“The H2O mass loss rate is only 5% of the CO2 output. This is 16 times more extreme than expected for a typical comet at the same distance from the Sun,” Harvard astrophysicist Avi Loeb wrote this week, condensing the mystery of 3I/ATLAS into one disorienting statistic.
First seen on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, the object is only the third-ever known interstellar visitor to be seen, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. But whereas its ancestors measured just a few dozen meters across, 3I/ATLAS is immense about 46 kilometers across according to measurements from NASA’s SPHEREx observatory standing a million times heavier than Borisov. Its size, along with its strange chemistry, is making astronomers rethink assumptions regarding what moves between stars.
High-resolution imaging with the James Webb Space Telescope (JWST), SPHEREx, and the Hubble Space Telescope has shown a carbon-dioxide dominated coma with only traces of water vapor and carbon monoxide. Near-infrared spectroscopy with JWST’s Near-Infrared Spectrograph (NIRSpec) detected CO₂, water ice, carbon monoxide, and carbonyl sulfide, but the CO₂-to-H₂O ratio is the largest ever seen in a comet. The inference is either that the nucleus is CO₂-enriched by nature possibly built up close to the carbon dioxide ice line of its parent protoplanetary disk or that its ices survived intense cosmic radiation never before experienced in the Solar System.
SPHEREx observations between August 8–12 recorded a CO₂ mass loss rate of about 70 kilograms per second, with no visible water coma greater than 4.5 kilograms per second. The outgassing behavior creates a symmetric CO₂ cloud stretching at least 348,000 kilometers from the nucleus, decreasing in density by radius to the -2.5 power. The net loss is equivalent to shedding just a millimeter-thick layer in ten years an external “skin” covering up the object’s underlying structure.
It is no less puzzling what isn’t there: 3I/ATLAS lacks any visible dust tail. Hubble imagery reveals a tight glow in front of its motion towards the Sun, which is taken as dust evaporation off its sun-facing surface, but the lack of a tail suggests little micron-scale dust ejection. The reddish color in reflected sunlight is probably due to the surface itself, not suspended particles.
Its path provides yet another layer of mystery. The orbit is remarkably flat, lying with the plane of the Solar System’s planets a chance with probabilities of roughly one in 500 for a random interstellar arrival. Loeb has made the case that this precision, coupled with the object’s unlikely size, might signify “its trajectory was precisely aligned to target the inner Solar System.” Most astronomers dismiss the idea of man-made origin, but the statistical improbability stimulates discussion.
Orbital dynamics assure us that 3I/ATLAS is not bound to the Sun, moving at a speed in excess of 130,000 mph (210,000 km/h) and set to leave the Solar System after its October 30th closest approach to Earth approximately 130 million miles. Alas, at the time it will be on the far side of the Sun from Earth, closing the best observing window.
That such a large interstellar body is so rare is highlighted by population models: based on the density of rocky material within interstellar space, surveys such as ATLAS ought to have detected millions of objects of Borisov’s size previously, before they were discovered at this size. That gulf between expectation and observation is fuel for speculation, mostly focusing on unusual formation regions in distant planetary systems within old protoplanetary disks to rare formation scenarios.
3I/ATLAS is also a time capsule of deep galactic history. Its precipitous approach trajectory implies formation in the thick disk of the Milky Way, which would indicate an age of around 7 billion years old three billion years older than the Solar System. Analysis of its composition provides a unique window into planetary system chemistry from an era and location very different from our own. For planetary scientists, the unprecedented CO₂ richness and damped water emission challenge current cometary models, suggesting thermal or structural barriers constraining heat penetration into the nucleus.
As the Sun’s heat strengthens in the weeks ahead, from JWST to SPHEREx, the instruments will keep monitoring its changing coma, for whatever shifts may unlock its hidden interior. Whatever the result, 3I/ATLAS will leave the Solar System with fewer secrets than it entered on though maybe not all of them.
