NASA's Swift telescope detected hydroxyl (OH), a UV signature of water, coming from interstellar comet 3I/ATLAS. The object — at least 7 billion years old and only the third interstellar comet observed — is losing about 40 kg of water per second while nearly three times farther from the Sun than Earth. Swift's ultraviolet observations, which ground telescopes cannot make, reveal that the chemical ingredients tied to life are likely common across other star systems.
NASA Detects Water Vapor on 7‑Billion‑Year‑Old Interstellar Comet 3I/ATLAS — A Rare Cosmic Messenger
NASA's Swift telescope detected hydroxyl (OH), a UV signature of water, coming from interstellar comet 3I/ATLAS. The object — at least 7 billion years old and only the third interstellar comet observed — is losing about 40 kg of water per second while nearly three times farther from the Sun than Earth. Swift's ultraviolet observations, which ground telescopes cannot make, reveal that the chemical ingredients tied to life are likely common across other star systems.
22:47, 03.11.2025Science
NASA Detects Water Vapor on 7‑Billion‑Year‑Old Interstellar Comet 3I/ATLAS
Water is common in space, but this detection is exceptional. NASA's Neil Gehrels Swift Observatory has identified hydroxyl (OH) — the ultraviolet tracer produced when water is broken apart — streaming from an interstellar comet designated 3I/ATLAS. This object is only the third confirmed interstellar comet to visit our Solar System and offers a rare, pristine sample from another star system.
Why this matters
3I/ATLAS passed through the inner Solar System in July 2025 on a one‑time flyby and is at least 7 billion years old, making it the oldest comet ever observed. Because it likely formed around a different star and has remained largely unaltered for billions of years, it serves as a time capsule of extrasolar chemistry.
What was observed
A team at Auburn University used Swift to detect a faint ultraviolet glow from hydroxyl (OH). Ground-based telescopes could not see this signal because Earth's atmosphere blocks the ultraviolet wavelengths Swift can observe from space. Remarkably, Swift recorded this activity while the comet was nearly three times farther from the Sun than Earth is (about 3 AU), where most Solar System comets are normally inactive.
At that distance, 3I/ATLAS is shedding roughly 40 kilograms of water every second — a substantial outflow for such a remote location.
'When we detect water — or even its faint ultraviolet echo, OH — from an interstellar comet, we're reading a note from another planetary system,' said Dennis Bodewits, a physics professor at Auburn University. 'It tells us that the building blocks of life's chemistry travel freely between stars.'
What scientists think
Researchers suggest that sunlight may be heating tiny icy grains that break off from the comet's nucleus; these grains would sublimate and create a diffuse cloud of gas around the object. That mechanism differs from the activity normally seen in Solar System comets and implies that planet‑forming environments can vary widely across the galaxy.
Each interstellar visitor so far has been different: 1I/'Oumuamua showed little sign of volatiles, 2I/Borisov was rich in carbon monoxide, and now 3I/ATLAS is releasing water far from the Sun. These contrasts are reshaping our understanding of how comets and planetary systems form.
Looking ahead
The comet has faded from view as it moves behind the Sun but is expected to reemerge and become observable again after mid‑November 2025. Swift's 30‑centimeter telescope — modest by ground‑based standards but uniquely capable of ultraviolet observations from space — was crucial for this discovery.
Bottom line: The detection of water-related signals from a 7‑billion‑year‑old interstellar comet strengthens the idea that key molecules for life form commonly around other stars and gives astronomers a practical way to probe the chemistry of extrasolar planetary systems.
Source: BBC Sky at Night Magazine; originally reported on GEEKSPIN.