Press enter or click to view image in full sizeNew images from the Nordic Optical Telescope, in the Canary Islands, Spain. The evolution of the glow around 3I/ATLAS shows a transition from an anti-tail towards the Sun to a tail away from the Sun. The brightest pixel is marked by a red dot. Sunward is West and distance from Sun at each date is expressed in units of the Earth-Sun separation (au). (Credit: Jewitt and Luu 2025)Press enter or click to view image in full size
New images of the interstellar object 3I/ATLAS, taken by the 2.5 meter diameter Nordic Optical Telescope, in the Canary Islands, Spain (accessible here), reveal that the anti-tail from 3I/ATLAS towards the Sun observed during July and August 2025 turned into a tail in September 2025. No terrestrial observations are possible during the month of October as 3I/ATLAS arrived too close to the Sun in the sky.
The inferred mass loss rate from 3I/ATLAS scales roughly with the solar radiation flux impinging on its surface. The authors, David Jewitt and Jane Luu, note that this dependence is consistent with carbon dioxide (CO2) being the main driver of activity, as already inferred from previous spectroscopic data by the SPHEREx space observatory and Webb space telescope (reported here and here, respectively). The exquisite spectrum measured by the Webb telescope (reported here) implied a mass loss rate of about 150 kilograms per second at a distance of 3.32 times the Earth-Sun separation (au), with a mass composition fractions of 87% being CO2, 9% being CO and about 4% being H2O.
The high-resolution imaging of 3I/ATLAS by the Hubble Space Telescope on July 21, 2025 (as reported here), revealed a pronounced sunward anti-tail with a projected 2:1 elongation and a viewing-angle corrected elongation of about 10:1 in three dimensions — resembling a jet. In collaboration with Eric Keto I wrote a paper (accessible here) ithat explained this sunward jet as being dominated by scattering of sunlight by H2O ice fragments ejected from the nucleus of 3I/ATLAS as a result of CO2 sublimation.
Today, Eric Keto and I posted a detailed theoretical model (accessible here) that explains the transition of the anti-tail of 3I/ATLAS to a tail in September 2025. In addition to the Nordic Optical Telescopes, observations with the Keck telescope (reported here) and Gemini South observatory (reported here) show a reduction in the anti-tail and the growth of a tail in a direction away from the Sun. Our model details the physics behind this evolution. As 3I/ATLAS approaches the Sun, the exponential temperature dependence of the sublimation rate causes a continuous increase in the production rate of ice fragments and a sharp decline in their residence time in the outflow. The combined effects produce a peak in total scattering cross-section due to H2O ice grains at a distance of 3 to 4 au from the Sun. At closer heliocentric distances, the scattering becomes dominated by longer-lived refractory dust particles and larger volatile grains with survival times long enough to form a tail that stretches away from the Sun.
The images taken on October 2, 2025 by the HiRISE camera onboard the Mars Reconnaissance Orbiter (still unavailable, possibly due to the U.S. government shutdown) should provide a sideway view of the glow around 3I/ATLAS with a pixel resolution of 30 kilometers. Once released, they would offer an excellent test for the Keto-Loeb model of an icy coma around 3I/ATLAS.
The total amount of mass lost from 3I/ATLAS during the months of July through October 2025, amounts to about 2 million tons. This amounts to a fraction lower than 0.00005 of the total mass of 3I/ATLAS. The interstellar object was inferred to possess a mass larger than 33 billion tons based on the lack of detectable non-gravitational recoil in its trajectory, as inferred in a paper I wrote in collaboration with Richard Cloete and Peter Veres (available here). The entire plume of gas around 3I/ATLAS requires the ablation of a surface layer with an average thickness of merely 4 centimeter out of a solid object with a diameter of 5 kilometers, comparable to the ratio between the length of the palm of your hand and the length of Manhattan Island. Needless to say, we cannot infer the true nature of 3I/ATLAS from the skin layer that it shed so far.
My colleague, Adam Hibberd, pointed out that if the object is an alien spacecraft slowing down, and the anti-tail is braking thrust, then this change from anti-tail to tail would be entirely expected near perihelion. In that case, the transition would constitute a technosignature in the form of an unexpected phenomenon indicative of controlled maneuvering, possibly with the intention of achieving a bound heliocentric orbit between Mars’s and Jupiter’s orbits.
Since the surface of 3I/ATLAS will be exposed to at least 33 gigawatts of solar radiation at perihelion, post-perihelion observations at its closest approach to Earth on December 19, 2025, will provide the most important clues about its nature. If, as a result of the intense solar heating, 3I/ATLAS will show all the features of a natural comet, I will reduce its rank to 2 on the Loeb scale (quantified here and here). The rank will not go down to 0 because the enormously larger mass of 3I/ATLAS relative to 1I/`Oumuamua and 2I/Borisov and its fine-tuned orbital alignment with the ecliptic plane, will never go away.
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.