Kyoto Sangyo University Captures Changes in Interstellar Comet 3I/ATLAS Before and After Solar Approach Using Subaru Telescope — CO₂/H₂O Ratio Reveals New Insights, Published in The Astronomical Journal

The Subaru Telescope has captured new evidence that the composition of interstellar comet 3I/ATLAS changed as it approached the Sun, according to research led by Kyoto Sangyo University. Observations made on January 7, 2026, after the comet’s perihelion, show a significantly lower ratio of carbon dioxide to water in its coma compared to earlier measurements taken before its closest approach to the Sun.

The research team, led by Yoshiharu Shinnaka of the Koyama Space Science Institute at Kyoto Sangyo University, used the Subaru Telescope’s instruments to analyze light from the comet’s coma. By examining emissions at specific wavelengths, they estimated the CO₂/H₂O ratio, which serves as a key indicator of the comet’s icy nucleus composition. This post-perihelion measurement was notably lower than values derived from space telescope data collected prior to perihelion, including observations from the James Webb Space Telescope on August 6, 2025.

These findings suggest that the chemistry of the comet’s coma is not static but evolves over time, likely due to solar heating triggering the release of different ices from the nucleus at varying rates. The observed shift in composition offers clues about the internal structure and formation history of 3I/ATLAS, which originated outside the Solar System and is only passing through our cosmic neighborhood on an escape trajectory.

Comet 3I/ATLAS (designated C/2025 N1) was first discovered in July 2025 and quickly identified as an interstellar object. Unlike most comets native to the Solar System, it exhibits unusual behaviors, including the rare development of an “anti-tail.” Earlier studies had indicated an exceptionally high CO₂/H₂O ratio, suggesting formation in a cold, CO₂-rich environment or exposure to intense radiation in its native planetary system.

The Subaru Telescope, located at the Mauna Kea Observatory in Hawai‘i, enabled ground-based follow-up observations that were critical for tracking these changes. Its ability to observe in visible and near-infrared wavelengths allowed researchers to isolate spectral signatures of key molecules in the coma. The study demonstrates how ground-based facilities can contribute to time-sensitive planetary science missions, particularly when coordinated with space-based observatories.

The results have been accepted for publication in The Astronomical Journal, a peer-reviewed journal published by the American Astronomical Society. This marks one of the first detailed compositional evolution studies of an interstellar comet using combined ground- and space-based observations.

Researchers note that such changes in volatile composition may affect how interstellar objects are classified and understood, especially as more such visitors are detected with modern survey telescopes. Continued monitoring of outgoing interstellar objects could provide further insights into the diversity of planetary systems beyond our own.