3I/ATLAS: The Strange Interstellar Comet — Weird Chemistry, Not Aliens

3I/ATLAS: An Unusual Interstellar Visitor

Discovered in July by a planetary defense survey about 420 million miles from Earth, 3I/ATLAS is the third confirmed interstellar object to enter our solar system. Initially faint and comet-like, it plunged inward over the summer, reached perihelion around October 30, and re-emerged from behind the Sun on November 10. Since then, astronomers using ground- and space-based observatories have documented both familiar cometary behavior and several surprising chemical signatures.

Key observations

Appearance and structure: Gemini South images in late August showed a broad coma and a dusty tail at least 35,000 miles long. After perihelion, observers recorded multiple narrow jets—localized outbursts of gas and dust—typical of active comets.

Unusually CO2-rich coma: Spectra from NASA's SPHEREx mission and the James Webb Space Telescope reveal the coma is strongly enriched in carbon dioxide relative to water. While comets can carry CO2 and CO at large distances, most comets become water-dominated as they warm. In 3I/ATLAS the CO2-to-water ratio is unusually high; one plausible explanation is a volatile-rich surface layer (a CO2-dominated crust) that temporarily inhibits sublimation of interior water ice.

Metal atoms far from the Sun: The Very Large Telescope (VLT) detected nickel and iron in the coma—metals normally expected to appear only when a comet is much closer to the Sun. Similar detections were reported in 2021 for several comets and for interstellar visitor 2I/Borisov. One hypothesis is that metals are bound in highly volatile molecules such as metal carbonyls, which can release metallic atoms at relatively low temperatures when they dissociate.

Natural radio emission and dismissal of speculation

Radio observations with the MeerKAT array picked up broad-band emissions consistent with molecules produced by photodissociation of water and other natural processes. No narrow-band, coherent transmissions — the hallmark of engineered radio signals — were detected. Despite public speculation (including comments from some high-profile figures), scientists emphasize there is no evidence that 3I/ATLAS is an artificial object. "There is zero evidence suggesting it’s anything other than a ball of vaporizing ice," says planetary astronomer Cristina Thomas.

Origin, size, and trajectory

Orbital data suggest 3I/ATLAS may hail from an older stellar environment—possibly a cluster roughly eight billion years old—making it potentially one of the galaxy’s older small bodies. Hubble and other observations constrain the nucleus to under about 3.5 miles across, possibly as small as ~1,450 feet. When Hubble observed it, the comet was moving at roughly 137,000 miles per hour and will continue under solar gravity as it leaves the inner system. Its closest approach to Earth is expected in December at about 170 million miles.

Why this matters

3I/ATLAS offers a rare opportunity to study material formed around another star and to compare it with comets from our own system. Its unusual CO2 dominance and early-metal signatures challenge simple models of comet activity and suggest a more complex mix of volatiles and metal-bearing compounds. Continued monitoring may reveal changes—outbursts or fragmentation—that could expose interior materials and yield better constraints on composition.

Bottom line: 3I/ATLAS is an intriguing, chemically odd interstellar comet, but all current evidence points to natural cometary processes rather than extraterrestrial technology. Astronomers will keep observing in the coming weeks to learn more.