Colin Orion Chandler, Pedro H. Bernardinelli, Mario Jurić, Devanshi Singh, Henry H. Hsieh, Ian Sullivan, R. Lynne Jones, Jacob A. Kurlander, Dmitrii Vavilov, Siegfried Eggl, Matthew J. Holman, F. Spoto, Megan E. Schwamb, Lauren A. MacArthur, Rahil Makadia, M. Micheli, A. Heinze, E. Christensen, Wilson Beebe, A. Roodman, Kian-Tat Lim, Tim Jenness, James Bosch, Brianna Smart, Eric C. Bellm, Sean Macbride, Meredith L. Rawls, Sarah Greenstreet, Colin T. Slater, Željko Ivezić, Robert Blum, Andrew J. Connolly, Gregory Daues, Michelle Gower, J. Bryce Kalmbach, Michele T. Bannister, L. Dones, Rosemary Dorsey, Davide Farnocchia, Wesley C. Fraser, John C. Forbes, César Fuentes, Carrie E. Holt, Laura Inno, G. H. Jones, Matthew M. Knight, Chris Lintott, Tim Lister, Robert H. Lupton, Mark Jesus M. Magbanua, Renu Malhotra, B. E. A. Mueller, J Murtagh, Nitya Pandey, W. T. Reach, Nalin H. Samarasinha, Darryl Z. Seligman, C. Snodgrass, Michael Solontoi, Gyula M. Szabó, Peter Vereš, Ellie White, Maria Womack, L. A. Young, Russ Allbery, Shreya Anand, Roberto Armellin, Éric Aubourg, Chrysa Avdellidou, Farrukh Azfar, James Bauer, K. Bechtol, Valerie R. Becker, Matthew Belyakov, Susan Benecchi, Ivano Bertini, Dennis Bodewits, Patricia Boeshaar, Bryce Bolin, Maitrayee Bose, A. Boucaud, R. C. Boufleur, D. Boutigny, Andrew Bradshaw, F. Braga-Ribas, J. Bregeon, Laura E. Buchanan, Daniel Calabrese, J. I. B. Camargo, Neven Čaplar, Jeffrey L. Carlin, B. Carry, Juan Pablo Carvajal, Ross Ceballo, Hsin‐Fang Chiang, Yumi Choi, C. Combet, L. N. da Costa, Preeti Cowan, John Franklin Crenshaw
Abstract We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object’s region of the sky during routine commissioning. Facilitated by Rubin’s high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of ∼70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of ∼3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at ∼0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-sectional ratio of η ≳ 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g − r = (0.657 ± 0.013) mag, r − i = (0.235 ± 0.018) mag, i − z = (0.147 ± 0.042) mag, and z − y = (0.047 ± 0.052) mag. These data represent the earliest observations of this object by a large (≳8 m class) telescope and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time will provide after it begins in early 2026.