3I/ATLAS, the third interstellar object, has a red TNO-like spectrum, an active dust coma, a rotation period of 16.79 hours, and a kinematic trace toward the Galactic thin disk.
Palomar and Apache Point Spectrophotometry of Interstellar Comet 3I/ATLAS
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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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Assessing interstellar comet 3I/ATLAS with the 10.4 m Gran Telescopio Canarias and the Two-meter Twin Telescope
3I/ATLAS, the third interstellar object, has a red TNO-like spectrum, an active dust coma, a rotation period of 16.79 hours, and a kinematic trace toward the Galactic thin disk.