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NASA’s Webb Finds Clues to Ancient, Distant Origin of Comet 3I/ATLAS

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June 22, 2026 11:00AM (EDT)Release ID: 2026-118
Comparison of three telescope images side by side. They are roughly spherical but pixelated, with more intense color saturation in the center. From left to right: smallest sphere is blue and labeled H2O, orange is larger and labeled CO2, and red is largest and labeled CO. A scale bar at the lower left is labeled 1300 km/1 arcsecond and is about one fourth of each of the three images. A compass at the lower right shows north pointing up to 12 o’clock, east pointing left to 9 o’clock, and a fainter arrow labeled to Sun pointing down to 8 o’clock.

Summary

The third identified interstellar comet in human history has a surprising chemical makeup, raising questions as to how common, or unusual, conditions in our own solar system may be. 

Billions of years ago in a nascent planetary system somewhere in the Milky Way galaxy, the icy fragment now known as 3I/ATLAS was getting knocked around as larger planets formed. Powerful gravitational interactions eventually catapulted it out of the system, and it began a journey through our Milky Way galaxy.

In July 2025, the comet was only the third interstellar object ever detected passing through our solar system – the meaning of the ‘3I’ in its name. Objects like 3I/ATLAS offer the unparalleled opportunity for up-close study of something from very far away. Astronomers used the NIRSpec (Near-Infrared Spectrograph) instrument on NASA’s James Webb Space Telescope to discover surprising details of 3I/ATLAS’s chemical makeup, with ratios of heavy carbon and heavy hydrogen never seen among comets in our solar system. Future study of interstellar objects will build on these findings toward a better understanding of how our solar system fits into the larger picture of our galaxy.

Full Article

As interstellar comet 3I/ATLAS began moving away from the Sun in December 2025, astronomers took the opportunity to turn NASA’s powerful James Webb Space Telescope in its direction and capture detailed measurements of its chemical components. The comet was freshly warmed from its closest pass by the Sun, and its ancient ice had been converted to a bright coma of gas ideal for observation.  Webb captured detailed data, including chemical ratios of carbon and deuterium, also known as heavy hydrogen, that are not found in solar system comets. The results surprised researchers. Working ...

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News releases highlighting the discoveries of NASA’s James Webb Space Telescope are produced for NASA by the Space Telescope Science Institute (STScI) in Baltimore, Maryland, under NASA Contract NAS5-03127. News release content is developed by the News Team in STScI’s Office of Public Outreach.

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