ATLAS Photometry of Interstellar Object 3I/ATLAS
John Tonry, Larry Denneau, Miguel Alarcon, Alejandro Clocchiatti, Nicolas Erasmus, Alan Fitzsimmons, Javier Licandro, Karen Meech, Robert Siverd, Henry Weiland
Published: 2025/9/6
Abstract
We present calibrated ATLAS photometry of the interstellar comet 3I/ATLAS (C/2025 N1) from 28 March through 29 Aug 2025, obtained with the five-site, robotic ATLAS network in the c (420-650~nm), o (560-820~nm), and Teide w (420-720~nm) bands. Stacked difference images yield reliable light curves measured in four fixed apertures that capture the evolving coma. We observe 3I/ATLAS transitioning in color from red (c-o)~0.7 before MJD 60860 to near-solar (c-o)~0.3 after MJD 60870, coincident with the appearance of a prominent anti-solar tail. The absolute magnitude curve H(t) shows a slope break near MJD 60890 at r~3.3 au from -0.035 to -0.014 mag/day, or in terms of coma cross section as a function of heliocentric distance, r^{-3.9} to r^{-1.2}. We interpret the evolution as a shift from dust lifted from a reddened surface to production of small, optically bright icy grains and changing coma optical depth. We release the aperture photometry with geometry and uncertainties to enable cross-instrument synthesis of 3I/ATLAS activity and color evolution.