Temporal Evolution of the Third Interstellar Comet 3I/ATLAS: Spin, Color, Spectra and Dust Activity (2508.00808v1)

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 the SALT telescope 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.

• The paper determines a precise 16.16-hour spin period, progressive color reddening, and a dust activity proxy (Afρ ~300 cm) for 3I/ATLAS.
• Multi-instrument photometry and low-resolution spectroscopy underpin a robust analysis of its spectral, photometric, and coma morphology properties.
• Findings indicate similarities with outer Solar System comets, refining our understanding of interstellar object composition and activity evolution.

Physical and Activity Characterization of the Interstellar Comet 3I/ATLAS

Introduction

The third confirmed interstellar object, comet 3I/ATLAS (C/2025 N1), provides a unique opportunity to probe the physical properties and activity of extrasolar planetesimals. This paper presents a comprehensive analysis of 3I/ATLAS during its first month of observation, focusing on its spin state, color evolution, spectral properties, and dust activity. The campaign utilized multi-band photometry and low-resolution spectroscopy from several ground-based facilities, establishing a pre-perihelion baseline for future comparative studies.

Spectral Properties and Color Evolution

Spectroscopic observations with SALT and NOT revealed a progressive reddening of 3I/ATLAS in the $0.4$–$0.7~\mu$m range, with the spectral gradient increasing from $17.1 \pm 0.2~\%/\mu$m (early July) to $22.8 \pm 0.1~\%/\mu$m (late July). Beyond $0.7~\mu$m, the spectrum flattens, indicating a transition to more neutral colors. The campaign-averaged $g'r'i'z'$ photometric fluxes corroborate this trend, with a clear reddening in $g-r$ and $g-i$ and a bluing in $i-z$ as the comet approached the Sun.

Figure 1: Comparison of the spectrum of 3I/ATLAS obtained with SALT and NOT against the reference spectrum, showing progressive reddening and spectral similarity to D-type asteroids and outer Solar System comets.

The measured colors are consistent with those of 2I/Borisov and distant Solar System comets at similar heliocentric distances, suggesting analogous dust properties. Notably, the colors are bluer than previously reported for 3I/ATLAS, contradicting earlier claims of strong red coloration. This discrepancy highlights the importance of multi-epoch, multi-instrument campaigns for robust color characterization.

Figure 2: Evolution of photometric colors over the campaign, showing a reddening trend in $g$, $r$, and $i$ bands, and a bluing trend in $i-z$.

Coma Morphology and Dust Activity

Intensity maps constructed from stacked $g'$ and $r'$ images reveal a compact, slightly asymmetric coma with no visible tail during most of the campaign. The absence of a tail is attributed to the observing geometry and low dust production, with the tail likely projected behind the nucleus due to the low phase angle. Digital enhancement techniques failed to reveal jet-like or other morphological features, indicating a lack of significant localized activity.

Figure 3: Intensity maps of 3I/ATLAS from FTN datasets, showing stable asymmetric coma and no visible tail on both 4 and 29 July.

Figure 4: Intensity maps in $g$ and $r$ SDSS filters, further illustrating the compact coma and absence of prominent tail structures.

Photometric analysis yielded an absolute magnitude of $H \sim 12$ mag and a dust activity proxy $Af\rho \sim 300$ cm, consistent with weakly active distant comets. The dust mass loss rate was estimated between $0.3$ and $4.2$ kg s$^{-1}$, slightly higher than previous reports for 3I/ATLAS but lower than typical Jupiter-family comets at similar distances. The inferred nucleus radius upper limit is $\sim$11 km, assuming a geometric albedo of $p_v = 0.04$.

Rotational State and Lightcurve Analysis

Time-series photometry in the SDSS $r'$ band enabled the construction of a rotational lightcurve, revealing a spin period of $16.16 \pm 0.01$ h and a peak-to-peak amplitude of $\sim$0.3 mag. The amplitude decreased to 0.2 mag over the campaign, likely due to increasing coma activity masking the nucleus's rotational signal. Fourier analysis identified two alias periods near 8 and 24 hours, but the 16.16 h period is robustly supported by the data.

Figure 5: Rotational lightcurve of 3I/ATLAS phased with a spin period of $16.16 \pm 0.01$ h, with multi-telescope photometric data and best-fitting Fourier model.

Figure 6: Periodogram from second-order Fourier analysis, showing a clear minimum at $16.16 \pm 0.01$ hours.

Segments of the lightcurve from certain telescopes exhibited flat profiles, attributed to observing conditions and enhanced coma activity diluting the rotational modulation. This effect underscores the challenges of nucleus characterization in active comets, especially those with weak or variable activity.

Implications and Future Directions

The physical and activity properties of 3I/ATLAS are consistent with a weakly active comet of outer Solar System origin, despite its confirmed interstellar trajectory. The progressive reddening and increasing dust activity as the comet approaches the Sun suggest ongoing surface or coma evolution, likely driven by solar heating and sublimation of volatile materials. The absence of a prominent tail and lack of morphological features indicate low dust production and minimal localized activity.

These findings have several implications:

• Compositional Diversity of ISOs: The similarity of 3I/ATLAS to outer Solar System comets supports the hypothesis that the population of interstellar comets may be dominated by bodies with analogous dust and volatile properties, rather than exhibiting extreme compositional diversity.
• Activity Evolution: The observed color and activity evolution provide constraints on the response of interstellar materials to solar irradiation, informing models of ISO surface processing and volatile depletion.
• Observational Strategies: The campaign demonstrates the necessity of rapid, multi-instrument follow-up for ISOs, especially given their transient observability and the potential for rapid activity changes.

Continued monitoring around perihelion, including polarimetric and high-resolution spectroscopic observations, will be essential for tracking further changes in activity and color, and for detecting possible emission features indicative of gas production. These data will enable more detailed comparisons with Solar System comets and refine our understanding of the physical processes governing ISO evolution.

Conclusion

The paper of 3I/ATLAS during its first month of observation reveals a weakly active, outer Solar System-like comet, with a well-constrained spin period, progressive reddening, and modest dust production. The results challenge previous reports of strong red coloration and highlight the importance of comprehensive, multi-epoch campaigns for ISO characterization. Future observations, particularly around perihelion, will be critical for elucidating the evolution of interstellar materials under solar radiation and for advancing our understanding of the compositional and activity diversity of extrasolar planetesimals.