As they spin, comet nuclei often show periodic variations of their brightness over timescales of order hours. Astronomers expect that to apply also to the new interstellar object 3I/ATLAS, since it was classified by the Minor Planet Center as a comet.
The first interstellar object, 1I/`Oumuamua was spinning with a period of about 8 hours, but its spin did not evolve as expected from a comet with its measured non-gravitational acceleration. Even more paradoxically, deep observations by the Spitzer space telescope did not find carbon-based molecules or dust in a cometary tail around 1I/`Oumuamua. As in Hans-Christian Andersen’s folktale, this “emperor” had no visible clothes but the astronomers pretending to be the adults in the room insist in a recent paper that 1I/’Oumuamua is a dark comet with invisible clothes.
As of now, there is no spectroscopic evidence for atomic or molecular gas around 3I/ATLAS either (see the papers here, here and here). In addition, data from the new Rubin Observatory on 3I/ATLAS shows no variability on hourly timescales. Why isn’t the flux from 3I/ATLAS changing periodically? Either 3I/ATLAS is not rotating or the rotation of its nucleus is hidden from view deep inside a thick cloud of dust.
Another interesting fact gleaned from images of 3I/ATLAS is that it has a leading glowing halo rather than a trailing tail. Why is there a glow ahead of 3I/ATLAS? One possible explanation is that the object does not spin and so its dayside is hot and its nightside is cold. As a result, any ice that accumulated on its surface during its freezing interstellar travel evaporated only at the front of the object facing the Sun. This would mean that the object’s surface is not hidden behind a veil of dust and so its diameter must be 10–20 kilometers to explain its brightness. This brings back the argument from my published paper that 3I/ATLAS cannot be a 10–20 kilometer rock because the available rocky material in interstellar space can only deliver a 10–20 kilometer rock to the survey volume of the ATLAS survey telescope once per 10,000 years rather than once per 10 years as required. If 3I/ATLAS is not a 10–20 kilometers rock, then what is the origin of this object?
One way to avoid the mass budget problem is by delivering 3I/ATLAS to the inner solar system through a fine-tuned trajectory. As it turns out, the trajectory of 3I/ATLAS has indeed rare features. It lines up with the orbital plane of the inner planets (with a likelihood of 0.2%) and comes unusually close to Jupiter, Mars and Venus (with a likelihood of 0.005%). Is 3I/ATLAS alien technology? This was the title of my second paper on 3I/ATLAS, in collaboration with Adam Hibberd and Adam Crowl from the Initiative for Interstellar Studies.
In recent days, I was independently asked by dozens of podcasters and reporters for my opinion on the probability that 3I/ATLAS is an object under intelligent control. I replied that it is difficult to assign a probability to the technological origin. It is natural to expect 3I/ATLAS to be a comet with a compact nucleus surrounded by a large dust cloud. Yet, 3I/ATLAS follows an exceptionally rare trajectory. And so, the final verdict depends on the significance assigned to the trajectory’s peculiarities. We will have a better assessment of this probability within a month or two, once 3I/ATLAS gets brighter and easier to observe as it approaches the Sun.
Given that the NSF-DOE Rubin Observatory is expected to find a new interstellar object every few months in the coming decade, I suggest that the time is ripe to establish a “Loeb scale” for the risk from interstellar objects similar to the Richter scale for earthquakes. The zero point of that scale would correspond to a verified comet or asteroid of a natural origin and a value of 10 would mean a verified technological object, possibly propelled by an engine which powers artificial lights. A team of policy experts, in consultation with scientists, will decide how to respond to an alien threat. And a team of psychologists will suggest how to communicate the message to the public without creating panic and a meltdown of world economies.
It is impossible to resolve 3I/ATLAS with telescopes on Earth, but we can get a better look of it with a camera on a spacecraft that gets close to its path. In my latest paper with Adam Hibberd and Adam Crowl, I proposed using the Juno spacecraft around Jupiter with this goal in mind. The Juno mission is currently scheduled to end its life in mid-September 2025. Instead of giving the spacecraft a push towards Jupiter’s atmosphere where it will burn up to ashes, the paper proposes giving it extra scientific life by instructing Juno’s engine to give it a push away from Jupiter — so that by mid-March 2026 it will intercept 3I/ATLAS at its closest approach to Jupiter. If the available fuel is insufficient for an intercept, even a mild burn of the engine in mid-September 2025 could bring Juno closer to the path of 3I ATLAS by mid-March 2026.
On July 29, 2025, I received a call from a member of the U.S. House of Representatives, the brilliant Anna Paulina Luna, who asked me to brief her about 3I/ATLAS. At the end of our conversation, she kindly offered to send a letter that encourages the leadership of NASA to use Juno as a probe of 3I/ATLAS.
A close encounter between Juno and 3I/ATLAS will provide a once-in-a-lifetime opportunity for human-made technologies to come within a short distance from a large interstellar object. This could actually happen in 2026 rather than appear in a Hollywood science-fiction script such as Stanley Kubrick’s film “2001: A Space Odyssey.” The encounter will also be an unprecedented public outreach opportunity for NASA to engage with the viral interest of followers of Joe Rogan and other podcasters in the nature of 3I/ATLAS.
As a token of my appreciation and gratitude for the vision of the Honorable Congresswoman Anna Paulina Luna, I attach below the letter that she sent to NASA today (also accessible here). The letter encourages NASA’s leadership to conduct a study of the feasibility of using Juno as a probe of 3I/ATLAS.
If aliens are watching Earth right now, they will likely raise humanity’s intelligence ranking among the stars after reading Congresswoman Luna’s inspiring letter.
Here’s hoping that a visit by an interstellar messenger will usher in a peaceful messianic era for all earthlings.
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.