3I/ATLAS, with a perihelion of 1.36 au, had a significantly more eccentric orbit than the first two known ISOs (e = 6.1); we discuss the implications of this in terms of its Galactic origin in Section 8.3. Early images of 3I/ATLAS already showed an extended source, indicating the existence of a dusty coma at a heliocentric distance of ∼4.5 au (Alarcon et al, 2025; Belyakov et al, 2025; Frincke et al, 2025; Hoogendam et al, 2025; Jewitt et al, 2025; Kareta et al, 2025; Opitom et al, 2025; Seligman et al, 2025). Precovery with Rubin Observatory Science Validation images (Chandler et al, 2025) and the Transiting Exoplanet Survey Satellite (TESS) (Farnham et al, 2025; Feinstein et al, 2025; Martinez-Palomera et al, 2025) show that this activity had started months before the discovery in July, as early as May 2025 at a distance of 6.4 au. There were other precovery observations by the Zwicky Transient Facility (Ye et al, 2025). HST observations later revealed a nucleus diameter of 1.2 km (Hui et al, 2026).
Pre-perihelion observations of 3I/ATLAS revealed the presence of CO2, CO, OCS, and an extended source of water, likely icy grains (Cordiner et al, 2025; Xing et al, 2025). 3I/ATLAS has a reddened colour most similar to D-type asteroids, and consistent with that of 1I/’Oumuamua and 2I/Borisov (Seligman et al, 2025). 3I/ATLAS exhibits properties that are generally consistent with Solar System comets at a similar distance, albeit with a slightly redder colour.
Like 2I/Borisov, its apparition was highly favourable for detailed characterization of its activity and composition. Pre-perihelion JWST observations at 3.3 au from the Sun revealed that the object’s activity was driven by CO2 and the presence of H2O, CO, OCS, and water ice (Cordiner et al, 2025). Contemporaneous observations with SPHEREx corroborated these observations, and also reported an extended CO2 coma and water ice absorption Lisse et al (2025). Its CO2/H2O abundance is far higher than that of Solar System comets (e.g. Harrington Pinto et al, 2022), even after accounting for the low sublimation efficiency of water at such large heliocentric distances. Water production was also detected/inferred early (Yang et al, 2025; Xing et al, 2025). The elevation of hypervolatiles has been hypothesized to be caused by ice composition alteration via processing of the surface and near surface of 3I/ATLAS during its travel through the interstellar medium (Maggiolo et al, 2026). Surprisingly, Ni was among the first species detected, appearing even before CN and HCN (Roth et al, 2025; Salazar Manzano et al, 2025; Coulson et al, 2026; Lazzarin et al, 2026; Paek et al, 2026), which is typically the earliest species identified in distant comets (Hutsem´ekers et al, 2026; Rahatgaonkar et al, 2025). Near-perihelion observations revealed that the object has brightened post perihelion Zhang and Battams (2026), presumably due to increased H2O activity. 3I/ATLAS had an extreme negative polarization signal in its dust coma (Gray et al, 2025).
Overall, and as with 2I/Borisov, the third known interstellar comet displayed a composition that was also consistent with formation at large stellocentric distance at cold ambient temperatures.

Thermal modelling of the nucleus indicated that hypervolatiles were likely driving the activity initially observed at large heliocentric distances (Yaginuma et al, 2026).