Harvard astronomer Avi Loeb has argued at length that 3I/ATLAS (the third interstellar object ever detected, currently hurtling through the solar system at incredible speed) could be a massive extraterrestrial spacecraft measuring kilometers in diameter.
While there is broad consensus in the scientific community that we are looking at a comet composed largely of carbon dioxide ice, Loeb maintains that the object has many unusual properties that make him suspect that it is something even more exotic.
On his blog, Loeb describes nine “anomalies” that he says support his surprising hypothesis that we could be looking at an “extraterrestrial mothership” that could be releasing “minidrones” as it passes behind the Sun, despite admitting that the most likely explanation is that 3I/ATLAS is simply a “naturally occurring comet.”
1. Its trajectory is closely aligned with the planets of the solar system.
Loeb points out that the trajectory of the mysterious visitor is only five degrees from the Earth’s path around the Sun, or the plane of the ecliptic. He maintains that there is only a 0.2 percent chance of this happening.
2. He visited several planets
In addition to its near-alignment with the ecliptic plane, 3I/ATLAS’ arrival time takes it on a course that passes just ahead of Mars and Jupiter (both worlds that intrigue our own scientists in the search for life beyond Earth) in what Loeb calls a “remarkable adjustment of the object’s trajectory.”
Earlier this month, the object passed by Mars close enough that two European Space Agency spacecraft orbiting the Red Planet could take pictures of it. It is expected to come within just 33.3 million miles of Jupiter in March 2026, which could allow NASA’s Juno spacecraft to intercept its trajectory as well, Loeb maintains.
3. He grew an anti-tail
Astronomers have also observed that 3I/ATLAS is growing a second tail, which appears to point in the direction of the Sun.
Many other comets have been observed to have developed an “antitail,” an optical illusion resulting from our relative positioning as it passes between the Earth and the Sun. However, Loeb maintains that 3I/ATLAS’s second tail is an outlier.
“This anomalous antitail, which is not the result of geometric perspective, has never before been reported in solar system comets,” Loeb wrote.
“The ice fragments evaporate after some time, but because of the greater mass loss on the side facing the Sun, a greater number of larger fragments can reach a great distance,” Loeb said. futurism in an email earlier this month.
4. It looks huge
According to Loeb’s calculations, the “diameter of its solid density core must be greater than (3.1 miles),” a measurement he deduced from an estimated mass of more than “33 billion tons.”
That would make it “three to five orders of magnitude” more massive than ‘Oumuamua and 2I/Borisov, the only two other interstellar objects that have been observed passing through our solar system, raising a confusing statistical question: Why haven’t we observed much smaller interstellar objects?
“Given the limited stock of heavy elements, we should have discovered on the order of a hundred thousand interstellar objects on the 0.1 kilometer scale of 1I/’Oumuamua before finding 3I/ATLAS, yet we only detected two interstellar objects previously,” he argued in a blog post in September.
5. Their nickel-to-iron ratios are off the charts.
Scientists have found that 3I/ATLAS shows an “extreme abundance ratio” of nickel and iron in its gas column, making it a major outlier compared to 2I/Borisov and other more familiar comets in the solar system.
“At the distances at which comets are observed, the temperature is too low to vaporize silicates, sulfides, and metal grains containing nickel and iron atoms,” wrote an international team of astronomers in a paper yet to be peer-reviewed. “Therefore, the presence of nickel and iron atoms in the cometary coma is extremely puzzling.”
Loeb maintains that the findings could indicate the presence of “industrially produced nickel alloys.”
6. Its column is mainly carbon dioxide ice
Researchers have concluded, by examining data from the Spectrophotometer for the History of the Universe, Reionization Epoch and Ice Explorer (SPHEREx), that 3I/ATLAS’s coma surrounding its nucleus appears to be just four percent water by mass, with a much higher proportion of carbon dioxide than other comets.
While Loeb claims this makes the object an outlier, scientists have argued otherwise.
“SPHEREx’s finding of very large amounts of vaporized carbon dioxide around 3I/ATLAS told us that it could be like a normal solar system comet,” said Carey Lisse, an astronomer at Johns Hopkins University. space.com last month.
In a separate preprint paper, an international team of researchers suggested that the object may contain “ices exposed to higher levels of radiation than comets in the Solar System,” or could have “formed near the CO2 ice line in its main protoplanetary disk.”
7. He is extremely negatively polarized
The mysterious object has also shown “extreme negative polarization,” as detailed in a September paper, making it a major outlier.
“The combination of a low flip angle and extreme negative polarization is unprecedented among comets and asteroids, making 3I/ATLAS the first known object with such polarimetric behavior and representing a previously unobserved population,” Loeb wrote in a blog post.
The findings suggest it has more in common with trans-Neptunian objects, minor planets and other smaller objects orbiting the Sun beyond Neptune’s orbit, according to the authors of the September research paper.
8. It could be behind the famous “WOW!” sign.
Loeb has gone so far as to suggest that 3I/ATLAS may have emitted the “Wow!” signal, a highly unusual narrowband radio signal that was detected by Ohio State University’s Big Ear radio telescope in 1977. The signal generated widespread excitement about the possibility that it came from an extraterrestrial entity, and has remained a mystery for decades.
Loeb maintains that “3I/ATLAS arrived from a direction coincident with the ‘Wow!’ radio signal. with a margin of error of nine degrees, with a probability of 0.6 percent.”
9. It is much bluer than the sun.
Initially, Loeb only listed eight reasons why he was suspicious of the true nature of 3I/ATLAS. But this week, as 3I/ATLAS approaches its closest point to the Sun, or its perihelion, scientists observed a “rapid increase” in its brightness, appearing “clearly bluer than the Sun,” Loeb wrote in a new blog post.
For the astronomer, it is a “very surprising” discovery. “The dust is expected to redden scattered sunlight, and the surface of the object is expected to be an order of magnitude cooler than the 5,800 degrees Kelvin in the Sun’s photosphere, which will give it a redder color than the Sun.”
“Therefore, we must add the blue color at perihelion as a ninth anomaly to the list of unexpected properties… of this strange interstellar object,” he wrote. “Does it use an energy source hotter than the Sun?”
If it smells like a comet…
It remains debatable whether these nine “anomalies” described by Loeb add up to form a sufficiently convincing argument that 3I/ATLAS is “technological” in nature.
Tom Statler, NASA’s senior scientist for small bodies in the solar system, said the guardian last month that there is a lot of evidence that we are simply observing a rare visitor in the form of a natural comet from another star system.
“It looks like a comet,” he said. “It does things related to comets. It looks very similar, in almost every way, to the comets we know.”
“It’s a comet,” Statler concluded at the time.
Meanwhile, Loeb argued on his blog that “we need to collect as much data as possible to discover the nature of this anomalous object.”
“The implication of extraterrestrial technology would be enormous and therefore we must take this possibility seriously,” he added. “Our largest rocket, (SpaceX’s) Starship, is a hundred times smaller than 3I/ATLAS, so if 3I/ATLAS were technological, its senders would have mastered capabilities far beyond our technologies.”
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