Before my morning jog at sunrise, I sketched a model for the dust outflow around the new interstellar object, 3I/ATLAS, which appears as a fuzzy glow in the image obtained by the Hubble Space Telescope on July 21, 2025.
One of the remarkable coincidences regarding 3I/ATLAS is the alignment of its trajectory with the ecliptic orbital plane of the planets around the Sun. This means that 3I/ATLAS is passing through the so-called interplanetary zodiacal dust that resides in that plane. As it gets rarefied at large distances, the outflow of dust from the surface of 3I/ATLAS is destined to eventually be stopped by the zodiacal medium.
The glow of scattered sunlight extends to a characteristic distance of about 3,000 kilometers on all sides of 3I/ATLAS. The mass density of the outflow gets diluted inversely with radius squared until it is stopped by the ram-pressure of the ambient medium. Streams of dust particles could interact electromagnetically as dust particles often acquire electric charge. If the radius of the nucleus of 3I/ATLAS is 10 kilometers, as inferred from its brightness for an albedo of 5%, then the outflow density is diluted by a factor of 90,000 at a distance of about 3,000 kilometers relative to its initial value near the nucleus surface.
What is the ejection speed of the outflow from the surface of 3I/ATLAS? The minimum value would be the rotation speed of that surface. The observed rotation period of 16 hours (reported here) gives a rotation speed of about 1 meter per second for a nucleus radius of 10 kilometers. But detailed analysis of the Hubble image (accessible here) suggests an ejection speed of about 20 meters per second for 1-micrometer sized dust particles.
The outflowing gas delivers to the ambient medium a ram pressure equal to its mass density times its ejection velocity squared. As the outflow gets rarefied, the ram pressure declines inversely with distance squared and eventually gets stopped by the zodiacal medium. In the frame of the object, the ambient medium moves at minus its velocity, 60 kilometers per second. Adopting a characteristic mass density for the zodiacal medium of about a proton mass per cubic centimeter and a speed of 60 kilometers per second for 3I/ATLAS relative to that medium, I calculated the implies mass density profile of the outflow so that it will get stopped at about 3,000 kilometers. Extrapolating the mass density of the outflow to the surface of the object and multiplying by the ejection speed of 20 meters per second and by the surface area of its Sun-facing side, gives a mass loss rate of about 10 kilograms per second. This is very close to the value inferred by other considerations from the detailed analysis of the Hubble image.
The inferred mass loss rate is independent of the assumed radius of the nucleus because the inferred mass density at the nucleus’ surface scales inversely with radius squared after being calibrated at the stopping radius. Since the mass loss rate scales as the mass density of the outflow at the nucleus’ surface times the surface area, which scales as radius squared, the derived mass loss rate does not depend on the radius of the nucleus.
The ram-pressure confinement and the small mass density in the outskirts of the dust cloud around 3I/ATLAS explain the lack of a prominent cometary tail behind 3I/ATLAS. The total dust mass required to maintain this cloud in a steady state over six months can be supplied by a surface layer that is only a millimeter in thickness on a 10-kilometer object. This dust layer could have been developed as a result of fragmentation of the surface from bombardment by interstellar dust and gas during a long interstellar journey.
The derived column density of the outflow is too low for it to be opaque. This suggests that the observed reddening of the spectrum of 3I/ATLAS reflects the red surface of 3I/ATLAS, as characteristic for the surfaces of Kuiper belt objects, like Arrokoth which was observed by the New Horizons spacecraft and believed to have organic molecules as a result of being exposed to interstellar cosmic rays and ultraviolet light.
Most importantly, the low opacity of the dust suggests that the reflected light originates mostly from the surface of 3I/ATLAS and not from the dust surrounding it. Given its brightness, the radius of 3I/ATLAS needs to be of order 10 kilometers for an albedo of 5% or a few times smaller for a perfect reflector.
As I pointed out in my first paper on 3I/ATLAS (accessible here), this conclusion raises two puzzles. The previous interstellar objects, 1I/`Oumuamua and 2I/Borisov, were both hundreds of meters in size. Based on the solar system reservoir of rocks, one would expect to find a hundred thousand rocks with a 100-meter diameter for each 20-kilometer rock. Clearly, we did not observe as many interstellar objects on the scale of 1I/`Oumuamua and 2I/Borisov before discovering 3I/ATLAS. In addition, the reservoir of rocky materials in interstellar space can only supply a 20-kilometer rock once per 10,000 years even if we assume that all this material is packaged in such rocks.
If 3I/ATLAS is not a rock made of heavy elements based on these considerations, what could it be? One possibility is that it is an iceberg made of hydrogen or helium, which are more abundant in interstellar space by several orders of magnitude. The problem is that a hydrogen iceberg would be easily evaporated by starlight, as I showed in a paper with Thiem Hoang (accessible here). Alternatively, 3I/ATLAS might have targeted the inner solar system and has nothing to do with the reservoir of rocks on random trajectories in interstellar space. A technological design would also explain the alignment of its trajectory with the ecliptic plane (likelihood of 0.2%) and its close passage to Mars, Venus and Jupiter (likelihood of 0.005%).
Here’s hoping that the data collected in the coming months will reveal more details about 3I/ATLAS as it approaches the Sun and gets brighter and warmer. Turning the heat on 3I/ATLAS may reveal its nature.
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.
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