Initial observations revealed a faint and diffuse coma that signalled cometary activity, although the lack of a pronounced tail hinted at unusual behaviour. Pre-discovery images from TESS indicated that 3I/ATLAS may have been active even at about 6.4 AU from the Sun, hinting at sublimation of volatile ices beyond water ice.
Spectroscopic data from multiple observatories have offered a clearer picture of its composition. JWST observations captured in August 2025 revealed a strikingly CO₂-rich coma, with the CO₂ to H₂O ratio among the highest observed in any comet. Lesser amounts of water, carbon monoxide, carbonyl sulfide, water ice, dust, and heavy CO₂ isotopes were also detected. SPHEREx confirmed abundant CO₂ emission, though it lacked the sensitivity to detect water vapor or CO reliably. Ultraviolet imaging by the Swift Observatory identified water-derived OH emission at around 3.5 AU, suggesting an active area representing over 20 percent of its nucleus surface—unusual for distant comet activity. Further ground-based spectrographs, notably using the Very Large Telescope, detected cyanide and nickel emissions, both increasing over time, while other common cometary gases remained undetected.
Estimates of its nucleus size vary—ranging from a few hundred metres to several kilometres—making size determination challenging due to the brightness of its dense coma. Orbital calculations indicate that 3I/ATLAS will pass closest to the Sun on 29 October 2025 at around 1.36 AU, then continue outward, never approaching closer than 1.8 AU from Earth, posing no threat. It will be in solar conjunction, invisible to Earth-based telescopes until it re-emerges later in the year.
Some voices, notably astrophysicist Avi Loeb, have raised provocative speculation that 3I/ATLAS could be a deliberately directed, possibly extraterrestrial probe, citing its alignment with the Solar System’s ecliptic and unusual composition. However, others in the scientific community have strongly rebutted those ideas, emphasising that the object displays classical comet behaviours and that extraordinary hypotheses necessitate extraordinary evidence.
Origin modelling suggests that 3I/ATLAS emerged from the Milky Way’s thick disk—a region dominated by ancient, low-metallicity stars. Age estimates range from around 3 to 14 billion years, placing it potentially among the oldest comets ever observed and pre-dating the Solar System by several billion years.
The coming months offer further opportunities. Mars-orbiting instruments like HiRISE could capture images during its fly‑by on 3 October 2025. Post-perihelion observations are slated with Hubble in ultraviolet, and with JWST in December, aiming to study the evolving coma composition and better constrain its structure.