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The biggest unresolved question about the interstellar object 3I/ATLAS involves the size of its nucleus. Yesterday, I emphasized this point in a recorded discussion with congresswoman Anna Paulina Luna and Peter Skafish (available here) and a podcast with Brian Keating and Michael Shermer (here).
One would have hoped to get clues about the nature of 3I/ATLAS from the plume of gas around it. Data from the Webb space telescope (reported here), the SPHEREx space observatory (here) and the Very Large Telescope (here), show that this plume stretches out to a vast scale of at least 350,000 kilometers and is composed primarily of carbon dioxide — CO2 (87% by mass), carbon monoxide — CO (9%), water — H2O (most of the remaining 4%), and traces of cyanide — CN, as well as nickel without iron (as found in the industrial production of nickel alloys through the carbonyl chemical pathway). However, such a plume could emanate from a natural icy rock or a technological source.
That this plume of gas 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 sharpest image of 3I/ATLAS so far was obtained on July 21, 2025 by the Hubble Space Telescope (reported here). It showed a glow of scattered sunlight ahead of the nucleus towards the Sun but no tail in the opposite direction. The sun-facing glow on that date could not have been dominated by refractory dust with a particle size comparable to the wavelength of sunlight because solar radiation pressure would have pushed the dust within a day to trail 3I/ATLAS in the shape of a typical cometary tail. That such a tail was not observed on July 21, 2025 beyond the transverse width of the glow implies that the scattering of sunlight was dominated by icy fragments that evaporated before they had an opportunity to trail 3I/ATLAS. The main open question is which fraction of the reflected sunlight originates from these icy fragments compared to the solid surface of the nucleus of 3I/ATLAS.
The brightness of 3I/ATLAS at a wavelength of 1 micrometer implies a nucleus diameter of 46 kilometer (as calculated here) for a typical albedo of 4%. However, if 99% of the brightness stems from icy fragments around 3I/ATLAS, then the nucleus diameter is ten times smaller, of order 5 kilometers. But even in this reduced size case, the mass carried by 3I/ATLAS is still a thousand times bigger than that carried by the previous interstellar object 2I/Borisov, whose diameter was estimated to be ~0.5 kilometers. The mass loss rate from 3I/ATLAS is a few times bigger than that from 2I/Borisov, whereas its surface area is larger by a factor ranging between ~100 for a 5-kilometer diameter to ~10,000 for a 46-kilometer diameter. The activity of 3I/ATLAS is very weak when calibrated by its large surface area.
In order to assess how anomalous 3I/ATLAS is, it is essential to measure the size of its nucleus. How can we accomplish this task?
On October 3, 2025, 3I/ATLAS will pass at a distance of 29 million kilometers from the HiRISE camera onboard the Mars Reconnaissance Orbiter. The camera’s 0.5-meter aperture will be able to image 3I/ATLAS with a resolution of 30 kilometers per pixel. The glowing cloud around 3I/ATLAS is optically thin (transparent). Hence, the total luminosity emanating from the central pixel in the HiRISE image will provide a strict upper limit on the nucleus brightness and hence its size, better by two orders of magnitude than the Hubble Space Telescope image.
Fortunately, the trajectory of 3I/ATLAS is aligned to within 5 degrees from the ecliptic plane, allowing it to arrive not only close to Mars but also within 54 million kilometers from Jupiter on March 16, 2026. These remarkable circumstances bring 3I/ATLAS also close to the Juno spacecraft around Jupiter, as discussed in my paper here and recognized by the visionary letter from congresswoman Luna here.
We cannot expect “the mountain to come to Muhammad” routinely in the context of future interstellar objects coming close to human-made probes, unless we are visited by alien probes which target solar system planets. For random trajectories with a high inclination angle relative to the ecliptic plane, such as in the cases of the first two interstellar objects, 1I/`Oumuamua (122.8 degrees) or 2I/Borisov (44 degrees), the task of resolving an interstellar object in a close-up image would be far more challenging than for 3I/ATLAS. It would be necessary to design an interceptor with a camera similar to HiRISE that maneuvers to arrive at the right time within a distance of a million kilometers from the expected path an interstellar object like 3I/ATLAS, in order to get a pixel resolution of 1-kilometer for the nucleus. A future NASA mission could aim to deploy such an interceptor in a waiting position for the future harvest of interstellar objects expected from the Rubin Observatory.
After all, a picture is worth a thousand words.
ABOUT THE AUTHOR
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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.