Would a single comet be able to turn our understanding of worlds beyond our Sun on its head?
The discovery of 3I/ATLAS the third confirmed interstellar object in history has put astronomers to work in an effort to unravel a mystery shrouded in a cloud of gas and dust. First spotted on July 1, 2025, by Chile’s ATLAS survey, the comet is not gravitationally bound to the Sun but is a visitor from a different star system. It is not its provenance that makes it remarkable, but the physical and chemical anomalies revealed as some of the globe’s most powerful observatories trained their gear upon it.
Measurements by NASA’s SPHEREx and the James Webb Space Telescope (JWST) have confirmed that the comet’s coma the thin veil of gas and dust surrounding its nucleus boasts the highest carbon dioxide to water ratio ever recorded in a comet. JWST’s Near-Infrared Spectrograph measured it at 8:1 when 3I/ATLAS was 3.32 astronomical units from the Sun, six standard deviations above the norm. “Our observations are compatible with an intrinsically CO₂-rich nucleus, which may indicate that 3I/ATLAS contains ices exposed to higher levels of radiation than Solar System comets, or that it formed close to the CO₂ ice line in its parent protoplanetary disk,” the astronomers said in their preprint.
The implications are deep. Compositional control of our solar system is governed by the timing and location of the body’s formation, and sublimation of volatile ices like carbon monoxide, carbon dioxide, and water at different temperatures. The dominance of CO₂ in 3I/ATLAS suggests a formative environment rich in this volatile or one with extended radiation-induced stripping of other ices. Johns Hopkins University astronomer Carey Lisse explained the observation of abundant CO₂ with water ice but little carbon monoxide by SPHEREx implies that the comet had been “well baked and boiled before being ejected from its parent solar system.”
3I/ATLAS was rule-breaking even before it was officially discovered. Archival data from NASA’s Transiting Exoplanet Survey Satellite (TESS) revealed it was already active in May 2025, at 6 AU from the Sun well past Jupiter’s orbit. Few comets activate outgassing until within 5 AU, where solar heat can cause water ice to sublimate. The early activity here points to CO₂, as it sublimates at much lower temperatures than water. SPHEREx August observations revealed the coma’s radius was 23 kilometers, that no tail or jets could be detected, and there were production levels sufficient to sustain its abnormal brightness.
Hubble imagery revealed an additional layer of intrigue: a teardrop-terminated dust coma in the absence of the characteristic sweeping tail. Harvard astronomer Avi Loeb described it as a “teardrop-shaped cocoon of dust coming off of the comet’s solid, icy nucleus,” a morphology that opposes the conventional solar wind-powered tail shape. The asymmetry could be due to localized outgassing regimes or comet rotation, but its persistence across observations hints at something more sophisticated in its dust dynamics.
Trajectory modeling shows 3I/ATLAS was traveling at over 130,000 mph (210,000 km/h) through the solar system on a very flat, linear path. That velocity, the fastest ever for a solar system visitor, means it has been traveling for billions of years, accelerated by gravitational encounters with stars and nebulae. Its diameter, as measured by Hubble at approximately 2.8 kilometers radius, also makes it the largest interstellar object to date. Some observations indicate it could be three billion years older than our solar system. Close-up opportunities for observation are rare.
3I/ATLAS will pass within 30 million kilometers of Mars on October 3, 2025 closer than it will ever come near Earth. ESA would utilize the High Resolution Stereo Camera on Mars Express and CaSSIS imager on the Trace Gas Orbiter of ExoMars to photograph its structure and activity. Spectrometers on these orbiters could detect molecular signatures such as water vapor or organics, although signal intensity is questionable. NASA’s Mars Reconnaissance Orbiter and MAVEN, and China’s Tianwen-1, also could join the campaign. The quest won’t stop there.
By March 2026, NASA’s Juno spacecraft could attempt an intercept when the comet flies by Jupiter. Flybys such as these would refine models of its structure, dust production, and volatile emissions rates. As NASA’s Jet Propulsion Laboratory’s SPHEREx project scientist Olivier Doré remarked, “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.”
For now, 3I/ATLAS remains a mystery a chilly ambassador from a different planetary system, with chemical signatures that muddy our descriptions of cometary origins and histories. Each new appearance deepens the mystery, to guarantee that this fleeting visitor will have a lasting impact on planetary science.
