Interstellar Interloper 3I/ATLAS: Nucleus Size, Photometry in RGB, Af(rho) and Antitail Structure Analysis

Abstract: Interstellar comet 3I/ATLAS (C/2025 N1) exhibits an unusual, tightly collimated dust feature in the sunward hemisphere which has been widely described as an anti-tail. At the same time, precise constraints on the nucleus size have been derived from a combination of high-resolution imaging and non-gravitational dynamics. In this work I present a unified analysis that combines existing constraints on the nucleus radius with new ground-based RGB imaging of the dust anti-tail obtained with a 0.25 m telescope at the Toni Scarmato Astronomical Observatory (MPC L92).

• The paper applies high-resolution imaging, non-gravitational dynamics, and photometric decomposition to constrain the nucleus size to approximately 0.26–0.37 km.
• The paper utilizes RGB photometry to demonstrate a dust-dominated, tightly collimated anti-tail extending up to 8×10^4 km.
• The paper’s results support a jet-driven, high-energy dust ejection mechanism, offering a new template for studying interstellar cometary structures.

Unified Photometric, Dynamical, and Structural Constraints on the Nucleus and Anti-tail Structure of Interstellar Comet 3I/ATLAS

Introduction

The study of the interstellar comet 3I/ATLAS (C/2025 N1) has provided an exceptional opportunity to probe the physical properties and dust environment of a cometary nucleus of extrasolar origin. The present work synthesizes key constraints on the nucleus size—derived from high-resolution imaging, non-gravitational dynamics, and thermophysical modeling—and introduces new RGB photometry and morphological analysis of the dust anti-tail using ground-based observations. The synthesis addresses consistency across photometric, dynamical, and colorimetric datasets, and advances understanding of the physical processes governing the formation, structure, and composition of 3I/ATLAS's prominent anti-tail.

Nucleus Size Constraints: Imaging and Dynamics

Three principal methodologies are applied to constrain the nucleus radius:

• HST Surface-brightness Profile Decomposition: Modeling the inner coma profile as the sum of an unresolved nucleus and a parametric coma component yields an upper bound on nucleus radius of $R_n \lesssim 2.8$ km (assuming $p_V \simeq 0.04$). The lower photometric detection limit is $R_n \gtrsim 0.16$ km, contingent on the inner coma's precise contribution.
• Non-gravitational Acceleration (NGA): Astrometry from interplanetary platforms constrains the dynamical mass and thus radius through measurement of $a_{\rm NG} \sim 5 \times 10^{-7}$ m s$^{-2}$, requiring $R_n \simeq 0.26$–0.37 km (diameter $0.52$–$0.74$ km) for plausible density and gas production parameters.
• Thermophysical/Structural Models: Assuming a compact, processed “crustal fossil” structure with $\rho = 1.5$–3 g cm$^{-3}$, the preferred radius is $R_n \sim 1$–3 km. Model uncertainties arise primarily from compositional and structural population priors.

A PSF–$\delta$ decomposition utilizing bicubic resampling on ground-based imaging independently recovers a radius range entirely consistent with those inferred from HST and NGA analysis (i.e., $0.16 \lesssim R_n \lesssim 2.8$ km, with strongest support for $R_n \simeq 0.26$–$0.37$ km). A detailed photometric application to a December 18, 2025, $R$-band stack yields $R_{\rm nuc} \simeq 0.25$ km, directly confirming the sub-kilometre nucleus hypothesis.

Ground-based RGB Imaging and Morphological Structure

A deep RGB sequence obtained with a 0.25 m Newtonian telescope and a cooled color CMOS camera was processed with 4$\times$4 bicubic resampling and a Larson–Sekanina rotational gradient filter, revealing the anti-tail’s morphology as a tightly collimated feature extending several $10^4$ km in projection.

A multi-channel analysis demonstrates:

• $R$-band (625 nm): The anti-tail is traceable to $\sim 5 \times 10^4$ km, exhibiting strong collimation along a position angle of $\sim 110^\circ$.
• $G$-band (530 nm): The anti-tail is visible to $\sim 8 \times 10^4$ km. The enhanced spatial extent and intensity reflect the contribution from both dust continuum and gas emission (notably, C$_2$, NH$_2$, [O I]).
• $B$-band (470 nm): The inner coma is comparatively compact and symmetric, dominated by gas emissions (CN, C$_3$, CH), with negligible anti-tail.

  Figure 1: Multi-channel view of the inner coma of 3I/ATLAS highlighting the collimated anti-tail jet in $R$ and $G$ channels and the compact gas-rich $B$ channel.

The color-resolved images provide stringent evidence that the anti-tail is primarily composed of dust grains, with the RGB dependence resulting from the interplay of continuum scattering and emission band strengths.

Colorimetric and Photometric Analysis

Photometry in the resampled RGB channels yields apparent magnitudes $m_R=11.40$, $m_G=11.68$, $m_B=12.60$. Comparison with solar colors indicates a moderate reddening in the (B–R) index ($\Delta(B-R)\simeq0.19$) and an anomalous blueing in the (G–R) index ($\Delta(G-R)\simeq-0.08$), attributable to gas emission enhancement in the green channel. Computed spectral slopes are consistent with dust-dominated cometary scattering continua ($S'_{B-R} \sim 12.3\%/100$ nm), but show strong curvature due to line contamination ($S'_{B-G} \sim 47\%/100$ nm).

The dust production proxy Af$\rho$ computed in the $R$ band ($\sim 1.1 \times 10^3$ cm in a $41.4''$ aperture at $r = 2.25$ au) quantifies a moderate dust output, fully typical of active comets at these heliocentric distances.

Dynamical Model for the Anti-tail

A one-dimensional nucleus-centered dust-flight model is used, with grains launched sunward at speed $v_0$ and subject to solar radiation pressure parameterized by $\beta$. The grains are decelerated and reach a maximum projected separation $L$ given by $L = v_0^2 / (2 \beta g_\odot)$. For $L_R \sim 5 \times 10^4$ km and $L_G \sim 8 \times 10^4$ km:

• For mm-sized grains ($\beta \sim 0.0057$–$0.02$), required launch velocities are $v_0 \sim 30$–$70$ m s$^{-1}$.
• For $0.1$ mm grains ($\beta \sim 0.057$), $v_0 \sim 90$–$120$ m s$^{-1}$.

These velocities vastly exceed typical subsonic dust velocities ($\lesssim 5$ m s$^{-1}$) adopted in cometary coma literature, but remain below the thermal velocities of sublimating volatiles. This points to a highly efficient, collimated jet mechanism capable of imparting unusually high kinetic energy to large, radiation-pressure-sensitive dust grains in the anti-tail direction.

Implications and Theoretical Relevance

These results imply that 3I/ATLAS possesses a highly anisotropic, energetic dust ejection mechanism, possibly realized as a precessing high-latitude jet. The findings are consistent with, and provide observational support for, several anti-tail formation scenarios: anisotropic survival of icy grains, jet-driven collimation, and decoupling of macroscopic fragments under outgassing torques. The existence of a long, dust-dominated anti-tail with color-morphology dependence implies a dynamic environment distinct from typical Solar System comets.

Practical implications include the demonstrated utility of careful ground-based photometry and small-aperture color imaging, when combined with advanced resampling and decomposition techniques, in extracting quantitative nucleus and coma parameters that are consistent with the most precise space-based astrometric results.

Conclusion

The analysis unifies photometric, dynamical, and colorimetric probes of 3I/ATLAS, finding:

• All photometric and dynamical measurements robustly indicate a sub-kilometre nucleus ($R_n \simeq 0.26$–$0.37$ km) with plausible densities.
• The tightly collimated, color-dependent anti-tail extends up to $8 \times 10^4$ km, with observed morphological and spectroscopic behaviors supporting a primarily dust-dominated, anisotropically-emitted structure.
• Modeling requires launch speeds for dust grains far exceeding canonical values, supporting the need for jet-driven collimated outflow models.
• The high efficiency and reliability of small-aperture, color-resolved imaging for quantitative assessment of cometary nuclei and dust environments is emphasized.

These results represent a comprehensive, internally consistent characterization of the nucleus and immediate dust environment of the third observed interstellar minor body and provide a template for analogous studies of future interstellar interlopers.