The interstellar comet 3I/ATLAS is wrapped in carbon dioxide fog, NASA space telescope reveals

On July 1, 2025, the Deep Random Survey remote telescope in Chile, part of the ATLAS (Asteroid Terrestrial-impact Last Alert System) project, spotted a new comet. But it wasn’t just any old comet: This one isn’t gravitationally bound by the sun, which means it originated outside of our solar system.

Named 3I/ATLAS, the comet is only the third known interstellar object to enter our solar system, and it’s the largest and brightest one yet. Perhaps unsurprisingly, researchers around the world are training every instrument at their ready on it, including NASA’s new space observatory SPHEREx.

SPHEREx — short for “Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer” — has detected an abundance of carbon dioxide gas in the fuzzy coma surrounding 3I/ATLAS, as well as water ice in the comet’s nucleus. The find marks a major step forward in understanding how comets from other star systems compare to those born in our own solar neighborhood, researchers said. And, in the case of 3I/ATLAS, there appears to be significant overlap.

“SPHEREx’s finding very large amounts of vaporized carbon dioxide gas around 3I/ATLAS told us it could be like a normal solar system comet,” Johns Hopkins University astronomer Carey Lisse, a member of the SPHEREx science team, told Space.com.

Comets comprise not only rocky dust but also water, carbon dioxide and carbon monoxide, which are the three most abundant icy constituents in our solar system.

“The amounts of each depend on where and when the comet was formed and evolved,” said Lisse. That’s because these ingredients sublimate, or boil off, into outer space at different temperatures.

“A comet formed at the very edges of our dawning solar system and then thrown quickly far away into the giant distant Oort Cloud reservoir should have all three ices in abundance,” he noted. “By contrast, a comet formed close to the sun and/or residing for a long time after its formation will lose its carbon monoxide and contain mainly water and carbon dioxide.”

SPHEREx’s observation of a carbon dioxide coma — notably one that lacks carbon monoxide — plus a water-ice nucleus indicates that 3I/ATLAS was “well baked and boiled before being ejected from its parent solar system,” according to Lisse. In other words, “3I/ATLAS is behaving like a normal, well-thermally processed, natural solar system cometary object does.”

But Lisse suspects this comet likely came from far, far away, within the thick disk of the Milky Way. As such, the comet is likely two to three times as old as any found in our solar system.

The new analysis of comet 3I/ATLAS’ composition highlights the unique capabilities of SPHEREx.

“SPHEREx’s forte is creating 102 different wavelength maps of the entire sky at the deep red to mid-infrared wavelengths of 0.75 to 5.0 microns,” said Lisse. Many of these wavelengths cannot be studied from Earth due to atmospheric absorption. For cometary scientists, this means unprecedented access to key chemical fingerprints in their research subjects.

Olivier Doré, SPHEREx project scientist at NASA’s Jet Propulsion Laboratory in Southern California, emphasized the mission’s broader impact.

By mapping the entire sky in 102 colors, we can study all kinds of celestial objects, from distant galaxies and nearby stars in the Milky Way to comets in our own solar system, whether interstellar visitors or not,” he said. “It really underscores the transformative power of the dataset we are now generating. The richness is extraordinary, and it will fuel discoveries across the astronomical community since our data are being continuously released.”

As SPHEREx continues to scan the night sky, it’ll keep an eye on 3I/ATLAS. In late October, the comet will reach perihelion, or its closest encounter with the sun, at approximately the distance of Mars. (3I/ATLAS poses no threat to Earth.)

As it approaches this point, scientists expect the water ice in its nucleus to sublimate, producing “a large water coma to match the carbon dioxide coma, as well as a much stronger dust coma and tail than we see now,” said Lisse. SPHEREx, as well as many other spacecraft in NASA’s fleet, will be watching closely, hoping to unlock more knowledge about our interstellar visitor.

The SPHEREx team published a brief on the findings in Research Notes of the American Astronomical Society and intends to publish a more detailed paper soon.