![Example of the stacking procedure for the Maidanak data obtained on 2025 August 15, based on ten 60 s exposures in each of the B, V , and R filters. Left: Comet-centered stack, showing the coma morphology without trailing of the comet signal. The pink circle indicates an aperture radius of 10 000 km. Right: Sidereal stack used for photometric calibration, in which the comet appears trailed.-- [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/08/The-Inbound.png?fit=1200%2C678&ssl=1)
[astro-ph.EP] Interstellar objects (ISOs) provide direct constraints on the composition and evolution of extrasolar planetary systems. As the third confirmed ISO, comet 3I/ATLAS offers a rare opportunity to study pristine material from another star system.
We present a systematic multi-band monitoring campaign conducted from 2025 July to September. To minimize systematic uncertainties, we utilized a homogeneous dataset of SDSS g′,r′,i′,zs and Johnson-Cousins BVR imaging obtained with MuSCAT3/4 on the 2.0-m Faulkes Telescopes and the 1.5-m Maidanak telescope.
We employed a novel multi-aperture technique providing a robust, distance-independent characterization of the coma, alongside optical spectroscopy from SALT and the Nordic Optical Telescope. 3I/ATLAS exhibited a stable reddish color with no significant secular variation during its inbound journey. Spectroscopic analysis reveals carbon-chain depletion, with CN as the only prominent molecular emission.
Coma morphology, analyzed via azimuthal averaging and rotational gradient filters, shows a persistent sunward jet at a position angle of ∼280∘, indicating sustained and localized outgassing from a high-latitude active region.
Light-scattering modeling demonstrates that the observed colors and polarization are consistent with dense aggregates of organic matter and silicate minerals, while water ice sublimation remained insufficient to alter the optical properties.
The stability of the coma structures and the homogeneous nature of our dataset indicate that 3I/ATLAS underwent a steady activation phase, preserving a volatile-rich but carbon-chain-poor composition. This characterization points to an object formed in a distinct protoplanetary environment and provides a definitive record of its pre-perihelion evolution.
Toni Santana-Ros, Alexey Sergeyev, Oleksandra Ivanova, Sofiia Mykhailova, Johannes Markkanen, Igor Luk’yanyk, Tomasz Kwiatkowski, Dagmara Oszkiewicz, Antti Penttilä, Nicolas Erasmus, Kamoliddin Ergashev, Yurij Krugly
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2608.18371 [astro-ph.EP] (or arXiv:2608.18371v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2608.18371
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Journal reference: T. Santana-Ros et al 2026, ApJ, 1007, 182
Related DOI:
https://doi.org/10.3847/1538-4357/ae8dbb
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Submission history
From: Toni Santana-Ros
[v1] Tue, 18 Aug 2026 22:57:26 UTC (2,973 KB)
https://arxiv.org/abs/2608.18371
Astrobiology, astrochemistry,
