NASA's Swift Observatory has detected water vapor being actively released by 3I/ATLAS, the third known interstellar object and second confirmed interstellar comet ever discovered — and the first interstellar visitor ever confirmed to contain and release water. The finding has profound implications for understanding how the essential ingredient for life as we know it is distributed across the galaxy and potentially seeded between star systems.
3I/ATLAS was first detected in August 2025 as it swept into the inner solar system on a hyperbolic trajectory that could only have originated from interstellar space. Its incoming velocity far exceeded anything bound by our Sun's gravity. As it approached closer to the Sun, its icy nucleus began sublimating, releasing gas and dust in a bright coma and tail visible to the naked eye from dark locations.
Chemistry From Another Star
The water in 3I/ATLAS comes with a distinctive chemical signature that sets it apart from solar system comets. It carries a notably higher ratio of carbon dioxide to water than typical native comets, suggesting it formed in a colder part of its original stellar system. Spectroscopic analysis also revealed the presence of nickel without the iron that almost always accompanies it in our local comets — a sign that the dust grains from which 3I/ATLAS assembled long ago had fundamentally different chemistry than the dust of our own protoplanetary disk.