Palomar and Apache Point Spectrophotometry of Interstellar Comet 3I/ATLASOPEN ACCESS 

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Matthew Belyakov, Christoffer Fremling, Matthew J. Graham, Bryce T. Bolin, Mukremin Kilic, Gracyn Jewett, Carey M. Lisse, Carl Ingebretsen, M. Ryleigh Davis, Ian Wong

Submitted to RNAAS, draft version July 17, 2025

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On July 1st 2025 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 C2.

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