[astro-ph.EP] We present 14 optical imaging and eight optical spectroscopic epochs of interstellar comet 3I/ATLAS from Keck/LRIS and Gemini/GMOS, two near-infrared (NIR) imaging epochs from Keck/MOSFIRE, and four NIR spectroscopic epochs from Gemini/Flamingos-2 and GNIRS, obtained between 2025 July and 2026 May.
Mean GMOS colors of g−r=0.732±0.029, r−i=0.237±0.025, and i−z=0.109±0.022 mag show no significant change across perihelion. Phase-corrected red-band activity follows Afρ∝r−2.10±0.14h inbound and r−3.34±0.43h outbound. Transverse-width and sunward-plume speeds imply dust losses of 2.7×109 and 1.0×1010 kg over 2025 July 3–2026 May 15 for 1-μm–1-mm grains, equivalent to global layers of 0.26 and 0.99 m under the adopted grain and nucleus assumptions.
The concentrated coma, compact sunward feature, and antisolar tail seen on 2025 August 27 broadened by 2025 November 26; extended dust persisted through 2026 February 17. C2/CN nearly doubled from 0.257±0.014 on 2025 September 14–15 to 0.481±0.080 on 2025 November 26, while C3/CN fell from 0.327±0.024 on 2025 August 28 to 0.214±0.009 on 2025 September 14–15.
MOSFIRE NIR imaging on 2025 July 11 gives Y−J=0.466±0.143, J−H=0.239±0.052, H−Ks=0.454±0.150, and J−Ks=0.693±0.202 mag. Spectra give J−H=0.24 mag on 2025 September 5–6 and J−H=0.27–0.28 and J−Ks=0.21–0.22 mag on 2025 December 1–2 over the retained passband intervals.
Model-calibrated imaging on 2026 February 6 gives near-solar J−Ks=0.303±0.051 mag. No water-ice absorption is robust to the tested continuum and calibration choices in the NIR spectra. These observations provide a lasting record of the comet’s evolution through the Solar System and a benchmark for studying and monitoring future interstellar objects.
Bryce T. Bolin, Jeffrey Cooke, Brian C. Lemaux, Marco Micheli, Davide Farnocchia, Laura-May Abron, Leo Y. Alcorn, Michael W. Coughlin, Christoffer Fremling, Josef Hanuš, Willem B. Hoogendam, Carl Ingebretsen, Oleksandra Ivanova, David O. Jones, Piera King, Ludmilla Kolokolova, Keith S. Noll, Filipp D. Romanov, Benjamin J. Shappee, Karleyne Silva, Matt Wahl, Pablo Candia, Kristin Chiboucas, Eleazar R. Carrasco, Veronica Firpo, Jeong-Eun Heo, Jameeka Marshall, Emily Peavy, Erin Figura, Denise Hung, Ariel Graykowski, Teo Močnik, Matias Rodriguez, David J. Tholen, Peter Vereš, Leila Alamos, Eunchong Kim, Jen Miller, Percy Gomez, Heather Hershley, Michael Lundquist, Rita Morris, Hyewon Suh, Emanuele Paolo Farina, Ryan Ridden, Mark Suhr
Comments: 53 pages, 21 figures, 9 tables. Submitted to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2610.32972 [astro-ph.EP] (or arXiv:2610.32972v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2610.32972
Focus to learn more
Submission history
From: Bryce Bolin
[v1] Fri, 23 Oct 2026 17:58:04 UTC (11,571 KB)
https://arxiv.org/abs/2610.32972
Astrobiology, Astrochemistry, Astrogeology,
