Perihelion Asymmetry In The Water Production Rate Of Tthe Interstellar Object 3I/ATLAS
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 QH2O≈4×1028 mol s−1, corresponding to a minimum active fraction of ∼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−5.9±0.8h scaling for the inbound rise, followed by a shallower r−3.3±0.3h scaling during the outbound decline, where rh 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.
Hanjie Tan (1), Xiaoran Yan (2), Jian-Yang Li (1 and 3) ((1) Planetary Environmental and Astrobiological Research Laboratory (PEARL), School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China, (2) National Research Council, Institute of Applied Physics “Nello Carrara” (IFAC-CNR), FI, Italy, (3) Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi, China)
Comments: 13 pages, 7 figures. Accepted for publication in The Astrophysical Journal Letters (ApJL)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2601.15443 [astro-ph.EP] (or arXiv:2601.15443v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2601.15443
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Submission history
From: Hanjie Tan
[v1] Wed, 21 Jan 2026 20:22:15 UTC (458 KB)
https://arxiv.org/abs/2601.15443
Astrobiology, Astrochemistry,
