Perihelion Asymmetry in the Water Production Rate of the Interstellar Object 3I/ATLAS

TL;DR

This work combines space-based SWAN Lyman-α measurements with a 3D Monte Carlo particle trajectory framework to quantify the water production rate $Q_{ m H_2O}$ of the interstellar object 3I/ATLAS across its post-perihelion phase. It reveals a strong perihelion asymmetry, with a pre-perihelion rise following $n_{ m pre} = 5.9 \pm 0.8$ and a post-perihelion decline with $n_{ m post} = 3.3 \pm 0.3$, and a post-perihelion peak near $Q_{ m H_2O} \approx 4 \times 10^{28}$ mol s$^{-1}$. The analysis supports a scenario where water production is driven by solar insolation on a stable active area, with a substantial extended source of icy grains in the coma, analogous to 103P/Hartley 2, and shows no signs of outbursts or rapid depletion. These findings imply a hyperactive, distributed-source coma for 3I/ATLAS and have implications for understanding activity in interstellar bodies under solar heating.

Abstract

3I/ATLAS is an interstellar object whose activity provides critical insights into its composition and origin. However, due to its orbital geometry, the object is too close to the Sun near perihelion to be observed from the ground, and space-based measurements are therefore required. Here we characterize the water production rate of 3I/ATLAS using SOHO/SWAN Lyman-$α$ observations from 2025 November to December (heliocentric distances 1.4 to 2.2 au) with 3D Monte Carlo modeling. We report a peak post-perihelion water production rate of $Q_{\mathrm{H_2O}} \approx 4 \times 10^{28}$ mol s$^{-1}$, corresponding to a minimum active fraction of $\sim$30\% (assuming a maximum nucleus radius of 2.8 km). Comparison of our post-perihelion measurements with published pre-perihelion results reveals a heliocentric asymmetry, with an $r_h^{-5.9 \pm 0.8}$ scaling for the inbound rise, followed by a shallower $r_h^{-3.3 \pm 0.3}$ scaling during the outbound decline, where $r_h$ is heliocentric distance. The post-perihelion behavior indicates that the water production of 3I/ATLAS was driven primarily by the varying solar insolation acting on a stable active area. Combined with other evidence, including comparison with the hyperactive comet 103P/Hartley 2, our findings suggest that its water production is likely dominated by a distributed source of icy grains. Furthermore, it displayed remarkable stability in the activity with no signs of outbursts or rapid depletion of water production.

Figures (7)

• Figure 1: Orbital geometry of 3I/ATLAS during the SWAN observing window. The low-inclination trajectory is shown relative to the ecliptic plane, with Sun, Earth and Mars orbits provided for scale.
• Figure 2: Representative SWAN full-sky Lyman-$\alpha$ map (2025 Nov 13), showing the simultaneous detection of comet C/2025 K1 (ATLAS) and numerous UV-bright stars. The zoomed inset displays the differential image centered on 3I/ATLAS, illustrating its stellar-like appearance after the pipeline subtracts the IPH background and faint stellar sources. Residual artifacts remain for luminous stars.
• Figure 3: Heliocentric dependence of the water production rate $Q(\mathrm{H_2O})$ for 3I/ATLAS (left axis, colored symbols), superimposed with the heliocentric magnitude (right axis, grey points). The $Q(\mathrm{H_2O})$ data follow a steep power-law trend pre-perihelion ($n_{\mathrm{pre}} = 5.9 \pm 0.8$), contrasting with a significantly shallower slope post-perihelion ($n_{\mathrm{post}} = 3.3 \pm 0.3$).
• Figure 4: Variation of the active fraction ($f_{\mathrm{A}}$) with heliocentric distance, calculated assuming a nucleus radius of $R_{\mathrm{N}} = 2.8$ km. The pre- and post-perihelion lines correspond to the power-law fits presented in Figure \ref{['fig:water_prod']}. The active fraction reaches a maximum of $\sim 47\%$ pre-perihelion and stabilizes at a constant value of $\sim 30\%$ post-perihelion.
• Figure 5: Comparison of water production rates for Jupiter-family comet 41P/Tuttle-Giacobini-Kresák derived in this work (red circles) with literature values. The data are plotted alongside the previous SWAN analysis by Combi2019 (blue squares), TRAPPIST OH observations from Moulane2018 (black diamonds), and Swift/UVOT measurements Bodewits2018 (green triangles).