ATLAS Photometry of Interstellar Object 3I/ATLAS
J. Tonry, L. Denneau, M. R. Alarcón, A. Clocchiatti, Nicolas Erasmus, A. Fitzsimmons, J. Licandro, K. J. Meech, Robert J. Siverd, H. Weiland
Abstract
Abstract We present calibrated Asteroid Terrestrial-impact Last Alert System (ATLAS) photometry of the interstellar comet 3I/ATLAS (C/2025 N1) from March 28 through 2025 August 29, 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 antisolar tail. The absolute magnitude curve H ( t ) shows a slope break near MJD 60890 at r ∼ 3.3 au from −0.035 to −0.012 mag day −1 , or in terms of coma cross section as a function of heliocentric distance, r −3.9 to r −1.1 . We release the aperture photometry with geometry and uncertainties to enable cross-instrument synthesis of 3I/ATLAS activity and color evolution.