Astronomers have detected water vapor emanating from interstellar comet 3I/ATLAS, marking the first confirmed detection of water from such an object. The finding, made using NASA’s Swift Observatory, suggests that the building blocks of life may be common across planetary systems.
The comet, discovered on July 1, 2025, by the NASA-funded ATLAS survey telescope in Chile, is only the third interstellar object confirmed to have passed through our solar system. The ATLAS system, part of NASA’s planetary defense network, initially identified the object, which was subsequently designated 3I/ATLAS – signifying it as the third interstellar (I) object discovered by the ATLAS telescope. Prior to its official discovery, the NSF-DOE Vera C. Rubin Observatory had already captured images of the comet in wide-field commissioning data, approximately ten days before the ATLAS announcement.
The detection of hydroxyl gas, a byproduct of water’s breakdown by solar radiation, was unexpected given the comet’s distance from the Sun. Typically, comets initiate releasing water vapor as they approach the Sun and its heat causes ices to sublimate. 3I/ATLAS, however, was observed emitting water at a rate of roughly 40 kilograms per second while still far from the Sun. This suggests the comet possesses a mechanism for releasing water even at significant distances, potentially indicating a higher abundance of volatile materials than previously assumed for interstellar objects.
Scientists determined 3I/ATLAS was interstellar based on its high velocity, and trajectory. Observations from the Hubble Space Telescope in August 2025 estimated the comet’s nucleus to be between 1,400 feet (440 meters) and 3.5 miles (5.6 kilometers) in diameter. At the time of discovery, it was traveling at approximately 137,000 miles per hour (221,000 kilometers per hour), a speed that increased to around 153,000 miles per hour (246,000 kilometers per hour) at its closest approach to the Sun, known as perihelion.
Despite its speed and interstellar origin, 3I/ATLAS poses no threat to Earth, with its closest approach to our planet being 170 million miles (270 million kilometers), or 1.8 astronomical units. The comet’s trajectory will carry it out of the solar system at the same speed it entered, continuing its long journey through interstellar space. The comet reached perihelion on October 29, 2025.
The characteristics of 3I/ATLAS – its color, speed, and direction – are consistent with expectations for an interstellar comet. Further observations are planned as the comet continues its outbound journey, but no further dedicated missions have been announced. The comet is currently moving away from the Sun and Earth, and its apparent magnitude was measured at 14.5 as of February 2026.
