arXiv · 2609.22021
Multiband Color Monitoring of 3I/ATLAS through Ground-Based Relay Observations
Abstract
We present multiband, long-baseline photometric observations of interstellar comet 3I throughout its 2025--2026 apparition using coordinated ground-based global relay observations. Our dataset combines measurements from professional observatories and citizen-operated Unistellar eVscopes distributed worldwide, providing dense temporal coverage from 2025 July 2 through 2026 April 1 and spanning the comet's pre- and post-perihelion trajectory. Broadband photometry was obtained in bandpasses equivalent to the Johnson--Cousins $B$ (436 nm), $V$ (545 nm), and $R$ (641 nm) filters and the Sloan $g$ (477 nm), $r$ (623 nm), and $i$ (763 nm) filters. The photometry was measured using projected aperture radii of approximately 10{,}000~km to provide a consistent probe of the inner coma across the heterogeneous dataset. We measure representative mean colors of $B-V=0.86\pm0.06$, $V-R=0.50\pm0.03$, $B-R=1.36\pm0.08$, and $g-r=0.58\pm0.08$, demonstrating a persistently red optical coma. Constant-color models provide an adequate description of the data, with little evidence for long-term color evolution with time or heliocentric distance despite substantial changes in the coma's brightness, gas production, and volatile composition. This suggests that the ensemble-averaged optical scattering properties of the coma remained relatively stable over the period sampled by our observations, even as other properties of the coma evolved. These observations provide the first densely sampled, apparition-long characterization of the broadband optical colors of an interstellar comet and establish a benchmark for comparison with future interstellar objects.
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Ariel Graykowski, Bryce Bolin, Laura-May Abron, Franck Marchis, Martin Mašek, Filipp D. Romanov, Ahmed M. Abdelaziz, Dimitrios Athanasopoulos, Matthew Belyakov, Luca Buzzi, Michael W. Coughlin, Ergün Ege, Nicolas Erasmus, Thomas M. Esposito, Christoffer Fremling, Josep M. L. Garcia, Marek Husárik, Oleksandra Ivanova, Tarek M. Kamel, Sergey Karpov, Myung-Jin Kim, Tomasz Kwiatkowski, H. -J. Lee, Sofiia Mykhailova, Alessandro Nastasi, Dagmara Oszkiewicz, John W. Pickering, Alexey Sergeyev, Roman Telesnikov, Josep M. Trigo-Rodríguez, Al Alicea, Jin Beniyama, Otabek A. Burkhonov, Kamoliddin E. Ergashev, Y. H. M. Hendy, Jeong-Han Kim, Cheng-Han Hsieh, M. Emir Kenger, Thomas Lehmann, Igor Luk'yanyk, Ali Takey, Dušan Tomko, Murat Tekkesinoglu, Shane Winter, Joel Alonzo, Michael Beddo, Axel Bienefeld, Mario Billiani, John K. Bradley, Olivier Clerget, Sabine Cremer, Nicolas Delaunoy, José Donas, John Dunn, Jiří Dvořáček, Michael Edwards, Taj Entabi, Raffaele Esposito, Karen Fischer, Robert Foster, Edward Friday, Keiichi Fukui, Francois Gagnon, Matthieu Garisson, George Ghreichi, Patrice Girard, Tateki Goto, Michael Grant, Chris Greenfield, Bruno Guillet, Josef Hanus, John Harrison, David Havell, Kevin Heider, Kirt Hufstedler, Kachi Iwai, Maxime Johnson, Christian Joisten, Scott Kardel, Suzanne Kavic, Vamshi Kesireddy, Rachel Knight, David Koster, Jericho Kuehl, Ryuichi Kukita, Jan Kunc, Petri Kuossari, Alain Lambert, Scott Lancelle, Jean-Marie Laugier, Stephen Lawrence, Pedro Lemos, Jeffrey Linn, Joseph Llanes, Steeve Loisel, Gaël Lombart, Margaret A. Loose, Yohann Lorand, Chris Mahar, Scott Manley. 2026-09-18. Multiband Color Monitoring of 3I/ATLAS through Ground-Based Relay Observations. https://arxiv.org/abs/2609.22021
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