Astronomers find interstellar comet 3I/ATLAS hiding in images taken before its official discovery

It turns out interstellar comet 3I/ATLAS was almost called 3I/Rubin, after researchers found that the giant survey telescope coincidentally spotted this visitor from the stars over a week before it was officially discovered.

3I/ATLAS was officially identified on July 1, 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS), which is a network of robotic telescopes in Hawaii, Chile and South Africa. But ten days before, the Vera C. Rubin Observatory, which is also in Chile, began its science validation phase ahead of entering full operation later that year. The science validation phase was designed to calibrate the 27.6-foot (8.4-meter) telescope and its instruments, to ensure that they were working correctly.

Curious as to whether Rubin had seen 3I/ATLAS before its official discovery date, a team led by Colin Orion Chandler of the University of Washington went sifting through the data from Rubin’s commissioning phase. Lo and behold, they found that Rubin had imaged 3I/ATLAS on its very first night of taking practice images, on June 20, ten days before ATLAS spotted it.

It wasn’t an easy task. Today Rubin has a very well planned out routine — called a ‘pipeline’ — for taking data and processing it for astronomers, but back during the validation phase the pipeline was not in operation. This meant that Chandler and his team had to devise their own custom pipeline to access the data.

Chandler estimates that if Rubin had begun its science validation phase a few weeks earlier, its data-handling pipelines might have been up and running in time to snag 3I/ATLAS before July 1.

The researchers found that Rubin proceeded to image the interstellar comet a further nine times between June 21 and July 2, and several more times between July 2 and July 20. The images clearly show that 3I/ATLAS was active even before ATLAS detected it, with an obvious coma — a cloud of dust and gas around the head of a comet that is liberated from the comet’s surface when it heats up as it nears the sun.

Rubin is designed to find up to 10,000 new comets over the 10-year lifetime of its initial Legacy Survey of Space and Time, and Rubin’s early detection of 3I/ATLAS bodes well for estimates that it could find, on average, one interstellar comet passing through our solar system each year. So while 3I/ATLAS doesn’t bear Rubin’s name, it’s a good bet that future interstellar comets will.

Jupiter probes team up

Meanwhile, 3I/ATLAS hasn’t left our solar system yet, and new information is still being revealed by spacecraft that have been keeping an eye on it. Observations by spacecraft came in particularly useful when the comet was hidden behind the sun from our point of view in October 2025, which coincided with perihelion (closest approach to the sun) when then comet was expected to be most active.

Scientists at the South-west Research Institute (SwRI) who lead the Ultraviolet Spectrograph (UVS) instruments on the European Space Agency’s JUICE mission and NASA’s Europa Clipper, both currently on their way to Jupiter, have revealed that the two spacecraft made joint observations of 3I/ATLAS in late 2025.

“As the comet passed between JUICE and Europa Clipper, we were able to informally coordinate observations between the two spacecraft,” said SwRI’s Kurt Retherford in a statement.

JUICE had a view of the interstellar comet’s dayside, while Europa Clipper saw its nightside, allowing researchers to see the same gas emissions from two different directions.

The joint observations by the UVS instrument on each spacecraft detected hydrogen, oxygen and carbon that were produced when molecular gases escaping 3I/ATLAS’s nucleus interacted with ultraviolet light from the sun, which broke the molecules apart into their component atoms. The abundance of carbon was higher than is typical for comets native to our solar system, which confirms James Webb Space Telescope observations that found excess carbon dioxide on 3I/ATLAS.

“By studying the ratio of water-ice and dry ice [i.e. carbon-dioxide ice], we can compare the composition of this interstellar comet to comets native to our solar system,” said SwRI’s Philippa Molyneux. “This helps us understand if the solar system where 3I/ATLAS formed is similar to ours or different.”

These findings add to the bounty of data on 3I/ATLAS already gathered by multiple space missions and ground-based observations. We know that the nucleus of 3I/ATLAS is about a kilometer (0.6 miles) wide, and its high velocity of 140,000 mph (61 kilometers per second) suggests that it is probably at least seven billion years old and possibly up to 12 billion years old, and has experienced many encounters with other stars that have boosted its velocity.

The analysis of Rubin’s observations of the comet were published on April 20 in The Astrophysical Journal Letters.