Is the oldest comet ever observed hidden in plain sight its secrets enshrouded in a cloud of carbon dioxide? Astronomers think so, thanks to an unusual interstellar visitor and a space telescope capable of reading the universe in more colors than the human eye can envision.
The ATLAS survey telescope in Chile spotted a mysterious object on July 1, 2025, unlike any Sun-bound object ever seen. Named 3I/ATLAS, it is the third known interstellar object to visit the solar system and the largest and brightest to date. Its highly inclined galactic trajectory indicates an origin within the thick disk of the Milky Way a population of stars more than two times as old as the Sun so this comet could be 7 billion years old, two or three times the age of any solar system comet.
NASA’s SPHEREx observatory, or Spectro-Photometer for “Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer,” has been key to discovering its make-up. Working between 0.75 and 5.0 microns, SPHEREx scans the whole sky in 102 infrared wavelengths, several of which are inaccessible to us from Earth because of absorption by the atmosphere. This range of spectra enables scientists to obtain chemical “fingerprints” of objects in the universe with unprecedented accuracy. “By mapping the entire sky in 102 colors, we can study all kinds of celestial objects… The richness is extraordinary,” said Olivier Doré, SPHEREx project scientist at NASA’s Jet Propulsion Laboratory.
In mid-August 2025, SPHEREx observed a rich carbon dioxide coma surrounding 3I/ATLAS, as well as water ice within its nucleus. The lack of carbon monoxide, which is sublimates at lower temperatures, indicates the comet was “well baked and boiled before being ejected from its parent solar system,” according to Johns Hopkins University astronomer Carey Lisse. This thermal processing signature is similar to that of long-lived solar system comets that have existed for eons near their parent stars before being expelled to deep space.
The carbon dioxide dominance is emphatic. Standalone measurements by the James Webb Space Telescope recorded a CO₂-to-H₂O ratio of 8 to 1 one of the highest ever detected in a comet. Such enrichment can trigger early activation at distances from the Sun; pre-discovery measurements by NASA’s TESS mission showed that 3I/ATLAS was already venting at 6 astronomical units, far past Jupiter’s orbit. Carbon dioxide sublimates at lower temperatures than water ice, allowing activity at these distances.
The coma’s radial measurement of 23 kilometers and absence of visible tails or jets at the time are indicative of high production rates. Data from Hubble show that the nucleus itself is approximately 2.8 kilometers in radius. As the comet gets closer to its October 29 perihelion, approximately at Mars’s orbital distance, scientists anticipate the sublimation of water ice strongly, leading to a large water coma and a more active dust tail.
The object’s history within the thick disk has consequences beyond age. This part of the galaxy evolved under a different set of chemical conditions than in the Sun’s neighborhood, perhaps depositing characteristic isotopic or molecular signatures in its ices. Resemblance to some peculiar solar system comets, e.g., C/2016 R2 with its rich CO, suggests cold formation environments where volatile preservation is enabled.
Infrared spectroscopy, the method behind SPHEREx’s success, operates by spreading light into its constituent wavelengths and finding absorption or emission features due to specific molecules. In cometary science, it allows direct measurements of volatile species key to understanding formation regions in protoplanetary disks. Detection of CO₂ and H₂O without CO in 3I/ATLAS places constraints on its thermal history and parent system architecture.
The scarcity of such interstellar guests adds to the pressure. ‘Oumuamua in 2017 and 2I/Borisov in 2019 provided limited windows for research. 3I/ATLAS, bigger and more energetic, is being monitored by a constellation of observatories, ranging from SPHEREx to ground-based behemoths like the Vera C. Rubin Observatory, set to detect dozens more in the next decade. As Lisse pointed out, “3I/ATLAS is behaving like a normal, well-thermally processed, natural solar system cometary object does,” yet its interstellar origin makes it a messenger from a part of the galaxy never before sampled up close.
As it passes by the Sun and comes out from behind its glare once again, Mars-orbiting spacecraft and even the Juno mission to Jupiter will get further views before it escapes into interstellar space again. Every spectrum taken until then will contribute to a chemical record that connects planetary science and galactic archaeology, charting the path of a frozen remnant from the ancient history of the Milky Way.
