Pre-perihelion Development of Interstellar Comet 3I/ATLAS

TL;DR

This study analyzes pre-perihelion optical observations of the interstellar comet 3I/ATLAS with the NOT to quantify activity onset and dust properties. Using fixed linear aperture photometry, the authors derive a heliocentric brightness index of $n \approx 3.8$ across $4.6$–$1.8$ au, consistent with a dust production rate scaling as $r_H^{-2}$ and CO$_2$-driven sublimation. The coma is dominated by large (∼100 μm) grains ejected slowly (∼5 m s$^{-1}$), explaining the delayed antisolar tail and yielding a dust mass loss rate near 2 au of ≈180 kg s$^{-1}$, extrapolating to ≈405 kg s$^{-1}$ at perihelion. Comparison with 2I/Borisov shows similar grain-size dominance but a different heliocentric index, indicating the index alone cannot reveal an interstellar object's dynamical age; overall, the work demonstrates the viability of ground-based optical photometry to constrain volatile content and dust dynamics in interstellar bodies.

Abstract

We describe pre-perihelion optical observations of interstellar comet 3I/ATLAS taken during July - September 2025 using the Nordic Optical Telescope. Fixed aperture photometry of the comet is well described by a power law function of heliocentric distance, rH, with the exponent (``index") n = 3.8+/-0.3 across the 4.6 au to 1.8 au distance range (phase function 0.04+/-0.02 magnitude/degree assumed). This indicates that the dust production rates vary in proportion to rH**(-1.8+/-0.3). An rH**(-2) variation is expected of a strongly volatile material, and consistent with independent spectroscopic observations showing that carbon dioxide is the primary driver of activity. The measured heliocentric index is unremarkable in the context of solar system comets, for which n is widely dispersed, and provides no basis on which to describe 3I as either dynamically old (thermally processed) or new (pristine). The morphology of the comet changes from a Sun-facing dust fan in the early 2025 July observations, to one dominated by an antisolar dust tail at later dates. We attribute the delayed emergence of the tail to the large size (effective radius 0.1 mm) and slow ejection (5 m/s) of the optically dominant dust particles, and their consequently sluggish response to solar radiation pressure. Small (micron-sized) particles may be present but not in numbers sufficient to dominate the scattering cross-section. Their relative depletion possibly reflects interparticle cohesion, which binds small particles more effectively than large ones. A similar preponderance of 0.1 mm grains was reported in 2I/Borisov. However, 2I differed from 3I in having a much smaller (asteroid-like) heliocentric index, n = 1.9+/-0.1. Dust production rates in 3I are 180 kg/s at 2 au, compared with 70 kg/s in 2I/Borisov at the same distance.

Figures (9)

• Figure 2: Sample images showing the morphological development of 3I/ATLAS. The image dates and heliocentric distances of the comet are shown in each panel, as is a 10$^{\prime\prime}$ scale bar. Sunward is West. The location of the brightest pixel is marked with a red dot, showing that the dust is extended asymmetrically towards the West in the early data, reversing to the East as the radiation pressure swept tail develops in the later data. See Table \ref{['geometry']} for additional details.
• Figure 3: Reduced photometry from NOT, assuming $\beta_R$ = 0.04 magnitudes degree$^{-1}$, as a function of heliocentric distance. The straight line is a least squares fit of slope $n$ = 3.8.
• Figure 4: Composite pre-perihelion lightcurve of 3I/ATLAS with error bars omitted for clarity. The solid line shows heliocentric index $n$ = 3.8 while the dashed lines above and below it represent the $\pm1\sigma$ uncertainties on $n$. Error bars are omitted for clarity of presentation.
• Figure 5: Heliocentric indices computed from equilibrium sublimation models for CO, CO$_2$ and H$_2$O ices as a function of heliocentric distance. Hemispherical sublimation is assumed, and the ices are taken to be perfectly absorbing and emissive. The measured index for 3I/ATLAS is shown. The vertical error on the 3I point denotes the $\pm$0.3 uncertainty on the heliocentric index and shows consistency with free sublimation of CO and CO$_2$ but not H$_2$O. The horizontal bar marks the range of distances over which 3I was reliably observed (c.f., Table \ref{['geometry']}).
• Figure 6: Composite pre-perihelion lightcurves of 2I/Borisov and 3I/ATLAS compared, with error bars omitted for clarity. The best-fit heliocentric indices are marked for each object.