3I/ATLAS, the third-ever recorded interstellar object in the solar system since 1I/Oumuamua (2017) and 2I/Borisov (2019), has been baffling the scientists and the lay alike ever since its discovery on July 1, 2025 by the ATLAS observatory. The highly anomalous exocomet entered its perihelion phase on October 29, 2025, getting the closest to the Sun by 1.4 AU and later flew past Earth at a distance of 1.8 AU or 270 million kilometers, on December 19, 2025. The pre-perihelion and post-perihelion observations of the interstellar interloper by Earth-based observatories and space-based probes have produced a wealth of data that the researchers are still trying to interpret. One thing is quite evident, though, that we are left with more questions than answers, as curiosity over 3I/ATLAS.
In the latest data from NASA’s Webb and Hubble observations between December 2025 and January 2026, the researchers could take another peek into the nucleus and post-perihelion activity of 3I/ATLAS. Here’s a rundown and interpretation by Avi Loeb of the comprehensive paper published by Man-To Hui & others from Webb’s MIRI spectrometer observations done on December 15-16 and 27, 2025.
The nucleus is inferred to have a diameter of 2.6 kilometers. This implies that
The 3I/ATLAS is 40 times massive than 2I/Borisov and 20,000 times larger than 1I/Oumuamua.
Unlike the established brightening trend, 3I/ATLAS faded more rapidly after perihelion
During its “full moon phase” on January 22, the exocomet showed an opposition surge of about 20%
Prof Loeb states
It is likely that comparably bright interstellar objects have passed through the inner solar system in the era of wide-field CCD surveys. This implies that multiple interstellar objects resembling 3I/ATLAS were likely missed even before the discovery of 1I/‘Oumuamua.
The spectra exhibit water (H₂O) in the wavelength range of 5.8–7.0 micrometers
carbon dioxide (CO₂) around 15 micrometers
nickel (Ni) at 7.507 micrometers
and methane (CH₄) at 7.6 micrometers
The Harvard scientist further adds
The most notable finding from the new data is the robust detection of methane (CH4) production. The production rates of methane molecules in the two observing epochs are 13.7% and 27% of the water molecular production rate, respectively.
Prof. Loeb flags the delayed onset of CH₄ production. What does it imply? He writes
This implies that methane ice near the surface of 3I/ATLAS would have been vigorously sublimating at the time of the first reports of outgassing from 3I/ATLAS before perihelion. However, neither the Webb observations nor the SPHEREx spectrophotometry from August 2025, detected methane. This suggests that methane is depleted in the outermost layers of 3I/ATLAS and was exposed to warming by sunlight only close to the Sun. Within this scenario, the early detection of carbon monoxide (CO) outgassing on 3I/ATLAS presents an apparent quandary as CO is more volatile than CH₄ and should therefore be depleted from the surface, yet it was detected prior to CH₄..
He concludes
In summary, the new Hubble and Webb data raise puzzles about the unprecedented mass and chemical composition of 3I/ATLAS. The more we learn about 3I/ATLAS, the more anomalous it looks.
Cover: NASA & Avi Loeb