The fundamental uncertainty about the new interstellar object 3I/ATLAS involves its size. With the typical albedo of 5% for an asteroid, its diameter needs to be 20 kilometers in order to account for its brightness. But as argued in my first paper about it, the reservoir of rocky material in interstellar space can only deliver a 20-kilometer rock once per 10,000 years, even in the unlikely case that all this interstellar material is packed in 20-kilometer rocks.
The alternative interpretation of the brightness of 3I/ATLAS is that most of the reflected sunlight originates from a dense cloud of dust that came out of a much smaller nucleus. This would associate the observed reddening of the reflected sunlight (as reported here, here and here) with small dust particles having a size of half a micron (half a millionth of a meter), the characteristic wavelength of sunlight being reddened.
But this association raises a new problem. For sub-micron particles, the radiation pressure from the Sun is stronger than gravity, pushing them away from the Sun to the form of a typical cometary tail that trails 3I/ATLAS. Given the acceleration from solar radiation pressure, the sub-micron dust particles would lag behind the nucleus by a few arcseconds within a single day. The resulting trail of dust should scatter sunlight efficiently but is not observed in the Hubble Space Telescope image, which only shows a glow ahead of 3I/ATLAS but no bright trail of dust behind it.
If most dust particles are larger than a micron in size, then they would encounter less push by radiation pressure because of their smaller surface to mass ratio. In that case, the reddening of 3I/ATLAS must be explained by the red surface of 3I/ATLAS, implying that the object’s diameter is 20-kilometers, bringing back the concern that it cannot be an interstellar rock because of the limited supply of rocky material in interstellar space. Instead, 3I/ATLAS might be a technological object which targets the inner solar system. This is consistent with its retrograde trajectory being aligned with the ecliptic plane of the planets around the Sun (0.2% probability), and its arrival time being perfectly matched to a close encounter with Mars, Venus and Jupiter (with a 0.0005% probability, as discussed here).
The amount of dust in the plume preceding 3I/ATLAS corresponds to a mass loss rate of up to 60 kilograms per second, which over a period of six months would be equivalent to eroding a millimeter-thick layer from a 20-kilometer object. Such a thin layer could have been dirt that accumulated on the surface of 3I/ATLAS as a result of it being bombarded by interstellar gas and dust particles during its journey over the past 10 billion years. Indeed, spectroscopic measurements show no evidence for molecular or atomic gas accompanying the glow around 3I/ATLAS as expected for a comet (see reports here, here and here, as well as the discussion about water ice here).
Might the dust plume around 3I/ATLAS be simply the result of microscopic fragmentation of the surface of a solid object which traveled for billions of years through interstellar space?
Impacts by interstellar dust particles and interstellar gas particles can easily create a thin layer of dust on the surface of 3I/ATLAS, with no accompanying ices if 3I/ATLAS is not a comet. The interstellar speed of 3I/ATLAS is ~60 kilometers per second relative to the local interstellar medium. Any impacting matter would deliver ten times more energy per impacting proton than required to break a single chemical bond within the solid surface. The microscopic breakup of the surface to super-micron fragments by interstellar impactors, such as dust, gas and cosmic-ray particles, might have led over billions of years to the formation of large dust particles that are released close to the Sun and account for the glow ahead of 3I/ATLAS in its Hubble image.
With this interpretation of the glow preceding 3I/ATLAS, the associated plume of dust does not make it a comet. In contrast with the interstellar comet 2I/Borisov, the glow around 3I/ATLAS is much smaller in size and does not show a bright trailing tail, even though the surface area of 3I/ATLAS is larger than that of 2I/Borisov by up to a factor of a few thousands. Moreover, 2I/Borisov showed clear evidence for H2O, CO, CN, C2 and C3 molecules in its cometary plume (see Tables 4 and 5 here) unlike 3I/ATLAS so far. This suggests that 3I/ATLAS is not an interstellar comet like 2I/Borisov, as it lacks common cometary ices on its surface.
3I/ATLAS will reach perihelion on October 29, 2025. This will provide a litmus test for its nature. If it happens to be a comet, it should erupt with enhanced outgassing as a result of the brighter illumination by the Sun. This would inform us beyond a reasonable doubt that 3I/ATLAS is natural in origin, bringing its rank to `0’ on the “Loeb scale” for the nature of interstellar objects. On the other hand, if 3I/ATLAS will not show the telltale characteristics of a comet but only those of a solid dusty object and also execute an unexpected maneuver near its closest approach to the Sun — when the Solar gravitational assist can amplify the thrust of its engine, then its rank on the Loeb scale will rise to `10’ — a number reserved for manifestly technological objects.
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.