🔭 Interstellar object 3I/ATLAS spotted before its official discovery

A curious coincidence marks the beginning of the story of the interstellar comet 3I/ATLAS: it was photographed by an observatory in its testing phase more than a week before its official detection.

On July 1, 2025, the ATLAS network announced the discovery of 3I/ATLAS. But ten days earlier, the Vera C. Rubin Observatory in Chile had already pointed its 28-foot (8.4-meter) mirror at the sky. That day, it was starting its science validation phase.


A team led by Colin Orion Chandler from the University of Washington sifted through Rubin's calibration images. And there, surprise: on June 20, the comet was already there, clearly visible. At the time, the data processing tools were not yet operational. So the researchers had to create their own.

Between June 21 and July 2, Rubin captured 3I/ATLAS nine more times. The images show a well-developed coma, proof that the comet was already active. If the data processing tools had been ready, this early detection might have allowed it to be announced before ATLAS.

Two probes bound for Jupiter then joined forces to observe the comet. ESA's JUICE and NASA's Europa Clipper, both equipped with the ultraviolet spectrograph UVS from the Southwest Research Institute, conducted joint observations in late 2025. The comet's passage between the two spacecraft allowed complementary views.

JUICE observed the day side of 3I/ATLAS, while Europa Clipper scanned its night side. This rare configuration allowed the detection of the same gas emissions from two different angles. Scientists thus measured hydrogen, oxygen, and carbon, produced by the fragmentation of gas molecules by the Sun's ultraviolet light.


The abundance of carbon turned out to be higher than in comets from our solar system. This confirms observations from the James Webb Space Telescope, which had already detected an excess of carbon dioxide. By comparing the proportions of water ice and carbon dioxide ice, researchers hope to better understand the comet's home star system.

The nucleus of 3I/ATLAS measures about 0.6 miles (1 km) in diameter. The comet is at least 7 billion years old, perhaps as much as 12 billion. It has had numerous encounters with other stars, which boosted its velocity to a measured speed of 140,000 mph (225,000 km/h). Rubin could detect on average one interstellar comet per year over the next ten years.