It’s like glimpsing a rifle bullet for a thousandth of a second, David Jewitt of the University of California, Los Angeles, described of the brief existence of interstellar comet 3I/ATLAS. The third confirmed interstellar visitor to the solar system, the object already has toppled centuries-old assumptions regarding cometary chemistry and activity.
Initially discovered on July 1, 2025, by the NASA-funded Asteroid Terrestrial-impact Last Alert System in Chile, 3I/ATLAS was characterized at 420 million miles from the Sun. Soon, four of the world’s most sophisticated astronomical observatories NASA’s Hubble Space Telescope, the James Webb Space Telescope (JWST), the Transiting Exoplanet Survey Satellite (TESS), and the new SPHEREx mission were pointed at it. The concerted effort has unveiled a comet unlike any ever seen.
SPHEREx and JWST spectroscopy revealed a carbon dioxide-dominant coma with a CO₂/H₂O ratio of 8:1 the highest yet measured in a comet. This is not only rare; it is six standard deviations above the norm. In comparison, the carbon monoxide level is nearer to that of normal cometary contents. Carbonyl sulfide, water ice, and dust were also verified to be present. Such a structure suggests an environment of formation either outside of the CO₂ ice line within its parent protoplanetary disk or in the presence of strong ultraviolet radiation, circumstances that might facilitate CO₂-rich ices while inhibiting water sublimation.
TESS archival data showed that 3I/ATLAS was already active in May 2025, when it was 6 astronomical units from the Sun far beyond Jupiter’s orbit. Comets typically only start to sublimate volatiles within 5 AU, where solar heating is enough to vaporize water. Premature activity at this distance indicates volatile species such as CO₂, which sublimate at lower temperatures, initiating outgassing much earlier than water ice is active.
Hubble imaging has also provided another twist. The spacecraft imaged a teardrop-shaped dust coma with no visible tail, a morphology untypical of active comets. It also quantified the coma’s dust-loss rate as typical of Sun-bound comets that were first seen approximately 300 million miles from the Sun, even though 3I/ATLAS is interstellar. Diameter estimates put the nucleus at between 320 meters and 5.6 kilometers in diameter, but the solid core is still hidden behind the light coma.
The comet’s kinematics are as astonishing as its chemistry. Flying at 130,000 miles per hour, 3I/ATLAS is the fastest solar system visitor ever recorded. This speed indicates billions of years of gravitational acceleration due to flybys of stars and nebulae throughout the Milky Way. Its path indicates a provenance in the galaxy’s thick disk, so it could be 7 billion years old three billion years older than the solar system itself.
Spectroscopic study of such an object is an unusual privilege. SPHEREx’s multi-spectral mapping and the Near-Infrared Spectrograph of JWST have broken some of the important molecular fingerprints not accessible from the Earth’s surface because of absorption by the atmosphere. With a comparison of the isotopic ratios, e.g., the identical Carbon-12 to Carbon-13 ratio as on Earth, scientists can investigate the chemical development of its parent system.
The window of observation is limited. Perihelion will take place on October 29, 2025, as the comet is hidden behind the Sun as viewed from Earth. Prior to that, Mars orbiters could pick up more information as 3I/ATLAS comes about two million miles from the planet. NASA’s Juno spacecraft in March 2026 might, in principle, catch the comet at Jupiter’s vicinity if a sophisticated trajectory maneuver perhaps involving a Jupiter Oberth burn is undertaken. But Juno’s age, fuel reserves, and recent technical problems make the prospect iffy.
For now, astronomers are racing to extract every possible detail before the comet vanishes into interstellar space. Each dataset whether from Hubble’s high-resolution imaging, JWST’s infrared spectra, SPHEREx’s compositional mapping, or TESS’s early light curves adds to a growing portrait of an object that challenges models of comet formation and evolution. As Jewitt noted, “This latest interstellar tourist is one of a previously undetected population of objects bursting onto the scene… We’ve crossed a threshold.”
