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The Massive Nucleus of 3I/ATLAS and its Puzzling Methane Outgassing, Based on New Data from the Hubble and Webb Telescopes

New data from the Hubble Space Telescope. Detection of the nucleus of 3I/ATLAS (third panel from the left) through the subtraction of the best-fit coma model (second panel) from the observed Hubble images (first panel) for each observational visit (indicated at right). In each row, the red and magenta arrows indicate local north and east, respectively, with the projected anti-solar direction and the negative heliocentric velocity of 3I/ATLAS represented by the yellow and cyan arrows, respectively. The horizontal white bar near the bottom marks a scale of an arcsecond in apparent length, corresponding to the range of 1,300–1,700 kilometers from top to bottom during this period. (Image credit: Man-To Hui et al. 2026)

What a glorious day! Today, new data on the interstellar object 3I/ATLAS was released from the Hubble and Webb space telescopes in two preprints posted here and here.

The Hubble Space Telescope report includes a successful detection of the nucleus of 3I/ATLAS based on post-perihelion data from December 2025 to January 2026. Most importantly, the nucleus is inferred to have an effective diameter of 2.6 (±0.4) kilometers for an assumed typical albedo value of 0.04.

Since mass scales as diameter cubed, this measurement implies that 3I/ATLAS is about 40 times more massive than 2I/Borisov whose diameter was inferred to be 0.7 (±0.3) kilometers, and at least 20,000 more massive than 1I/`Oumuamua, whose length was estimated <0.2 kilometers and its thickness is at least ~10 times smaller.

The derived nucleus diameter is consistent with an independent estimate derived from the reported non-gravitational acceleration and mass-loss rates of 3I/ATLAS based on the rocket effect from the observed outgassing.

Compared to the pre-perihelion brightening trend, 3I/ATLAS faded more rapidly after its closest approach to the Sun on October 29, 2025. This activity asymmetry is further corroborated by a post-perihelion surface brightness profile that is significantly shallower than its pre-perihelion counterpart.

The nucleus light-curve exhibits evidence of temporal variations, attributable to rotation modulation, as inferred in my paper with Toni Scarmato here.

When the Sun, Earth and 3I/ATLAS aligned on January 22, 2026, the scattered-light by dust grains displayed a statistically significant opposition surge of about 20%, characterized by an e-folding width of 3 degrees, as predicted in a recent paper that I co-authored with Mauro Barbieri here.

The authors estimate a lower limit of more than one 3I/ATLAS-sized interstellar object within a heliocentric distance of 4.5 times the Earth-Sun separation (AU) at any instant. This is likely a conservative lower bound as inactive interstellar objects of this size would be significantly more difficult to detect. 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.

New data from the James Webb Space Telescope. Top panels: stacked images derived from six successful observations of 3I/ATLAS by the MIRI system. The sunward and velocity directions are indicated. The panels are labeled with the corresponding date and spectral grating setting. Bottom panel: spectra of 3I/ATLAS from a heliocentric distance of 2.20 to 2.54 AU. The main H2O, CO2, CH4, and Ni spectral features are marked. The inset panel provides a zoomed-in view of the CO2 bands. (Image credit: Matthew Belyakov et al. 2026)

The new Webb telescope paper presents the first spectroscopic characterization of 3I/ATLAS after perihelion using the MIRI spectrometer on December 15–16 and 27, 2025, when the object was at heliocentric distances of 2.20 and 2.54 AU, respectively. 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. Comparison of the volatile production rates measured during the two epochs indicate a significant reduction in overall outgassing over 12 days, with the measured H2O activity level dropping more steeply than other species. 3I/ATLAS continues to display an extended source of water production from icy grains.

Webb images of H2O, CO2, and CH4 in the gas plume around 3I/ATLAS. The sunward and target velocity directions are denoted by the white arrows. For H2O and CO2, the white contours correspond to emission levels of 75%, 50%, and 25% relative to the maximum value. (Image credit: Matthew Belyakov et al. 2026)

Pre-perihelion Webb observations from August 2025 (as reported here) found that 3I/ATLAS is unusually rich in carbon-dioxide (CO2) relative to water (H2O) carrying 87% versus 4% of the total mass loss rate in gas phase, respectively, with most of the remaining 9% being carbon monoxide (CO). The new post-perihelion data implies a CO2/H2O ratio that is half that much or similar for the two epochs of JWST/MIRI spectroscopy, respectively.

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.

The delayed onset of CH4 production raises interesting questions regarding the history of 3I/ATLAS. Solid-phase methane is hyper-volatile, with a significantly lower sublimation temperature than carbon dioxide (CO2). 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.

In summary, the new Hubble and Webb data raises puzzles about the unprecedented mass and chemical composition of 3I/ATLAS. The more we learn about 3I/ATLAS, the more anomalous it looks. Perhaps this is all natural for early encounters with interstellar objects, akin to early partners in blind dates from other worlds. But perhaps we are also missing something important.

ABOUT THE AUTHOR

Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.

https://avi-loeb.medium.com/
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