3I/ATLAS and the other interstellar visitors: alien probe, or ice and dust?

An interstellar object is a body that formed outside the solar system. It arrives too fast to be captured by the Sun's gravity, swings past on an open path and leaves. Only three have been identified so far; the "I" in the designation stands for interstellar, and the number is its place in line.

It came in at about 58 km per second relative to the Sun, before the Sun's pull sped it up. On October 3, 2025, it passed about 29 million km from Mars; on October 29 it reached perihelion, about 203 million km from the Sun, just outside Mars's orbit; and on December 19 it made its closest approach to Earth, about 270 million km, roughly 1.8 times the Earth-Sun distance. On March 16, 2026, it passed about 54 million km from Jupiter. It never posed any danger.

Its nucleus is small: between 440 meters and 5.6 km according to Hubble, and probably under a kilometer. The James Webb Space Telescope found it giving off far more carbon dioxide than water, an unusual ratio for a comet, and the Very Large Telescope detected nickel around it. ESA's Juice spacecraft measured about 2,000 kg of water vapor per second in November 2025 and a cloud of gas and dust stretching more than 5 million km. Age estimates range from 3 to 14 billion years, and some put it older than the Sun.

The research continues. In late October 2026 a team led by Silvia Protopapa of the Southwest Research Institute published three sets of Webb observations: the ice grains in the comet's cloud are mixed with a lot of dust and resemble those of some icy bodies beyond Neptune. Large telescopes are still tracking it, but it is far too faint for any backyard telescope.

Avi Loeb, an Israeli-born Harvard astrophysicist, is the best-known scientist arguing that a technological origin deserves a look. In 2018 he and Shmuel Bialy wrote that 'Oumuamua's push could come from sunlight pressing on a very thin sheet, under a millimeter thick, like an artificial light sail. In 2021 he founded the Galileo Project to look for evidence of alien technology. More on Loeb and his claims.

On 3I/ATLAS he posted a paper on July 16, 2025, with Adam Hibberd and Richard Crowl, asking whether it might be alien technology, "possibly hostile," along the lines of the dark forest idea. The authors themselves called it largely a pedagogical exercise. They noted that its path lies close to the plane of the planets, that it passes improbably close to Venus, Mars and Jupiter, and that at perihelion it was hidden from Earth behind the Sun, where it could have maneuvered unseen. Loeb later added more "anomalies" on his blog, such as a sunward-pointing tail, nickel without iron, and a Jupiter flyby distance matching the planet's gravitational sphere of influence. On his own 0 (certainly natural) to 10 (certainly technological) scale, he rated it 4 at most.

After the Earth flyby he softened: "At this point, given all the data that we have, I would agree that it's most likely natural," he said in December 2025, adding that it "did not maneuver or display any unusual activity" at its closest approach to us.

Most astronomers rejected the idea from the start. Oxford's Chris Lintott criticized it immediately, and Darryl Seligman, who led the first study of the comet, said the observations showed ordinary cometary activity. Astronomer Samantha Lawler said the evidence in the paper "is absolutely not extraordinary." ESA says the object's "shape and behaviour indicate that it is a comet." At a NASA briefing on November 19, 2025, delayed by the US government shutdown, the agency showed images from its spacecraft at Mars, and associate administrator Amit Kshatriya said plainly: "3I/ATLAS is a comet."

'Oumuamua got a natural explanation too. An international team that reviewed all the data concluded in 2019 that no artificial explanation was needed, and in 2023 Jennifer Bergner and Darryl Seligman showed in Nature that hydrogen released from ice irradiated over millions of years could produce the push without a visible cloud.

And if anyone aboard were transmitting, we would know. The Green Bank Telescope heard nothing from 'Oumuamua at a sensitivity that would have caught a 0.08-watt transmitter. 3I/ATLAS was scanned by the SETI Institute's Allen Telescope Array, South Africa's MeerKAT and the Green Bank Telescope, which on December 18, 2025, was sensitive to a transmitter of only about 0.1 watt. No artificial signal was found. Every alien-signal candidate so far.

Astronomers don't rule out an artificial object. Under the definition the International Academy of Astronautics adopted in June 2026, a technological artifact is one of the kinds of signs SETI looks for, and if one were found, the verification and announcement rules of the first contact protocol would apply. These are the signs that would raise real suspicion:

All three visitors were found by survey telescopes hunting for hazardous asteroids, and the International Astronomical Union's Minor Planet Center assigns their names and numbers. The odds of new finds just jumped: the Vera C. Rubin Observatory in Chile began a ten-year survey on June 30, 2026, imaging the entire southern sky every few nights. Estimates of how many interstellar objects it will find range from almost none to about 70 a year, according to a 2023 study by Dušan Marčeta and Darryl Seligman.

The next one will be called 4I, and eventually something will be waiting for it in space: ESA plans to launch Comet Interceptor in late 2028 or early 2029 to wait at the Sun-Earth L2 point until a suitable comet, possibly a visitor from another star, comes along. When the next one is found there will be headlines about a "spaceship heading for Earth." The questions to ask: does it have a gas cloud? Has any signal been detected? And is its path explained by gravity and outgassing?