NASA’s Swift Ultraviolet/Optical Telescope (UVOT) observed interstellar comet 3I/ATLAS during two visits in July and August 2025. The panels show visible-light (left) and ultraviolet (right) images, where the faint glow of hydroxyl (OH) traces water vapour escaping from the comet. Each image combines dozens of short, three-minute exposures, painstakingly stacked to reach total integration times of about 42 minutes in visible light and 2.3 hours in ultraviolet. Swift’s vantage point above Earth’s atmosphere allowed astronomers to detect these ultraviolet emissions that are normally invisible from the ground. Credit: Dennis Bodewits, Auburn University.

An international team, led by Auburn University, has detected water’s ultraviolet fingerprint in interstellar comet 3I/ATLAS.

For millions of years, a fragment of ice and dust drifted between the stars – like a sealed bottle cast into the cosmic ocean.

This summer, that bottle finally washed ashore in our Solar System and was designated 3I/ATLAS, only the third known interstellar comet.

When Auburn University scientists pointed NASA’s Neil Gehrels Swift Observatory toward it, they made a remarkable find: the first detection of hydroxyl (OH) gas from this object, a chemical fingerprint of water.

Swift’s space-based telescope could spot the faint ultraviolet glow that ground observatories can’t see – because, high above Earth’s atmosphere, it captures light that never reaches the Earth’s surface.

Breakthrough

Detecting water – through its ultraviolet by-product, hydroxyl – is a major breakthrough for understanding how interstellar comets evolve.

In solar-system comets, water is the yardstick by which scientists measure their overall activity and track how sunlight drives the release of other gases.

It’s the chemical benchmark that anchors every comparison of volatile ices in a comet’s nucleus.

Finding the same signal in an interstellar object means that, for the first time, astronomers can begin to place 3I/ATLAS on the same scale used to study native solar-system comets – a step toward comparing the chemistry of planetary systems across the galaxy.

What makes 3I/ATLAS remarkable is where this water activity occurs.

The Swift observations detected OH when the comet was nearly three times further from the Sun than Earth – well beyond the region where water ice on a comet’s surface can easily sublimate – and measured a water-loss rate of about 40 kilograms per second – roughly the output of a fire hose running at full blast.

At those distances, most solar-system comets remain quiet.

The strong ultraviolet signal from ATLAS suggests that something else is at work: perhaps sunlight is heating small icy grains released from the nucleus, allowing them to vaporise and feed the surrounding cloud of gas.

Such extended sources of water have been seen only in a handful of distant comets and point to complex, layered ices that preserve clues to how these objects formed.

Dramatic variation

Each interstellar comet discovered so far has revealed a different side of planetary chemistry beyond our Sun.

Together, they demonstrate that the building blocks of comets – and the volatile ices that shape them – can vary dramatically from one star system to another.

These differences hint at how diverse planet-forming environments can be, and how processes like temperature, radiation, and composition sculpt the materials that ultimately seed planets and, potentially, life.

Technical triumph

Catching that whisper of ultraviolet light from 3I/ATLAS was a technical triumph in itself.

NASA’s Neil Gehrels Swift Observatory carries a modest 30-centimetre telescope, but in orbit above Earth’s atmosphere it can see ultraviolet wavelengths that are almost completely absorbed before reaching the ground.

Free from the sky’s glare and air’s interference, Swift’s Ultraviolet/Optical Telescope achieves the sensitivity of a 4-metre-class ground telescope for these wavelengths.

Its rapid-targeting capability allowed the Auburn team to observe the comet within weeks of discovery – long before it grew too faint or too close to the Sun to study from space.

Dennis Bodewits, professor of physics at Auburn, said: “When we detect water – or even its faint ultraviolet echo, OH – from an interstellar comet, we’re reading a note from another planetary system.

“It tells us that the ingredients for life’s chemistry are not unique to our own.”

Lead author Zexi Xing, a postdoctoral researcher, said: “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.”

Another chance

3I/ATLAS has now faded from view but will become observable again after mid-November, offering another chance to track how its activity evolves as it approaches the Sun.

The current detection of OH provides the first clear evidence that the comet is releasing water at large heliocentric distances.

It also shows how a small space-based telescope, free from Earth’s atmospheric absorption, can reveal faint ultraviolet signals that link this visitor to the wider family of comets – and to the planetary systems from which they are born.

The study is published in The Astrophysical Journal Letters.