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

TL;DR

This paper presents initial spectrophotometric observations of interstellar comet 3I/ATLAS, revealing its spectral slope and lack of emission features, providing insights into its composition and properties.

Contribution

It reports the first visible to near-infrared spectrophotometry of 3I/ATLAS using Palomar and Apache Point telescopes, highlighting its spectral characteristics.

Findings

Spectral slope of 19%/100 nm in 420-700 nm range

Spectral slope of 6%/100 nm in 700-1000 nm range

No notable emission features detected

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$_2$.