A fragment of ancient ice and dust from another star system has arrived in our cosmic neighborhood as 3I/ATLAS, only the third interstellar comet ever found. Using NASA's Neil Gehrels Swift Observatory, Auburn University physicists detected hydroxyl gas, the ultraviolet by-product of water, confirming active water release from this visitor.
Swift's vantage point above Earth's atmosphere en by Clarence Oxford
Los Angeles CA (SPX) Oct 09, 2025
A fragment of ancient ice and dust from another star system has arrived in our cosmic neighborhood as 3I/ATLAS, only the third interstellar comet ever found. Using NASA's Neil Gehrels Swift Observatory, Auburn University physicists detected hydroxyl gas, the ultraviolet by-product of water, confirming active water release from this visitor.
Swift's vantage point above Earth's atmosphere enabled ultraviolet measurements that ground telescopes cannot make. The team captured the comet's faint OH glow, demonstrating that a modest 30 cm space telescope can achieve UV sensitivity comparable to large ground facilities while rapidly targeting newly discovered objects within weeks of announcement.
The observations show water activity at an unexpected distance. The OH signal appeared when 3I/ATLAS was nearly three times farther from the Sun than Earth and suggested a water-loss rate near 40 kilograms per second. At such ranges most solar-system comets remain quiet, implying an unusual mechanism powering the outflow.
Researchers propose that sunlight may heat small icy grains released from the nucleus, allowing them to vaporize and feed an extended cloud of water around the comet. Similar extended sources have been seen only in a few distant comets, hinting at complex, layered ices that preserve formation clues.
Each interstellar visitor has told a different chemical story. "'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," said lead author Zexi Xing. "When we detect water - or even its faint ultraviolet echo, OH - from an interstellar comet, we're reading a note from another planetary system," added Dennis Bodewits.
3I/ATLAS has faded for now but should be observable again after mid November, allowing scientists to track how its activity changes on approach to the Sun. The new detection, reported in The Astrophysical Journal Letters, links this object to the broader comet family and to the star systems where such bodies originate.
Research Report:Water Production Rates of the Interstellar Object 3I/ATLAS
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Auburn University Department of Physics
Stellar Chemistry, The Universe And All Within It