Press enter or click to view image in full sizeThe highest-resolution image of 3I/ATLAS was taken by the Hubble Space Telescope on July 21, 2025, when the interstellar object was at a heliocentric distance of 3.8 times the Earth-Sun separation. It shows an anti-tail, with the glow of light extending towards the Sun-facing side, opposite to the common situation in comets. (Credit: E. Keto & A. Loeb 2025, reproduced from D. Jewitt et al. 2025)
A tantalizing feature of the highest-resolution image of the interstellar object 3I/ATLAS is the unexpected appearance of an anti-tail. This image of 3I/ATLAS was taken by the Hubble Space Telescope on July 21, 2025 (as reported here), when the object was at a heliocentric distance of 3.8 times the Earth-Sun separation (AU). The anti-tail is an extension of the glow of scattered sunlight around 3I/ATLAS towards the Sun and not away from it — as typically the case for comets. This anomalous anti-tail, not a result of geometric perspective, had never been reported before for solar system comets.
A new paper (available here) that I co-authored with my brilliant colleague, Eric Keto, provides an explanation for the development of an anti-tail based on a simple physical model.
In short, our new model associates the glow around 3I/ATLAS with scattering of sunlight by ice fragments that are shed from its surface, rather than refractory dust particles as previously assumed. Most of the scattering stems from ice grains with a size comparable to the wavelength of sunlight, about half a micrometer. The ice fragments evaporate after some time but because of the enhanced mass loss in the Sun-facing side, more of the bigger fragments can reach a large distance — giving rise to an anti-tail. In other words, the anti-tail represents an extension of the snow line, or survival distance of a sublimating ice grain, in the direction of the Sun. The extension of the glow is due to the difference in the sublimation mass flux in the solar and perpendicular directions caused by the change in the illumination angle of the object’s surface. The stronger sublimation mass flux in the solar direction results in ice grains with larger sizes, longer sublimation lifetimes, and a snow line at a larger distance with respect to other directions. The observed surface brightness profiles as a function of illumination angle are well reproduced by our model for a constant outflow velocity and sublimating ice grains with angularly dependent survival lengths.
The paper models the glow as scattering by a spherical outflow of sublimating ice grains as a function of angle around 3I/ATLAS. Whereas the sublimation mass flux off the object’s surface is predominantly carbon dioxide (CO2) as observed by the Webb Telescope (here) and the SPHEREx space observatory (here), the longest-surviving grains are mostly made of water (H2O) ice which evaporates more slowly than carbon dioxide (CO2) ice.
From a chain of simple considerations, the new model determines the maximum ice grain size and terminal velocity as a function of the illumination angle to derive the survival length. The survival length dictates the extent of the glow of scattered sunlight around 3I/ATLAS as a function of illumination angle.
Press enter or click to view image in full sizeBrightness profiles in different directions around 3I/ATLAS in the Hubble Space Telescope image taken on July 21, 2025. The angles of 10, 100, and 190 degrees represent the direction towards the Sun, perpendicular to that direction and the direction opposite to the Sun, respectively. The observed profiles are overlaid with fits to a model. The spatial axes have units of pixels (0.04 arcsecond per pixel) and counts (detected electrons per second). (Credit: E. Keto & A. Loeb 2025)
The sublimation of ice grains, driven by the sublimation mass flux of CO2 gas from the nucleus, determines the observed curvature of the profiles and the angular anisotropy.
Within a spherical outflow model, the anti-tail emerges naturally as a consequence of anisotropic illumination. The angular dependence of the characteristic length scale for the destruction is primarily due to the illumination angle of the object’s surface which affects the sublimation mass flux of CO2 gas. The mass flux determines both the terminal velocity of the ice grains and the maximum size in the grain size distribution and therefore the characteristic survival length of the ice grains in the outflow.
The plume of gas extending out to at least 350,000 kilometers around 3I/ATLAS is dominated by carbon dioxide — CO2 (87% by mass) with traces of carbon monoxide — CO (9%), and water — H2O (most of the remaining 4%). Together with the path of 3I/ATLAS being aligned with the ecliptic plane of the planets around the Sun — at a chance probability of a fifth of a percent, the above characteristics make 3I/ATLAS anomalous relative to familiar icy rocks.
The development of an observable anti-tail depends on a favorable combination of composition and solar insolation that both affect the sublimation mass flux off the object’s surface as well as the destruction of ice grains by sublimation. In particular, the total scattering cross-section is dominated by volatile ice grains. The sublimation mass flux and radiation pressure should not be so large as to create a traditional cometary tail, both indicating that an anti-tail is more likely to be observed at larger heliocentric distances. This in turn requires high angular resolution observations, such as the image taken by the Hubble Space Telescope. As 3I/ATLAS approaches the Sun, the physical conditions and shape of the glow around it will change.
That the plume of gas and ice is shaped by the solar radiation and solar wind to a teardrop shape, as observed last week by the Gemini South telescope (here), is a straightforward consequence of gas dynamics and not a clue about the nature of the nucleus. The situation is akin to observing a plume of smoke carried by the wind. Without a resolved image of the source of the smoke, we cannot tell whether it originates from a burning log of wood or the exhaust of a car.
The transition from red to green-blue colors of 3I/ATLAS observed in recent days (and described here) might be associated with the steep rise in the production of cyanide (CN) as reported by the Very Large Telescope on August 25, 2025 (here). The production of both cyanide and nickel without iron (as known for industrial production of nickel alloys), was found to increase dramatically with decreasing heliocentric distance to the power of about 9 (+/-1).
This morning, the ATLAS telescope team released data dating back to March 28, 2025 (accessible here), showing that the 3I/ATLAS was active much earlier than previously thought. The cross section of scattered light around 3I/ATLAS grew inversely with heliocentric distance to the power of 3.9 when the object was farther than 3.3 times the Earth-Sun separation (AU). Once 3I/ATLAS got closer, the growth was moderated to a power-law index of 1.2. The ATLAS team interprets this anomalous evolution as a shift from scattering of sunlight by dust lifted from a reddened surface to the production of small, optically bright icy grains, which changed the opacity of the plume of materials shed off by 3I/ATLAS. The latter conclusion is consistent with our new model, except that the model argues for the dominance of ice of refractory dust at a heliocentric distance of 3.8 AU.
Altogether, 3I/ATLAS is different from the first interstellar object 1I/`Oumuamua which displayed no signs of gas or dust around it but nevertheless exhibited non-gravitational acceleration. It is also different from the second interstellar object 2I/Borisov which behaved like a familiar comet. In the coming months we will learn much more about the nature of 3I/ATLAS. Evidence-based science is fun as long as scientists who interpret this evidence do not pretend that there is nothing new under the Sun.
ABOUT THE AUTHOR
Press enter or click to view image in full size(Image Credit: Chris Michel, National Academy of Sciences, 2023)
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.