The New Interstellar Object 3I/ATLAS is Smaller or Rarer Than it Looks
The interstellar object 3I/ATLAS was discovered on July 1, 2025 with an orbital eccentricity of 6.2, a distance of closest approach to the Sun (perihelion) of 1.35 times the Earth-Sun separation (=astronomical unit, abbreviated as `au’) — expected in the last week of October 2025, orbital inclination of 175 degrees relative to the Earth’s orbital plane around the Sun, and a hyperbolic velocity of 60 kilometers per second.
Following its discovery, Darryl Seligman and collaborators argued that 3I/ATLAS shows a weak cometary activity and used the measured absolute magnitude of 3I/ATLAS, H=12, to infer a nuclear radius of R~10 kilometers assuming an asteroid-like albedo of 5%. They also derived a spatial number density of n_0~10^{-3} per cubic au for objects similar or bigger than 3I/ATLAS.
In a new paper written this morning during my July 4th vacation in Aruba, and following on a BBC News interview last night, I showed that the combination of these two results leads to an untenable mass density in interstellar objects, unless 3I/ATLAS is a comet with a much smaller core (radius lower than 400 meters) or an object that favored a radial orbit into the inner solar system and represents a population with a much lower number density (less than 5x10^{-8} per cubic au) than the value inferred by Seligman et al. (2025).
The Galactic mass density of interstellar objects similar or bigger than 3I/ATLAS is given by n_0*(4*pi/3}R³)*D, where D is the intrinsic mean mass density of objects of radius bigger than R. Adopting R as the radius of the solid core of 3I/ATLAS yields a Galactic mass density of 0.02 solar masses per cubic parsec for the fiducial values inferred by Seligman et al. (2025).
For comparison, the Galactic mass density for stars in the neighborhood of the Sun is twice that value and the mass density in heavy elements that may source interstellar asteroids is 4% of that value. But we can consider an even stricter condition, as it is reasonable to expect the mass density of interstellar asteroids to be lower than the mass density of rocky materials around stars, which is 20,000 times smaller, as I showed in a recent paper that I wrote with Morgan MacLeod.
The resulting limit from the mass budget in Galactic rocks implies that either the core radius of 3I/ATLAS is smaller than 0.4 kilometers for the inferred number density or the number density is smaller than 5x10^{-8} per cubic au for interstellar objects with radius above 10 kilometers. In the first case, the radius is similar to the size limit on the core of the interstellar comet 2I/Borisov.
In my new paper, I concluded that the radius and number density inferred by Seligman et al. (2025) for interstellar objects like 3I/ATLAS cannot be both valid. The Galactic mass budget in rocky materials found in planetary systems argues that either: (i) the reflection of sunlight by 3I/ATLAS originates from its cometary plume whereas most of its mass is carried by a solid core with an effective surface area smaller by nearly three orders of magnitude than the value inferred by Seligman et al. (2025) (with radius R less than 0.4 kilometers); or (ii) 3I/ATLAS is a solid object with a radius of 10 kilometers but the number density of objects of its size or bigger is lower than 5x10^{-8} per cubic au. In both cases, at least one of these numbers is much smaller than the fiducial values inferred by Seligman et al. (2025).
If 3I/ATLAS is a comet, then it will get brighter as it comes closer to the Sun and evaporates more vigorously. Upcoming data from state-of-the-art telescopes, including the Vera C. Rubin Observatory and the James Webb Space Telescope, will be able to test the first possibility mentioned above. In case the first possibility is ruled out and 3I/ATLAS is a solid object with a physical radius of 10 kilometers, then the remaining possibility is that its trajectory was not drawn from a random distribution and might have favored a radial plunge towards the inner Solar system. Future data will tell.
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.
