Astronomers first spotted it in early July 2025. Since then, the interstellar object known as 3I/Atlas has been raising eyebrows—and questions. And despite a growing body of data, the mystery around it only deepens.
For weeks now, the world’s most advanced instruments have taken turns observing this strange traveler speeding through our solar system. It’s only the third confirmed interstellar object ever detected in our cosmic backyard.
The latest tools trained on it? NASA’s SPHEREx telescope (short for Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer) and, of course, the James Webb Space Telescope. Their shared mission: gather data on the object’s size, physical properties, and—perhaps most importantly—its chemical composition.
As NASA continues to study the interstellar comet 3I/ATLAS, two space telescopes have made new observations, together providing more information about its size, physical properties, and chemical makeup.@NASAWebb observed the object with its Near-Infrared Spectrograph instrument… pic.twitter.com/LwNd5C4MJI
— NASA Solar System (@NASASolarSystem) August 25, 2025
What its coma reveals
When the comet was about 470 million kilometers (roughly 292 million miles) from the Sun, SPHEREx managed to collect data shedding light on its makeup. According to a note published by the American Astronomical Society, scientists detected large amounts of carbon dioxide (CO₂) in the comet’s coma—the hazy cloud of gas and dust that surrounds its core. They also found traces of water ice in the nucleus itself.
Data from the James Webb Telescope backed this up: 3I/Atlas appears to have a very high CO₂ content, but much less water than expected. In fact, the CO₂-to-H₂O ratio is one of the highest ever recorded in any comet, which researchers found particularly intriguing.
El James Webb Space Telescope observó el cometa interestelar 3I/ATLAS el 6 de agosto con su Near-Infrared Spectrograph instrument: https://t.co/B159i2HMc0 pic.twitter.com/QDQdlXXnv0
— Gaston Giribet (@GastonGiribet) August 26, 2025
What about the core?
There’s more. Astronomers believe that most of the comet’s brightness is likely coming from the coma—not the nucleus. If the light were coming from the core, 3I/Atlas would need to be nearly 30 miles wide and have a mass one million times that of the last interstellar comet studied, 2I/Borisov.
In other words, that would make it a cosmic giant. For now, though, the mystery continues—and scientists will be watching closely to see what 3I/Atlas reveals next.
Nathalie Mayer
Journalist
Born in Lorraine on a freezing winter night, storytelling has always inspired me, first through my grandmother’s tales and later Stephen King’s imagination. A physicist turned science communicator, I’ve collaborated with institutions like CEA, Total, Engie, and Futura. Today, I focus on unraveling Earth’s complex environmental and energy challenges, blending science with storytelling to illuminate solutions.
