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

TL;DR

This study characterizes the interstellar comet 3I/ATLAS during its first month after discovery using time-series photometry and low-resolution spectroscopy from multiple telescopes. It finds a rotation period of $16.16 \pm 0.01$ h with a lightcurve amplitude of $\sim$0.3 mag, increasing dust activity, and reddening optical colors, with no visible tail detected. Dust mass-loss rates are estimated to be $\sim$0.3–4.2 kg s$^{-1}$ and $Af\rho \sim 300$ cm, consistent with weak activity at large heliocentric distances; spectral colors resemble outer Solar System comets and differ from earlier 3I/ATLAS measurements. The results support a weakly active, outer Solar System–like object of interstellar origin, and they highlight the value of continued monitoring around perihelion to understand interstellar material evolution under solar radiation and to enable comparisons with Solar System counterparts.

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.

Figures (5)

• Figure 1: Comparison of the spectrum of 3I/ATLAS obtained with the Southern African Large Telescope (SALT) and the Nordic Optical Telescope (NOT) against the reference spectrum reported by Seligman25. Overlaid on the spectral data are the campaign-averaged $g'r'i'z'$ photometric fluxes observed with the Faulkes Telescope North (FTN). The D-type asteroid spectrum DeMeo2009 is plotted for reference. All the datasets are normalized to 0.55 $\mu\mathrm{m}$.
• Figure 2: Evolution of photometric colors over the campaign from 02-29 July. A reddening trend is observed in the wavelength range covered by the g, r, and i bands, while a bluing trend is seen in the i–z color.
• Figure 3: Rotational lightcurve of comet 3I/ATLAS phased with a spin period of $16.16 \pm 0.01$ h. Photometric data points from multiple telescopes are shown with error bars; their Minor Planet Center (MPC) codes are listed in the box within the figure. The red solid line represents the best-fitting second-order Fourier model to the phased lightcurve. The MPC codes correspond to the following telescopes: C65 (Telescopi Joan Oró), E10 (Faulkes Telescope South), F65 (Faulkes Telescope North), M28 (Lesedi Telescope), and Z23 (Nordic Optical Telescope).
• Figure 4: Intensity maps of comet 3I/ATLAS in the $g$ and $r$ SDSS filters. The color scale does not reflect the absolute brightness of the comet. Arrows indicate the directions toward the Sun, celestial north, east, and the negative velocity vector of the comet projected onto the plane of the sky.
• Figure 5: Periodogram obtained through a second-order Fourier analysis of the lightcurve data. The black curve represents the root mean square (RMS) residuals as a function of the tested rotation period, measured in hours. A clear minimum is observed at $16.16 \pm 0.01$ hours, indicated by the red shaded region.