Dark Comets or Alien Tech?
An alien civilization which happens to be interested in monitoring the evolution of life in the inner solar system, will not just fly through it once and for all. It would more likely send a mothership that deploys mini-probes into bound orbits around the Sun. This deployment can be achieved by launching the mini-probes with a velocity partly opposite that of the mothership, so that their net speed relative to the Sun would be below the escape speed from the Solar system.
The mini-probes will likely exhibit non-gravitational acceleration without the telltale signature of comets, a cometary tail. Even the largest ground-based telescope as well as the Webb space telescope cannot resolve reasonable probe sizes. Our largest spacecraft, 10 meters in length, would occupy an angular size of order 10 micro-arcseconds at a distance comparable to the Earth-Sun separation. Even if the it emitted millimeter-wavelength radiation from its ends, our state-of-the-art Event Horizon Telescope with an aperture comparable to the entire size of the Earth, would be unable to resolve the probe.
How would astronomers label an unknown population of pointlike technological objects?
The answer was provided in a 2024 paper that reported about two populations of 14 “dark comets” near Earth, namely objects that show no evidence for cometary activity but exhibit non-gravitational accelerations. For technological probes, the use of the term “dark comet” is equivalent to calling an elephant “a stripeless zebra”. Technological probes have nothing in common with comets, but comet experts wish to explain everything in the sky in terms of what they know. As William Shakespeare noted: “a rose by any other name would smell as sweet.” As far as I am concerned: “an alien probe by the name of a `dark comet’ would appear as intriguing”.
This inappropriate labeling by comet experts comes as no surprise. On January 2, 2025, the Minor Planet Center reported about a new near-Earth asteroid. Within a day, it retracted the claim when astronomers realized that this asteroid follows the orbit of the Tesla Roadster car, launched as a dummy payload by SpaceX on February 6, 2018. Recognizing that this object is not a rock but a technologically-manufactured car required knowledge that SpaceX launched such a car. Without that knowledge, as would be the case for an interstellar probe, this car would have been cataloged, even today, as a rocky asteroid by the Minor Planet Center.
On September 17, 2020, the Pan-STARRS telescope in Hawaii discovered the near-Earth object 2020 SO, which showed a non-gravitational acceleration as a result of solar radiation pressure. A few months later, NASA’s Infrared Telescope Facility identified spectral features similar to those of stainless steel and confirmed that 2020 SO is a Centaur upper stage of NASA’s lunar mission Surveyor 2 from 1966. On October 19, 2017, Pan-STARRS discovered the first interstellar object, 1I/`Oumuamua, which also exhibited a non-gravitational acceleration consistent with the effect of solar radiation pressure. Nevertheless, astronomers still regard 1I/`Oumuamua as a dark interstellar comet because its orbital parameters do not match any known space mission from Earth. The only problem with the dark comet interpretation of 1I/`Oumuamua is that deep observations by the Spitzer space telescope did not detect any gas or dust around 1I/`Oumuamua. This emperor had no detectable clothes, yet comet experts argued decisively that the clothes exist but are invisible.
The anomalous trajectory and brightness of the new interstellar object 3I/ATLAS and the absence of a cometary tail behind it raise the question of whether it too might be alien technology. We should have better information about the nature of 3I/ATLAS as it approaches perihelion on October 29, 2025. Nevertheless, one lesson is clear from the above-mentioned examples: the fact that astronomers call something a comet, does not mean that it is a comet.
Is there any evidence for anomalous objects closer to home, in the Earth’s atmosphere? Such objects are classified by the intelligence agencies as Unidentified Anomalous Phenomena (UAP). Whether they relate to alien technology depends on their detailed properties which need to be better documented. The Galileo Project under my leadership is currently constructing three observatories that would monitor the entire sky and potentially detect millions of objects per year over Massachusetts, Pennsylvania and Nevada, in the infrared, visible, radio and audio bands. We aim to measure distances of outliers as UAP candidates, and analyze their properties with state-of-the-art artificial intelligence algorithms.
The public is extremely interested in more data that will test whether 3I/ATLAS is alien technology, for the reasons I outlined here. As 3I/ATLAS gets closer to the Sun, we might know whether it is an interstellar comet like 2I/Borisov or perhaps an alien mothership. At any event, I tasked the research team of the Galileo Project to search for new UAP activity as 3I/ATLAS gets closest to Earth.
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.
