Interstellar Comet 3I/ATLAS Exhibits Unprecedented Water Emission

NASA's Swift Ultraviolet/Optical Telescope has detected substantial water vapor emissions from interstellar comet 3I/ATLAS, marking the first ultraviolet signature of water in such an object. Observations conducted in July and August 2025 revealed hydroxyl emissions—a byproduct of water molecules broken apart by sunlight—indicating that the comet is releasing water vapor at a rate of approximately 40 kilograms per second.

This activity was observed when 3I/ATLAS was about 2.9 astronomical units (AU) from the Sun, a distance where water ice typically remains frozen. The detection challenges existing models of cometary behavior, suggesting that small icy grains ejected from the comet's nucleus may be heating up and releasing water vapor, even though the comet itself remains too cold for surface ice to sublimate directly.

Zexi Xing, a postdoctoral researcher at Auburn University and lead author of the study, commented on the findings:

"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."

The discovery of water in 3I/ATLAS provides valuable insights into the composition and behavior of interstellar objects, offering a unique opportunity to study materials from other planetary systems. The comet is expected to become observable again after mid-November 2025, allowing for further study of its activity as it approaches perihelion.