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

TL;DR

This paper reports the discovery and initial characterization of the third known interstellar object, 3I/ATLAS, including its orbit, physical properties, and observational prospects, highlighting its similarities to other interstellar bodies.

Contribution

The paper presents the first detailed observations and analysis of 3I/ATLAS, providing insights into its orbit, composition, and observational opportunities, which are novel for this interstellar object.

Findings

3I/ATLAS has a hyperbolic orbit with eccentricity ~6.1.

Spectral slope similar to other interstellar objects and primitive bodies.

Limited light curve variation over 4 days, indicating a stable brightness.

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.