile.

How Does 3I/ATLAS Compare to Past Visitors?

Detecting interstellar objects is very new. Before 2017’s 1I/’Oumuamua, all known objects were native to our solar system.⁶ With only three confirmed visitors, a pattern of incredible diversity is emerging.

1I/’Oumuamua (2017): Looked like a dry, rocky asteroid with a very strange elongated shape. Showed no coma or gas release, but had a slight acceleration unexplained by gravity alone.
2I/Borisov (2019): Clearly a comet with a coma and tail.⁷ Its chemistry was weird, though. It was extremely rich in carbon monoxide (CO), unlike our comets, suggesting it formed in a very cold place.
3I/ATLAS (2025): A comet defined by its distant water activity. Further study with the James Webb Space Telescope (JWST) found it has the highest ratio of carbon dioxide (CO₂) to water ever seen in a comet, yet is strangely low in cyanogen (CN).

These huge differences tell us that the “building codes” for planetary systems vary wildly across the galaxy. Each visitor is a unique sample from a different cosmic construction site.

Three-panel infrared image from JWST showing breakdown of Comet 3I/ATLAS light, highlighting CO2 and water signatures.

What Do Its Chemicals Tell Us About Its Home?

The unique chemistry offers clues. The high CO₂ suggests ATLAS may have formed far from its star, beyond the point where CO₂ freezes solid (the “CO₂ ice line”). The low cyanogen might mean it formed near a star poor in carbon, or perhaps in an ancient star system. Tracing its path back suggests it might originate from the Milky Way’s “thick disk,” a population of much older stars than our Sun.⁸ This raises the possibility that 3I/ATLAS could be billions of years older than our own solar system.

Was It Really an Alien Probe?

The strange nature of interstellar objects often sparks speculation about alien technology, fueled partly by Harvard astrophysicist Avi Loeb. For 3I/ATLAS, some pointed to its size, path, and the (theoretical) chance it could maneuver while hidden by the Sun. However, the scientific consensus firmly rejects this hypothesis.

Why? First, it clearly acts like a natural comet, with a visible coma and measured water outgassing. As NASA scientist Tom Statler put it, “It looks like a comet. It does comet things… the evidence is overwhelmingly pointing to this object being a natural body.”⁹ Second, its trajectory poses absolutely no threat to Earth, passing nearly twice as far away as the Sun. Third, its “anomalous” chemistry points to a different natural origin, not artificial construction. Finally, extraordinary claims need extraordinary proof, and there is zero positive evidence (like radio signals or unnatural movements) suggesting it’s anything but a comet. The real story is about natural cosmic diversity, not aliens.

What Happens Next with 3I/ATLAS?

The study continues. The comet recently passed Mars, where ESA’s ExoMars Trace Gas Orbiter captured images.¹⁰ Its closest approach to the Sun (perihelion) is on October 29, 2025. Shortly after, in November 2025, ESA’s JUICE spacecraft, en route to Jupiter, will get a prime view of the comet at its peak activity.¹¹ Due to data transmission limits, we likely won’t see that data until early 2026. After that, it swings past Jupiter in March 2026 and heads back to interstellar space forever.

The discovery of 3I/ATLAS shows interstellar astronomy is maturing fast. New telescopes like the Vera C. Rubin Observatory will soon find many more visitors, perhaps dozens per year. Missions like ESA’s Comet Interceptor are being designed to potentially fly to one of these objects. 3I/ATLAS is a reminder that our solar system is connected to a vast, varied galaxy, and we’re just beginning to read the messages these travelers bring.