Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS

Darryl Z. Seligman; Marco Micheli; Davide Farnocchia; Larry Denneau; John W. Noonan; Henry H. Hsieh; Toni Santana-Ros; John Tonry; Katie Auchettl; Luca Conversi; Maxime Devogèle; Laura Faggioli; Adina D. Feinstein; Marco Fenucci; Marin Ferrais; Tessa Frincke; Michaël Gillon; Olivier R. Hainaut; Kyle Hart; Andrew Hoffman; Carrie E. Holt; Willem B. Hoogendam; Mark E. Huber; Emmanuel Jehin; Theodore Kareta; Jacqueline V. Keane; Michael S. P. Kelley; Tim Lister; Kathleen Mandt; Jean Manfroid; Dušan Marčeta; Karen J. Meech; Mohamed Amine Miftah; Marvin Morgan; Francisco Ocaña; Eloy Peña-Asensio; Benjamin J. Shappee; Robert J. Siverd; Aster G. Taylor; Michael A. Tucker; Richard Wainscoat; Robert Weryk; James J. Wray; Atsuhiro Yaginuma; Bin Yang; Quanzhi Ye; Qicheng Zhang

arXiv · 2507.02757

Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS

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Abstract

We report initial observations aimed at the characterization of a third interstellar object. This object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered on 2025 July 1 UT and has an orbital eccentricity of $e\sim6.1$, perihelion of $q\sim 1.36$ au, inclination of $\sim175^\circ$, and hyperbolic velocity of $V_\infty\sim 58$ km s$^{-1}$. We report deep stacked images obtained using the Canada-France-Hawaii Telescope and the Very Large Telescope that resolve a compact coma. Using images obtained from several smaller ground-based telescopes, we find minimal light curve variation for the object over a $\sim4$ day time span. The visible/near-infrared spectral slope of the object is 17.1$\pm$0.2 %/100 nm, comparable to other interstellar objects and primitive solar system small bodies (comets and D-type asteroids). 3I/ATLAS will be observable through early September 2025, then unobservable by Earth-based observatories near perihelion due to low solar elongation. It will be observable again from the ground in late November 2025. Although this limitation unfortunately prohibits detailed Earth-based observations at perihelion when the activity of 3I/ATLAS is likely to peak, spacecraft at Mars could be used to make valuable observations at this time.

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Darryl Z. Seligman, Marco Micheli, Davide Farnocchia, Larry Denneau, John W. Noonan, Henry H. Hsieh, Toni Santana-Ros, John Tonry, Katie Auchettl, Luca Conversi, Maxime Devogèle, Laura Faggioli, Adina D. Feinstein, Marco Fenucci, Marin Ferrais, Tessa Frincke, Michaël Gillon, Olivier R. Hainaut, Kyle Hart, Andrew Hoffman, Carrie E. Holt, Willem B. Hoogendam, Mark E. Huber, Emmanuel Jehin, Theodore Kareta, Jacqueline V. Keane, Michael S. P. Kelley, Tim Lister, Kathleen Mandt, Jean Manfroid, Dušan Marčeta, Karen J. Meech, Mohamed Amine Miftah, Marvin Morgan, Francisco Ocaña, Eloy Peña-Asensio, Benjamin J. Shappee, Robert J. Siverd, Aster G. Taylor, Michael A. Tucker, Richard Wainscoat, Robert Weryk, James J. Wray, Atsuhiro Yaginuma, Bin Yang, Quanzhi Ye, Qicheng Zhang. 2025-07-24. Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS. https://arxiv.org/abs/2507.02757

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