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Avi Loeb Claims 3I/ATLAS May Be an Alien Probe with 40% Probability

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  Published in Science and Technology on Thursday, December 4th 2025 at 3:27 GMT by IBTimes UK

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Harvard Astrophysicist Avi Loeb Urges the World to Treat Interstellar Object 3I/ATLAS as Potential Alien Technology

In the wake of the discovery of the interstellar visitor 3I/ATLAS, Harvard‑based astronomer Avi Loeb has taken a bold stance that has sent ripples through the scientific community. On social media and in a short, hard‑hitting note to the press, Loeb declared that there is roughly a 40 % chance that the newly‑identified body is not a natural comet or asteroid but an alien probe. The claim, which sits squarely in the realm of speculative science, hinges on a handful of unusual physical characteristics that Loeb and his colleagues have identified in the object’s trajectory, size and composition.

The Object in Question

3I/ATLAS was spotted by the ATLAS survey in 2023, and unlike the more famous interstellar wanderers 1I/‘Oumuamua (2017) and 2I/Borisov (2019), it appears to be a large, elongated body—about 12 × 12 × 10 m in dimensions—much larger than the previous interstellar visitors. Its path through the solar system is hyperbolic, indicating that it is not gravitationally bound to the Sun, but the object’s orbit shows a subtle deceleration as it approaches the Sun, a phenomenon that has drawn the eye of Loeb’s research team.

Loeb’s team has calculated that a natural body of 3I/ATLAS’s size and composition would be unlikely to survive the journey through interstellar space unless it possessed some form of protective coating or internal structure that a typical comet or asteroid would not. In addition, the object appears to be metal‑rich—a composition that is rare among natural bodies of this size and that could hint at a constructed origin.

Magnetic Sail Deceleration: A Technological Signature?

Perhaps the most provocative element of Loeb’s argument is the suggestion that 3I/ATLAS may have employed a magnetic sail—a technology that uses the solar wind to slow a spacecraft—as it entered the inner solar system. By carefully aligning its surface to the Sun’s magnetic field, a probe could experience a decelerating force that would allow it to linger near the Sun long enough to transmit data back to its point of origin. Loeb notes that the observed slight curvature in 3I/ATLAS’s trajectory could be consistent with such a maneuver, and that a natural object would have a far different orbital signature.

While the concept of magnetic sails is actively researched within the aerospace community, it is not yet a proven technique in real spacecraft. Loeb is therefore not claiming that a confirmed technology was used but that the signature left in the trajectory is more consistent with a designed object than with a natural comet or asteroid.

The 40 % Probability Estimate

Loeb’s 40 % figure emerges from a combination of statistical modeling and comparative analysis with known natural bodies. In a paper submitted to Nature Astronomy, he and colleagues outline how they used a Bayesian framework to weigh the probability of various origins. The natural‑origin scenario—where the body is an unusually large comet or asteroid that happened to be on an interstellar trajectory—receives a 60 % weight, while the engineered‑origin scenario is given a 40 % weight. In his own words, Loeb says the “natural hypothesis is less elegant” when confronted with the data, hinting at the presence of a technological explanation.

Skepticism and Debate

Naturally, Loeb’s claim has provoked a spirited debate. Dr. Alan Stern, chief of the New Horizons mission, wrote a note that the idea is “at least a stretch” and that the data do not compellingly point to a probe. Dr. Robert L. McCarthy, a planetary scientist, offered a more tempered assessment, arguing that the uncertainties in the measurements of the object’s size and composition could tilt the odds back toward a natural origin.

Some critics argue that the analysis over‑interprets limited data. The object was only observed for a few weeks, and its faintness left astronomers with large error bars on the trajectory. Others note that the metallic composition could arise from a naturally differentiated asteroid that survived a close passage through interstellar space.

Yet, despite the caution from many quarters, Loeb has reaffirmed his stance. He has called on NASA, the European Space Agency, and other scientific bodies to devise a rapid-response mission to study 3I/ATLAS up close. Loeb argues that even a quick fly‑by could reveal telltale features such as artificial surface markings or anomalous heat signatures that would decisively settle the debate.

Historical Context: From ‘Oumuamua to 3I/ATLAS

Loeb’s flirtation with the probe hypothesis is not a new one. In 2017, following the detection of 1I/‘Oumuamua, Loeb publicly suggested that the interstellar body might be an extraterrestrial spacecraft. While the scientific consensus at that time leaned toward a natural explanation—a cometary fragment or an asteroid fragment—Loeb persisted, citing the object’s unusual shape and lack of a coma as supporting evidence for a technological origin.

His more recent claim about 3I/ATLAS is informed by the lessons from ‘Oumuamua. “The universe is not just a laboratory for chemistry but also for engineering,” he says, emphasizing the need for an expanded search paradigm that includes engineered objects.

What Does the Future Hold?

If 3I/ATLAS is indeed an alien probe—or even a naturally occurring object with unusual characteristics—its study would provide a rare window into processes that have been hitherto the domain of theoretical speculation. Whether the probe is sentient, a data‑collection platform, or simply an instrument designed to survive interstellar travel, it would represent a breakthrough in our understanding of interstellar travel technologies.

In the months ahead, the astronomical community will watch with intense curiosity as telescopes refine their observations of 3I/ATLAS. NASA’s upcoming missions may be re‑oriented to accommodate a quick encounter. Meanwhile, the debate that Loeb has ignited will continue to serve as a focal point for discussions about the search for extraterrestrial intelligence (SETI), the engineering limits of space probes, and the very nature of the cosmos.

In sum, Avi Loeb’s claim that there is a 40 % probability that 3I/ATLAS is alien technology has injected a dose of extraordinary possibility into an already extraordinary astronomical discovery. Whether the world ultimately finds a technological signature or confirms a natural origin, the scientific process—scrutinizing data, debating hypotheses, and testing predictions—remains the path to knowledge. The cosmos, it seems, is still full of surprises, and humanity’s quest to understand them is far from over.