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±0.06, V−R=0.50±0.03, B−R=1.36±0.08, and g−r=0.58±0.08, 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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BibTeX
@misc{graykowski2026multiband,
title = {{Multiband Color Monitoring of 3I/ATLAS through Ground-Based Relay Observations}},
author = {Ariel Graykowski and Bryce Bolin and Laura-May Abron and Franck Marchis and Martin Mašek and Filipp D. Romanov and Ahmed M. Abdelaziz and Dimitrios Athanasopoulos and Matthew Belyakov and Luca Buzzi and Michael W. Coughlin and Ergün Ege and Nicolas Erasmus and Thomas M. Esposito and Christoffer Fremling and Josep M. L. Garcia and Marek Husárik and Oleksandra Ivanova and Tarek M. Kamel and Sergey Karpov and Myung-Jin Kim and Tomasz Kwiatkowski and H. -J. Lee and Sofiia Mykhailova and Alessandro Nastasi and Dagmara Oszkiewicz and John W. Pickering and Alexey Sergeyev and Roman Telesnikov and Josep M. Trigo-Rodríguez and Al Alicea and Jin Beniyama and Otabek A. Burkhonov and Kamoliddin E. Ergashev and Y. H. M. Hendy and Jeong-Han Kim and Cheng-Han Hsieh and M. Emir Kenger and Thomas Lehmann and Igor Luk'yanyk and Ali Takey and Dušan Tomko and Murat Tekkesinoglu and Shane Winter and Joel Alonzo and Michael Beddo and Axel Bienefeld and Mario Billiani and John K. Bradley and Olivier Clerget and Sabine Cremer and Nicolas Delaunoy and José Donas and John Dunn and Jiří Dvořáček and Michael Edwards and Taj Entabi and Raffaele Esposito and Karen Fischer and Robert Foster},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.22021},
eprint = {2609.22021},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.22021},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Multiband Color Monitoring of 3I/ATLAS through Ground-Based Relay Observations
AU - Ariel Graykowski
AU - Bryce Bolin
AU - Laura-May Abron
AU - Franck Marchis
AU - Martin Mašek
AU - Filipp D. Romanov
AU - Ahmed M. Abdelaziz
AU - Dimitrios Athanasopoulos
AU - Matthew Belyakov
AU - Luca Buzzi
AU - Michael W. Coughlin
AU - Ergün Ege
AU - Nicolas Erasmus
AU - Thomas M. Esposito
AU - Christoffer Fremling
AU - Josep M. L. Garcia
AU - Marek Husárik
AU - Oleksandra Ivanova
AU - Tarek M. Kamel
AU - Sergey Karpov
AU - Myung-Jin Kim
AU - Tomasz Kwiatkowski
AU - H. -J. Lee
AU - Sofiia Mykhailova
AU - Alessandro Nastasi
AU - Dagmara Oszkiewicz
AU - John W. Pickering
AU - Alexey Sergeyev
AU - Roman Telesnikov
AU - Josep M. Trigo-Rodríguez
AU - Al Alicea
AU - Jin Beniyama
AU - Otabek A. Burkhonov
AU - Kamoliddin E. Ergashev
AU - Y. H. M. Hendy
AU - Jeong-Han Kim
AU - Cheng-Han Hsieh
AU - M. Emir Kenger
AU - Thomas Lehmann
AU - Igor Luk'yanyk
AU - Ali Takey
AU - Dušan Tomko
AU - Murat Tekkesinoglu
AU - Shane Winter
AU - Joel Alonzo
AU - Michael Beddo
AU - Axel Bienefeld
AU - Mario Billiani
AU - John K. Bradley
AU - Olivier Clerget
AU - Sabine Cremer
AU - Nicolas Delaunoy
AU - José Donas
AU - John Dunn
AU - Jiří Dvořáček
AU - Michael Edwards
AU - Taj Entabi
AU - Raffaele Esposito
AU - Karen Fischer
AU - Robert Foster
PY - 2026
DA - 2026/09/18
PB - arXiv
DO - 10.48550/arxiv.2609.22021
UR - https://arxiv.org/abs/2609.22021
AB - 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±0.06, V−R=0.50±0.03, B−R=1.36±0.08, and g−r=0.58±0.08, 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.
ER -