Why Avi Loeb Thinks 3I/ATLAS is More Than Just a Comet

On July 1, 2025, the Asteroid Terrestrial-Impact Last Alert System telescope in Río Hurtado, Chile, noticed something strange streaking across the sky. Most astronomers knew what it was: a comet. It was the kind of self-assured, instinctive diagnosis that results from years of pattern recognition. However, while seated in his Harvard office, Avi Loeb examined the same object—now known as 3I/ATLAS—and came to a different conclusion. Not quite a definitive conclusion. It was more of a demand that the question itself be kept open.

Depending on who you ask, this insistence has made him either the most persistently problematic or the most intellectually courageous astronomer working today. He has previously worn this reputation. When Oumuamua, the first known interstellar visitor, traveled through the solar system in 2017 without a visible cometary tail and with a non-gravitational acceleration that no one could adequately explain, Loeb speculated that it might be a remnant of alien technology—possibly a light sail from a far-off civilization. The idea was mainly rejected by the scientific community. However, the public was infatuated with it.

A similar script has been used by 3I/ATLAS, but it is louder and involves greater stakes. The massive object, which could have a diameter of 20 kilometers if it were a solid body, is moving at about 130,000 miles per hour, making it the fastest interstellar visitor to ever enter our solar system. On December 19, 2025, it made its closest approach to Earth. In the weeks preceding that flyby, the discussion about its true nature had evolved into something more akin to a cultural phenomenon. Loeb made an appearance on the podcast of Joe Rogan. His posts on Medium received thousands of likes. His co-authored paper, which presented the item as a potential technological artifact, went viral and infuriated the very people it was intended to provoke.

The anomalies, which Loeb refers to as thirteen and eventually fourteen, are what he keeps coming back to. The object had no cometary tail, at least in the first few weeks following its discovery. With only a five-degree tilt from the ecliptic, its trajectory is precisely aligned with the planets’ orbital plane—an unusual alignment for something coming from deep interstellar space. Its plume was found to contain high concentrations of the industrial element nickel. The anti-tail, a characteristic that ought to point away from the sun, instead pointed straight toward it. This orientation was so precise and consistent that Loeb estimated the likelihood of it happening naturally to be about 0.5%. He finds it difficult to reconcile a simple pocket of evaporating ice with a sunward jet that is tightly collimated and ten times longer than it is wide.

Each of these facts is not a smoking gun in and of itself, according to Loeb. However, he contends that when taken as a whole, they add up to something that calls for careful consideration rather than a hasty rejection. “You might argue it’s an animal when you see a cloud of dust in the desert because animals, when they run in the desert, make a cloud of dust,” he said to reporters in December. However, it might also be an automobile. He assigned 3I/ATLAS a “4” on what he refers to as the Loeb Scale of interstellar technological threats, a scale he created that goes from zero to ten. This indicates that there is about a 40% chance the object is not natural. It’s a strange thing to say aloud. Saying it on tape is even less common.

The general public’s reaction has been measured and occasionally blatantly doubtful. The object “looks like a comet,” does “comet things,” and “very, very strongly resembles” what NASA knows about solar system comets, according to Tom Statler, the agency’s lead scientist for solar system small bodies. The non-gravitational acceleration of 3I/ATLAS, the rocket-like propulsion Loeb identified as potentially technological, was found to be consistent with the slight push comets naturally receive from outgassing near the sun, according to a new study published in Research Notes of the AAS in December. “3I/ATLAS is exotic and wonderful,” was the direct conclusion of the paper. It’s a comet, too. That study was deemed superficial by Loeb. He claimed that by focusing only on how the object behaves like something familiar, it neglected to address the thirteen anomalies. “What they need to do,” he stated, “is explain the anomalies.”

It’s important to note that Loeb is not asserting that 3I/ATLAS is unquestionably alien, something that occasionally gets overlooked in the commotion surrounding him. He asserts that the question is worthwhile and that it might be. Even though it doesn’t always translate into headlines, there is a significant difference. In a blog post, he admitted that “the simplest hypothesis is that 3I/ATLAS is a comet.” He clearly presents his alternative interpretation as a challenge to preconceived notions rather than a definitive conclusion. However, detractors contend—with some merit—that the hedging accomplishes two goals: it preserves scientific deniability while creating enough interest to remain relevant in a crowded media environment. Reddit users have noted that setting the likelihood of alien origin at 40% is precise enough to sound scientific but ambiguous enough to be hard to refute in the near future.

Nevertheless, Loeb’s framing of the larger stakes has some merit. He has maintained that humanity’s propensity to believe that all interstellar objects are rocks is comparable to an archaeologist believing that all dirt contains only dirt, and that if this assumption is incorrect, it may result in the discovery of something remarkable. In late 2025, he told a group of wealthy investors that access to extraterrestrial technology could shorten centuries of development into decades. He has also been open about the financial realities of fringe-adjacent research. It’s an argument that is equally entrepreneurial and scientific. Around the time of 3I/ATLAS’s closest approach, he hired four new postdoctoral fellows at the Galileo Project. According to him, each of them was eager to work at the cutting edge of what is deemed acceptable inquiry.

Observing all of this, it seems as though Loeb holds a peculiar and genuinely uncomfortable place in contemporary science. Despite having more than a thousand published papers and a Harvard professorship, he uses his credentials to promote ideas that most of his peers consider to be excessive. Because the comet hypothesis fits the available data fairly well and extraordinary claims require extraordinary evidence, the scientific community tends to close ranks around it, not necessarily out of narrow-mindedness. Loeb’s evidence isn’t particularly compelling. It’s a group of anomalies that, when considered collectively, may have significance or, if better instruments are used for a longer period of time, may dissolve into perfectly natural explanations.

Since then, 3I/ATLAS has shifted away from Earth and returned to interstellar space. The time frame for studying is getting smaller. It is genuinely unclear if the anomalies Loeb listed will eventually be explained away or if they will build up into something more peculiar and obstinate. The fundamental question he keeps posing is more difficult to ignore: is it the proper scientific instinct to go straight to the most well-known explanation when something appears from outside the solar system, traveling faster than anything we’ve ever seen, and having characteristics we don’t fully understand? Or is it to actually look while sitting with the uncertainty for a bit?

Regardless of one’s opinion of him, Avi Loeb consistently selects the latter. That may be his strongest argument to date.