Snapshot of a new interstellar comet: 3I/ATLAS has a red and featureless spectrum

TL;DR

This paper presents initial optical spectroscopic observations of the interstellar comet 3I/ATLAS, revealing a red, dust-dominated coma with no detectable gas emissions, providing a baseline for future studies of its activity and composition.

Contribution

First spectroscopic analysis of 3I/ATLAS shortly after discovery, establishing its red, dusty coma and setting a baseline for future interstellar comet studies.

Findings

Comet 3I/ATLAS has a red, featureless spectrum.

No gas emissions detected from C2, NH2, CN, or [OI].

Coma appears entirely dusty at current distance.

Abstract

The interstellar comet 3I/ATLAS is only the third interstellar object to be discovered. Pre-perihelion measurements provide a unique opportunity to study its activity and composition, which may alter as it is heated in the coming months. We provide an initial baseline from optical spectroscopic observations obtained only two days after discovery, using the MUSE instrument on the VLT on 2025 July 3, while 3I was at 4.47 au from the Sun and 3.46 au from the Earth. These observations confirm the cometary nature of 3I, and reveal a red coma with a spectral slope of $(18\pm4)\%/1000$~\AA, redder than most Solar System comets but similar to the surface colour of some Solar System Trans-Neptunian Objects or Centaurs. We searched for but did not detect gas emission from C$_2$, NH$_2$, CN, and [OI], which is consistent with volatile non-detections for Solar System comets at this heliocentric distance. At present, the coma appears entirely dusty. Future observations of 3I as it comes closer to the Sun will provide an invaluable opportunity to witness the evolution of its activity, study its composition, test predictions of interstellar object population models, and compare 3I to Solar System comets.