Temporal Evolution of the Third Interstellar Comet 3I/ATLAS: Spin, Color, Spectra and Dust Activity

TL;DR

This study characterizes the physical and activity properties of interstellar comet 3I/ATLAS through spectroscopic and photometric observations, revealing its rotation, dust activity, and spectral characteristics over the first month after discovery.

Contribution

It provides the first detailed analysis of 3I/ATLAS's rotation, dust activity, and spectral properties, offering insights into its nature and comparison with Solar System comets.

Findings

Spin period of 16.16 hours with 0.3 mag amplitude

Dust activity and reddening colors observed

Spectral colors similar to outer Solar System comets

Abstract

We aim to characterize the physical and activity properties of the interstellar comet 3I/ATLAS through spectroscopic and photometric observations during the first month after its discovery. We performed time-series photometry and long-slit spectroscopy between 2 and 29 July 2025 using multiple ground-based telescopes. Photometric data were calibrated against field stars from the ATLAS and APASS catalogs, and Fourier analysis was applied to derive the comet's rotational period. Spectral data were obtained using SALT and Nordic Optical Telescope. We report a spin period of $16.16 \pm 0.01$ h with a lightcurve amplitude of approximately 0.3 mag. The comet exhibits increasing dust activity and reddening colors during the observation period, with no visible tail detected, likely due to viewing geometry and low dust production. Dust mass loss rates are estimated between 0.3 and 4.2 kg s^-1, consistent with weakly active distant comets. Spectral colors are similar to those of outer Solar System comets and differ from previously reported values for 3I/ATLAS. The morphological and photometric properties of 3I/ATLAS are consistent with a weakly active comet of outer Solar System origin, despite its interstellar provenance. Continued monitoring around perihelion is necessary to track changes in activity, color, which will provide insights into the evolution of interstellar materials under solar radiation.