Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLASOPEN ACCESS
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, 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, 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
Draft version July 8, 2025, submitted to AAS Journals
“We report initial observations aimed at the characterization of a third interstellar object candidate. This object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered on 2025 July 1 UT and has an orbital eccentricity of e ∼ 6.1, perihelion of q ∼ 1.36 au, inclination of ∼ 175◦, and hyperbolic velocity of V∞ ∼ 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 ∼ 4 day time span. The visible/near-infrared spectral slope of the object is 17.1±0.2 %/100 nm, comparable to other interstellar objects and primitive solar system small bodies (comets and D-type asteroids), although this result is likely affected by some coma contamination. 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. Additional photometric, spectroscopic, and polarimetric monitoring of 3I/ATLAS by ground- and space-based telescopes, and possibly spacecraft based at Mars, are highly encouraged for characterizing 3I/ATLAS’s rotational light curve, activity evolution, nongravitational acceleration, and compositional indicators of formation conditions.”
Comment on “Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS”
Abraham Loeb
Comment (Draft version July 9, 2025) on “Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS”
“The interstellar object 3I/ATLAS shows a weak cometary activity. Its brightness suggests a maximum radius of ∼ 10 km (A/0.05)−1/2 for an asteroid with an albedo A. I show that interstellar objects with that radius would amount to an interstellar mass density that is well above the expected mass budget of interstellar comets or asteroids. Given this budget, the detection rate of objects like 3I/ATLAS implies that it is a comet with a small core radius < 0.6 km, or a member of a rare population with a number density ≲ 5 × 10−8 au−3 for R ≳ 10 km. The second possibility would suggest that the rare population of 3I/ATLAS objects favors plunging orbits towards the inner solar system to accommodate their inferred detection rate.”
