Astronomers made an exciting discovery regarding the interstellar comet 3I/ATLAS. Although it was officially detected on July 1, 2025, images taken by the Vera C. Rubin Observatory in Chile show that this comet was already active and visible ten days earlier. This early capture gives us a unique look into how interstellar objects travel through our solar system.
The Vera C. Rubin Observatory started its science validation phase on June 20, 2025. On its very first night, it caught the comet on camera, even before the Asteroid Terrestrial-impact Last Alert System (ATLAS) identified it. Colin Orion Chandler from the University of Washington and his team used a special processing method to analyze this early data since the telescope’s standard system wasn’t yet operational.
Chandler points out the significance of this feat. If the observatory had started its operations a few weeks earlier, it might have made the first discovery of 3I/ATLAS. Over time, Rubin imaged the comet several times, capturing the cloud of dust and gas around it as it heated up near the sun. This suggests that Rubin could identify many more interstellar comets in the future, possibly about one each year, thanks to its Legacy Survey of Space and Time mission.
Meanwhile, 3I/ATLAS has been under careful observation by spacecraft headed to Jupiter, including ESA’s JUICE and NASA’s Europa Clipper. These missions coordinated their efforts when the comet passed between them in late 2025. Kurt Retherford from the Southwest Research Institute summarized the collaboration, noting how both spacecraft effectively shared observation data.
Using ultraviolet spectrographs, the missions detected elements like hydrogen, oxygen, and carbon released from the comet as its gases were broken down by sunlight. Interestingly, the carbon levels in 3I/ATLAS are higher than those typically found in solar system comets, echoing findings from the James Webb Space Telescope. This information helps researchers pin down the conditions in which these interstellar bodies formed.
Philippa Molyneux from the Southwest Research Institute highlighted the importance of studying the composition of 3I/ATLAS. By comparing the ratio of water ice to carbon-dioxide ice, scientists can learn whether the comet’s origin was similar to that of our own solar system or if it came from a different environment altogether.
Further analysis suggests that 3I/ATLAS is likely between seven and twelve billion years old, with a nucleus about one kilometer wide. It travels at a remarkable speed of 140,000 mph (61 km/s) and has likely encountered various stars before reaching us. Observations during its closest approach to the sun in October 2025 revealed active gas outgassing, reinforcing earlier compositional findings.
The combined data from the Rubin Observatory and the missions studying 3I/ATLAS paints a clearer picture of this interstellar voyager. As scientists continue to analyze this comet, they expect these findings to help predict and identify future interstellar travelers, allowing for quicker and more accurate characterization of these celestial visitors.
For further in-depth reading, you can check The Astrophysical Journal Letters.
