Is 3I/ATLAS an alien spacecraft? What astronomers have found

What is 3I/ATLAS?

On 1 July 2025 the ATLAS survey telescope at Río Hurtado in Chile, part of a NASA-funded network that sweeps the sky for asteroids, picked up a faint moving object in four 30-second exposures and reported it to the IAU Minor Planet Center. The center's electronic circular of 2 July gave it two names: the comet designation C/2025 N1 (ATLAS), and 3I/ATLAS, where the I stands for interstellar and the 3 means it was the third such object on record. Its orbit is strongly hyperbolic, with an eccentricity of about 6.3, so it was never bound to the Sun. Once astronomers knew where to look, older survey images showed it as far back as 14 June 2025.

Only two interstellar visitors had been seen before: 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. ʻOumuamua showed no coma at all and set off a long argument about whether it could be artificial, which Real or Legend covers in its own file on ʻOumuamua. 3I/ATLAS arrived much faster. NASA puts its speed at roughly 221,000 km/h when it was found and about 246,000 km/h at its closest to the Sun. ESA says a speed like that suggests the comet had been drifting between the stars for a very long time, possibly billions of years.

3I/ATLAS photos: what the telescopes and spacecraft saw

The witnesses here are instruments. Hubble imaged it on 21 July 2025 and showed a cocoon of dust around a small nucleus; from those data NASA estimated the nucleus at somewhere between 440 metres and 5.6 km across. The Webb telescope took its spectrum on 6 August, and NASA's SPHEREx survey telescope watched it from 7 to 15 August.

The best views came from Mars. The comet passed within about 29 million km of the planet on 3 October 2025. A day earlier NASA's Mars Reconnaissance Orbiter photographed it with its HiRISE camera, showing the fuzzy coma around the nucleus, and on 4 October the Perseverance rover caught it as a faint smudge against the stars. ESA's ExoMars Trace Gas Orbiter and Mars Express observed it too, and the orbiter's data sharpened predictions of where the comet would be. In November ESA's Juice spacecraft, on its way to Jupiter, recorded the comet with a clear tail.

Then it disappeared from Earth's view. From the end of September 2025 it sat too close to the Sun in the sky for ground telescopes. It passed perihelion on 29 October 2025, about 1.4 times Earth's distance from the Sun and just inside the orbit of Mars, and reappeared in early December. Its nearest point to Earth, about 270 million km, came on 19 December 2025. NASA states that it posed no threat.

The 3I/ATLAS alien spacecraft idea and Avi Loeb

The alien-technology suggestion came mainly from Avi Loeb, an astrophysicist in Harvard University's astronomy department. On 16 July 2025, about two weeks after discovery, he and two co-authors from the Initiative for Interstellar Studies, Adam Hibberd and Adam Crowl, posted a preprint titled "Is the Interstellar Object 3I/ATLAS Alien Technology?" on the arXiv server, before any peer review. The arXiv record now lists it as published in 2025 in the first issue of the International Journal of Aerodynamic Control & Avionic Mechanics.

The paper rests on orbital coincidences. Its authors noted that the comet's path is tilted only slightly from the plane the planets share, although it travels around the Sun in the opposite direction; that it passes unusually close to Venus, Mars and Jupiter; and that the Sun would hide it from Earth around perihelion. From these they built a hypothesis that it could be technological, and even hostile, in line with the so-called Dark Forest answer to the Fermi paradox. The same paper describes itself as largely a pedagogical exercise, and its conclusion states that by far the most likely outcome is a completely natural object, probably a comet.

The official explanation: 3I/ATLAS is a comet

NASA set out its position at a briefing at the Goddard Space Flight Center on 19 November 2025, where it released images from its missions. "It looks and behaves like a comet," said Associate Administrator Amit Kshatriya, adding that all the evidence pointed that way. Nicky Fox, who heads NASA's Science Mission Directorate, said the agency had seen no technosignatures, the term scientists use for signs of technology. ESA's own explainer describes 3I/ATLAS as an icy body shedding dust and gas whose shape and behaviour mark it as a comet.

The chemistry points the same way. According to ESA, Webb found carbon dioxide, water, carbon monoxide, carbonyl sulphide and water ice in late August 2025, and NASA says SPHEREx saw dust, water, organic molecules and carbon dioxide in the coma. What stood out was the balance: Tom Statler, NASA's lead scientist for small bodies, said at the briefing that it was giving off more carbon dioxide than water. NASA says the comet's characteristics are all consistent with a comet, and presents differences like this as clues to the chemistry of the star system it formed in.

Where scientists stand as of October 2026

The most direct test of the spacecraft idea was a listening one. A Breakthrough Listen team led by Benjamin Jacobson-Bell of the University of California, Berkeley, pointed the 100-metre Green Bank Telescope at the comet to search for narrowband radio signals. The search could have detected a transmitter as weak as about 0.1 watts, roughly a tenth of a mobile phone. It flagged nine candidate signals, and every one was traced to transmitters on Earth. "In the end, there were no surprises," Jacobson-Bell told Space.com in January 2026. The team's paper was posted as a preprint.

As of October 2026, NASA and ESA both still describe 3I/ATLAS as a natural comet, and no published observation has shown a technological signal. The comet is outbound and will not return. What remains is the genuine 3I/ATLAS mystery for comet scientists: how a body this old, and this rich in carbon dioxide, formed around another star.

Verdict

3I/ATLAS explained in a line: a comet from another star system, found on 1 July 2025, closest to the Sun on 29 October 2025, and now leaving for good. The alien spacecraft claim rested on orbital coincidences in a paper whose own authors judged a natural comet the likeliest outcome; every instrument that looked saw a comet, and the radio search heard nothing. Real or Legend stamps the alien-spacecraft claim debunked.