Avi Loeb's Interstellar Probes: 'Oumuamua, the Ocean Spherules and 3I/ATLAS

October 19, 2017. The Pan-STARRS1 telescope, run by the University of Hawaii, spots something no one has ever confirmed before: an object from another star, passing through our solar system.

It gets the name 'Oumuamua. NASA describes it as cigar-shaped, reddish, up to about 400 meters long and roughly ten times longer than it is wide.

Then it does something odd. As it heads away from the Sun, it speeds up slightly, more than gravity alone can explain. And no one sees the tail of gas and dust that would normally make a comet behave that way.

A year later, a Harvard astrophysicist named Avi Loeb put an extraordinary idea into a peer-reviewed journal. Maybe 'Oumuamua was built.

The light-sail paper

Give Loeb credit for the logic. In June 2018, a team led by Marco Micheli reported in Nature a strong detection of non-gravitational acceleration in 'Oumuamua's motion. They favored weak outgassing, the way comets push themselves, as a viable explanation.

But no outgassing was seen. So in November 2018, Bialy and Loeb asked in The Astrophysical Journal Letters whether solar radiation pressure, the gentle push of sunlight, could do the job instead.

It could, they calculated, if the object were extremely thin, about 0.3 to 0.9 millimeters thick for a flat sheet. Then they went further. "Considering an artificial origin, one possibility is that 'Oumuamua is a lightsail," they wrote, perhaps drifting as debris from advanced technology.

They added an even bolder line: "a more exotic scenario is that 'Oumuamua may be a fully operational probe" sent toward Earth on purpose.

Here's the fair reading. The paper didn't claim 'Oumuamua was alien. It showed a thin-sail model fits one puzzling number, and it said so out loud. That's speculation, clearly labeled, sitting next to a real calculation.

The natural explanations pile up

Mainstream astronomy took the puzzle seriously and went looking for natural answers.

In 2019, an international team convened at the International Space Science Institute in Bern reviewed everything known about 'Oumuamua. Their Nature Astronomy paper concluded the observations are consistent with a purely natural origin.

Then came the ice proposals. In 2020, Darryl Seligman and Gregory Laughlin suggested 'Oumuamua was made of molecular hydrogen ice. Its invisible vapor could have provided the push without a visible coma. Loeb answered with Thiem Hoang that same year, arguing in ApJ Letters that starlight would destroy such an iceberg in under 10 million years.

In 2021, Alan Jackson and Steven Desch of Arizona State proposed nitrogen ice, a chip knocked off the surface of a Pluto-like planet in another system. In their analysis, nitrogen ice best fit 'Oumuamua's size, brightness and acceleration.

And in 2023, Jennifer Bergner and Seligman offered a version in Nature that needs nothing exotic. Cosmic rays split water in an icy body, and hydrogen gets trapped in the ice. When the object warms near the Sun, that hydrogen escapes, producing a small push without a dusty tail.

None of these is proven. But each is a known physical process. That's the core of the consensus: weird isn't the same as artificial.

Into the Pacific

Loeb didn't stop at 'Oumuamua. In 2019, he and Harvard student Amir Siraj flagged a fireball that blazed over the Pacific off Papua New Guinea on January 8, 2014. Government sensors put its speed at about 45 kilometers per second. They argued that was fast enough to mean it came from outside the solar system.

In 2022, a memo from U.S. Space Command, signed by Lt. Gen. John Shaw, said the velocity estimate was accurate enough to indicate an interstellar trajectory. NASA's planetary defense officer, Lindley Johnson, was more cautious: "Quite frankly, we can't confirm that it's interstellar."

In June 2023, Loeb led an expedition that towed magnets across the seafloor near the fireball's estimated path. His team reported recovering hundreds of tiny metallic spherules, about 850 in a later count. A small subset showed what they called a "BeLaU" pattern, with excesses of beryllium, lanthanum and uranium. The team argued that pattern supports an interstellar origin and might come from a differentiated planet or "more exotic sources."

"The implication of alien technology would be huge and therefore we must take it seriously."

The critics hit back

The IM1 claims drew fire from several directions.

Astronomer Peter Brown and Jiří Borovička argued in The Astrophysical Journal in 2023 that the government velocity data carry large uncertainties at high speeds, and that the fireball's speed was likely substantially overestimated. If so, the interstellar case collapses.

Desch and Jackson made a blunt physical point in a research note: if interstellar, practically none of the object would have survived entry.

Patricio Gallardo compared the spherules' nickel, beryllium, lanthanum and uranium with a public coal database. In Research Notes of the AAS, he found them consistent with coal ash, a common human-made contaminant. Loeb's team countered that the BeLaU pattern differs from coal fly ash.

And in March 2024, Johns Hopkins seismologist Benjamin Fernando presented work suggesting the Manus Island seismic signal used to refine the search area was likely a passing truck. "The signal changed directions over time, exactly matching a road that runs past the seismometer," he told Astronomy magazine. His team estimated the fireball probably fell about 100 miles from where Loeb's team searched. Loeb replied that the search zone was mainly set by military satellite data.

The Galileo Project and 3I/ATLAS

Loeb's response to skeptics has been, fairly, to collect more data. In July 2021 he launched the Galileo Project, with nearly $2 million in private funding, to build AI-assisted telescopes that watch the sky for UAP and study 'Oumuamua-like objects. Sites now include Harvard, Pennsylvania and Las Vegas, according to The Harvard Crimson.

On July 1, 2025, the NASA-funded ATLAS survey telescope in Rio Hurtado, Chile, reported a third interstellar object: 3I/ATLAS. Loeb publicly floated a technological origin. Before its closest approach to the Sun, around October 29 to 30, he called it most likely a natural comet but gave it a 4 out of 10 on his own "Loeb Scale," citing anomalies. He said a manufactured object "might maneuver or release mini-probes."

NASA calls 3I/ATLAS a comet. It passed no closer than about 1.8 astronomical units from Earth, and NASA says it posed no threat.

What we know, and what we don't

What we know

• 'Oumuamua showed a small non-gravitational acceleration, and Bialy and Loeb proposed solar radiation pressure on a thin sail as one explanation.
• Peer-reviewed natural models, including hydrogen ice, nitrogen ice and trapped hydrogen in water ice, have been proposed to explain it.
• IM1's interstellar status, its seismic location and the origin of the recovered spherules are all disputed in the literature.

What we don't

• 'Oumuamua's true composition. It's gone, and no future observation can check it.
• Whether IM1 was really interstellar, given the uncertainties in classified sensor data.
• Whether any interstellar object will ever show a feature no natural process can explain.

The real test is coming

Here's what makes Loeb's program worth watching, even for skeptics. It's falsifiable. A real alien probe would eventually show something comets can't fake: a maneuver, a non-natural spectrum, a structure in a close-up image.

Three interstellar visitors have now been confirmed, and each new one is another chance to look. Every one that behaves like an ordinary comet weakens the probe hypothesis a little more.

So where do you land: is Avi Loeb a needed gadfly pushing science to look harder, or is he crying wolf? Tell us in the comments.

Sources

• 'Oumuamua — NASA Science
• Non-gravitational acceleration in the trajectory of 1I/2017 U1 ('Oumuamua) — Micheli et al., Nature 559, 2018
• Could Solar Radiation Pressure Explain 'Oumuamua's Peculiar Acceleration? — Shmuel Bialy and Abraham Loeb, The Astrophysical Journal Letters, 2018, doi:10.3847/2041-8213/aaeda8
• The natural history of 'Oumuamua — 'Oumuamua ISSI Team, Nature Astronomy, 2019
• Natural origin for 'Oumuamua — University of Hawaii, 2019
• Evidence that 1I/2017 U1 ('Oumuamua) was Composed of Molecular Hydrogen Ice — Darryl Seligman and Gregory Laughlin, The Astrophysical Journal Letters, 2020
• Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 ('Oumuamua) — Thiem Hoang and Abraham Loeb, The Astrophysical Journal Letters, 2020
• 1I/'Oumuamua as an N2 Ice Fragment of an exo-Pluto Surface: I. Size and Compositional Constraints — Alan P. Jackson and Steven J. Desch, Journal of Geophysical Research: Planets, 2021
• 'Oumuamua may be an icy fragment of a Pluto-like exoplanet — Eos, on Jackson and Desch, Journal of Geophysical Research: Planets, 2021
• Scientists think they know why interstellar object 'Oumuamua moved so strangely — NPR, on Bergner and Seligman, Nature, 2023
• Memo from U.S. Space Command confirms Harvard scientists' findings — Harvard Gazette, 2022
• U.S. officials confirm first known interstellar meteor struck Earth in 2014 — NBC News, 2022
• Discovery of Spherules of Likely Extrasolar Composition in the Pacific Ocean Site of the CNEOS 2014-01-08 (IM1) Bolide — Loeb et al., arXiv, 2023
• Recovery and Classification of Spherules from the Pacific Ocean Site of the CNEOS 2014 January 8 (IM1) Bolide — Loeb et al., Research Notes of the AAS 8, 2024
• On the Proposed Interstellar Origin of the USG 20140108 Fireball — Peter G. Brown and Jiří Borovička, The Astrophysical Journal 953, 2023
• Anthropogenic Coal Ash as a Contaminant in a Micro-meteoritic Underwater Search — Patricio A. Gallardo, Research Notes of the AAS 7, 2023
• Is industrial pollution, not alien technology, behind these spheres? — Interesting Engineering, 2023
• The rumblings that led to the 'alien spheres' probably came from a truck — Astronomy, 2024
• Was It an Alien Spacecraft—Or a Delivery Truck? — Lamont-Doherty Earth Observatory, 2024
• Avi Loeb's Galileo Project Will Search for Evidence of Alien Visitation — Scientific American, 2021
• Harvard Astrophysicist Puts Observatory on Top of the Las Vegas Sphere to Search for UAPs — The Harvard Crimson, 2025
• 3I/ATLAS — NASA Science
• Is Comet 3I/ATLAS Really an Alien Spacecraft? We're About To Find Out — Newsweek, 2025