Palomar and Apache Point Spectrophotometry of Interstellar Comet 3I/ATLAS

Abstract

Abstract On 2025 July 1st the third interstellar object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered, with an eccentricity of e = 6.15 ± 0.01 and perihelion of q = 1.357 ± 0.001 au. We report our initial visible to near-infrared (420–1000 nm) spectrophotometry of 3I/ATLAS using both the Palomar 200 inch telescope and Apache Point Observatory. We measure 3I/ATLAS to have a red spectral slope of 19%/100 nm in the 420–700 nm range, and a more neutral 6%/100 nm slope over 700–1000 nm. We detect no notable emission features such as from C 2 .