NASA’s James Webb Space Telescope (JWST) has delivered unprecedented insights into the interstellar comet 3I/ATLAS, revealing that it is both smaller and far more chemically unusual than scientists initially believed. Early estimates suggested the comet’s nucleus might be as large as 3.5 miles (5.6 kilometers) across, but refined observations from JWST, combined with data from ground-based telescopes, now indicate the solid nucleus is likely less than 1 mile (1.6 kilometers) in diameter, with earlier measurements inflated by the surrounding coma of gas and dust.

Even more striking are JWST’s chemical findings: observations from its Near-Infrared Spectrograph show the coma to be extraordinarily rich in carbon dioxide, with a CO₂-to-water ratio of roughly 8:1—around sixteen times higher than that seen in typical solar system comets—suggesting 3I/ATLAS formed in a radically different environment, possibly near a CO₂ ice line or under intense radiation conditions. Discovered on July 1, 2025, by the ATLAS sky survey, this rare object is only the third confirmed interstellar visitor ever observed passing through our solar system, traveling at speeds exceeding 130,000 mph (210,000 km/h) and expected to reach perihelion around October 29, 2025. As it continues its passage, astronomers see 3I/ATLAS as a unique time capsule—an opportunity to study material that may predate our solar system and gain new insight into the physical and chemical conditions of the early galaxy.