For the first time in the history of astronomy, a space telescope has detected water vapor coming from an interstellar comet at a distance from the Sun where these celestial objects normally remain inactive. Neil Gehrels Swift Observatory NASA has captured the ultraviolet signal of hydroxyl (OH) emitted by 3I/ATLAS when the comet was almost three times the distance between the Earth and the Sun. The discovery, published on September 30th on The Astrophysical Journal Letters from the team led by Dennis Bodewits ofAuburn University, revolutionizes what we know about cometary activity and opens up new questions about the nature of interstellar objects.
Water where it shouldn't be
Swift detected the hydroxyl when 3I/ATLAS was nearly 450 million kilometers from the Sun, well beyond the region where cometary surface ice can easily sublimate. At that distance, most comets in the Solar System remain silent. measurement recorded a water loss rate of approximately 40 kilograms per second, a significant value for an object so far from our star. It's like a leaking freezer in Antarctica: technically possible, but decidedly strange.
The ultraviolet signal captured by Swift suggests that something else is fueling the comet's activity. Perhaps solar radiation is heating small grains of ice released from the nucleus, allowing them to vaporize and feed the surrounding gas cloud. These extensive water sources They have been observed in only a handful of distant comets and indicate the presence of complex, layered ices that preserve clues to how these objects formed.
et al ., doi: 10.3847/2041-8213/ae08ab.
Every interstellar comet is a surprise: 3I/Atlas is no exception.
Each interstellar object discovered so far has revealed a different side of planetary chemistry beyond our Sun. 1I/'First, the first interstellar visitor confirmed in 2017, appeared dry and devoid of visible cometary activity. 2I / Borisov, discovered in 2019, was rich in carbon monoxide. And now 3I/ATLAS releases water at a distance it shouldn't. As he explains Zexi Xing, postdoctoral researcher atAuburn University: "Every interstellar comet so far has been a surprise. 'Oumuamua was dry, Borisov was rich in carbon monoxide, and now ATLAS is releasing water at a distance we didn't expect.".
Detecting water, or even just its faint ultraviolet (OH) echo, from an interstellar comet means reading a message from another planetary system. These objects demonstrate that The basic building blocks of comets, and the volatile ices that shape them, can vary dramatically from star system to star system. These differences offer clues to how diverse planet-forming environments may be and how processes such as temperature, radiation, and composition shape the materials that eventually seed planets and, potentially, life.
A small but powerful telescope
Capturing that whisper of ultraviolet light from 3I/ATLAS was a technical triumph. Swift carries a modest 30-centimeter 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 glare of the sky and interference from the air, the Ultraviolet/Optical Telescope Swift's telescope achieves the sensitivity of a 4-meter ground-based telescope for these wavelengths. Its rapid pointing ability allowed astronomers to observe the comet within weeks of its discovery, long before it became too faint or too close to the Sun to be studied from space.
3I/ATLAS & Messages from Other Worlds
As emphasized Dennis Bodewits: "When we detect water, or even just its faint ultraviolet (OH) echo, from an interstellar comet, we're reading a note from another planetary system. It tells us that the ingredients for life's chemistry aren't unique to ours."The discovery published on The Astrophysical Journal Letters confirms that 3I/ATLAS can be directly compared to native comets in the Solar System using the same yardstick: water. As we previously reported, this comet has an anomalous CO₂/water ratio of 8:1, among the highest ever recorded.
Interstellar objects are the only things we've ever physically observed within the Solar System that originated outside it. Each new discovery rewrites what we thought we knew about how planets and comets form around stars. 3I/ATLAS will continue its journey through the Solar System, passing behind the Sun at the end of October and venturing beyond Jupiter in March 2026, before returning to interstellar space from which it came. But it has already left its mark: water can exist where it shouldn't, and the rules we know only apply to our own backyard.
Wheww… I managed to talk about an interstellar object without talking about Avi Loeb and aliens! I pat myself on the back.
