Rubin data uncovers pre-discovery images of interstellar comet 3I/ATLAS

Astronomers found images of the interstellar comet 3I/ATLAS in Vera Rubin Observatory commissioning data from June 20, 2025, ten days before its official discovery by ATLAS on July 1, 2025. Rubin captured nine additional images between June 21 and July 2, with a clear coma, but a non-operational data pipeline during validation required a custom pipeline to extract the signals. The discovery hints Rubin LSST could detect about one interstellar comet per year. Joint observations by JUICE and Europa Clipper later detected hydrogen, oxygen, and carbon from 3I/ATLAS, with JWST data suggesting excess carbon dioxide; these findings help compare the comet’s origin to solar-system bodies. The nucleus is ~1 km wide and traveling ~140,000 mph, implying an age of several billion years and multiple stellar encounters. The study was published in The Astrophysical Journal Letters.