The discovery of interstellar objects over the past decade raises an important question that could shape the future of humanity: how to distinguish extraterrestrial spacecraft from interstellar asteroids? Both types of objects reflect sunlight. However, no telescope on Earth can resolve a hundred-meter object — the scale of our largest rocket — Starship, from a distance of about a billion kilometers — the distance where 3I/ATLAS was discovered on July 1, 2025.
Unfortunately, we cannot rely on sky watchers to alert us to the possibility that a spacecraft just entered the solar system. Even after the first reported interstellar object, 1I/`Oumuamua, showed the anomalies of a flat shape and a non-gravitational acceleration without a cometary tail that distinguished it from any known asteroid or comet, it was nevertheless labeled as a “dark comet”, namely a comet without the unique signature that would flag it as a comet: a visible plume of gas and dust. Given this definition, any object launched by humans to space — which is pushed by rocket fuel or solar radiation pressure, is a dark comet.
The best we can hope for is courageous astronomers that would admit anomalies exhibited by outliers, namely features that may fit better the description of a technologically manufactured object than a natural rock.
Recently, I listed the anomalies of the new interstellar object 3I/ATLAS. This object is anomalously bright, implying a diameter of ~20 kilometers for the typical reflectance of asteroids. The implied diameter and detection rate are untenable by the mass budget in interstellar asteroids, as I showed in a new paper — just published in Research Notes of the American Astronomical Society. If 3I/ATLAS ends up being a comet, its nucleus must be an order of magnitude smaller. But if it happens not to possess a large cometary plume of dust or gas, what is the nature of this object?
Without asking this question, humanity will remain in the “stone age,” regarding interstellar objects. Even if 3I/ATLAS will show up as a genuine comet, like 2I/Borisov, as it gets closer to the Sun and heats up, we should always ask this question about future interstellar objects.
An interstellar comet is easy to identify by its tail. But what are the markers that would distinguish a technological interstellar object — a spacecraft, from an asteroid? Here is a list of some of them:
1. Propulsion: a central engine or solar radiation pressure (as I suggested in a paper with Shmuel Bialy for 1I/’Oumuamua) would cause a technological object to deviate from a Keplerian hyperbolic orbit, dictated solely by gravity.
2. Trajectory: the path of the object could selectively target the inner planets in the Solar system. For example, the orbital plane of 3I/ATLAS was within 5 degrees of the ecliptic plane for the Earth’s orbit around the Sun. The likelihood for these orbital angular momenta to be aligned so well is ~0.001, as I had mentioned in my recent anomalies essay.
3. Artificial lights: Reflection of sunlight can be distinguished from artificial light by its spectrum and through its faster decline with increasing distance from the Sun, as I discussed in a paper with Ed Turner.
4. Shape: An artificially-designed shape can be inferred from the light curve of reflected sunlight as the object rotates. This is how 1I/’Oumuamua was inferred in a paper by Sergei Mashchenko to have a disk-like shape.
5. Image from a flyby: Resolved details of the object’s surface could instantly distinguish a technological object from a rock. Such an image can be taken by a camera on a dedicated intercept mission or in case the object happens to be passing very close to Earth. Landing on a technological object through a rendezvous mission like OSIRIS-REx would offer the benefit of a direct inspection, including the privilege of pressing buttons on it.
6. Surface composition: remote spectroscopy of the surface might show signatures of bombardment by cosmic-rays, interstellar dust particles and interstellar protons. The energy deposition rate scales as velocity cubed and travel duration. Faster or older objects should be more scarred by interstellar damage.
7. Signals: A functioning technological device might transmit electromagnetic signs that terrestrial telescopes could search for over a broad range of frequencies from the radio to gamma-rays.
8. Launch of Mini-Probes from a mothership: An efficient way to seed habitable planets with probes is to pass near them and release small devices at the right time and place with the appropriate velocity kick, so that they will intercept the planets while the mothership continues on its journey to the next star.
Ironically, 3I/ATLAS was discovered by the small ATLAS telescope with an aperture diameter of half a meter, during the same month that the 8.36-meter aperture of the Rubin Observatory started to search for interstellar objects from nearly the same location in Chile. Over the next decade, the Rubin observatory is expected to find tens of new interstellar objects.
My advocacy is simple. We should study the Rubin data with an open mind to the possibility that it may discover technological objects from extraterrestrial civilizations. If we insist that all interstellar objects are asteroids and comets with the outliers catalogued as dark comets, then the answer to the question “Are we alone?” would be “Yes, by choice.” Some of the loneliest people in the world are those who stopped searching for a partner. Their status is a self-fulfilling prophecy. In order to find our cosmic partners, we must allow them to exist in our mind as we inspect the Rubin data.
Surely, interstellar objects were passing overhead in the sky in 1950 when Enrico Fermi asked: “where is everybody?” As an experimental physicist, his oversight was not to build a large telescope to search for them.
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.