The mystery surrounding the interstellar object 3I/ATLAS has intensified following a bold assertion by Avi Loeb, a Harvard astrophysicist, who proposed that this cosmic traveler might be composed of antimatter. He described antimatter as “the most efficient fuel” in existence, suggesting that if 3I/ATLAS indeed contains this substance, it could produce detectable gamma-ray or X-ray emissions as it moves through the solar wind. These emissions might be observable by NASA“s Fermi Gamma-Ray Space Telescope.
In a recent article published on Medium, Loeb emphasized that while his theory is primarily educational, it has reignited discussions about the potential nature of this interstellar visitor, which is officially designated C/2025 N1. Discovered on July 1 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Hawaii, 3I/ATLAS is only the third confirmed interstellar object to enter our Solar System, following the discoveries of ʻOumuamua in 2017 and 2I/Borisov in 2019.
According to Sky & Telescope, the orbit of 3I/ATLAS is tilted at 175° to the ecliptic, placing it within approximately 5° of the plane inhabited by the Solar System”s planets. Observations reported by The Economist indicated that the object appeared “bluer than the Sun” as it approached its closest point to the Sun, a phenomenon Loeb interpreted as potential evidence of internal energy generation rather than simple sunlight reflection.
Loeb also pointed out that 3I/ATLAS exhibited non-gravitational acceleration, hinting at an additional force acting on the object. In an interview referenced by Newsweek, he suggested this could indicate a propulsion mechanism or an energy source beyond sunlight, although he refrained from asserting that it was evidence of extraterrestrial technology.
In his Medium essay, Loeb argued that if 3I/ATLAS were comprised of antimatter, the annihilation of this material with solar particles could account for its acceleration and distinctive glow. He noted that the combination of matter and antimatter would yield unparalleled fuel efficiency, proclaiming that “complete mass-to-energy conversion is unmatched by any chemical or nuclear reaction.” However, he acknowledged that antimatter is both expensive and unstable, estimating that producing just one gram of antimatter on Earth would cost around five quadrillion dollars, highlighting its extreme rarity.
Despite the speculative nature of this idea, Loeb believes it is “testable” through future observations, especially by looking for high-energy emissions or unusual spectral signatures as the object continues its journey through the inner Solar System. Many mainstream scientists have expressed skepticism. Dr. Samantha Lawler, an astrophysicist at the University of Regina in Canada, remarked that “extraordinary claims require extraordinary evidence,” and she found the evidence presented by Loeb to be lacking in extraordinary merit.
In comments reported by The Guardian, NASA stated that 3I/ATLAS “behaves like a natural comet in all observable ways,” indicating no credible data that would suggest an artificial or exotic composition. Additionally, astronomers have noted that the non-gravitational accelerations observed are common among comets when they are heated by sunlight, creating gas jets that can slightly alter their trajectories. Cosmologists have also asserted that significant amounts of antimatter in the universe are unlikely, a view supported by gamma-ray observations published on arXiv. Loeb himself acknowledged that large bodies of antimatter should have been annihilated shortly after the Big Bang.
Nonetheless, scientists agree that 3I/ATLAS merits ongoing observation. Research teams worldwide plan to monitor its brightness, tail development, and trajectory to ascertain whether the anomalies noted by Loeb persist or can be reconciled with established physical principles. In a subsequent post titled “Should We Be Happier if 3I/ATLAS Is a Comet?”, Loeb suggested that even if the object is purely natural, it would still be a cause for celebration, as nature continues to astonish us without needing to invoke aliens or antimatter. As 3I/ATLAS speeds away from the Sun, the discussion it has sparked highlights the vast unknowns regarding interstellar materials traversing our cosmic neighborhood and the lengths to which astronomers will go in pursuit of understanding.
