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

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

arXiv · 2507.11720

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

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

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Abstract

On July 1st 2025 the third interstellar object, 3I/ATLAS or C/2025 N1 (ATLAS), was discovered, with an eccentricity of $e=6.15 \pm 0.01$ and perihelion of $q=1.357\pm0.001$ 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$_2$.

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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. 2025-07-15. Palomar and Apache Point Spectrophotometry of Interstellar Comet 3I/ATLAS. https://arxiv.org/abs/2507.11720

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