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

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 $e\sim 6.1$, perihelion of $q\sim 1.36$ au, inclination of $\sim 175^{\circ }$, 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 $\sim 4$ 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.

Accepted for publication in ApJL. 15 pages, 8 figures, 2 tables. Community follow-up organization can be found here: https://3i-atlas.github.io/ The showyourwork! version of the manuscript can be found here: https://github.com/3I-ATLAS/discovery-paper