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

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 Solar and Heliospheric Observatory/SWAN Ly α observations from 2025 November to December (heliocentric distances 1.4–2.2 au) with 3D Monte Carlo modeling. We report a peak postperihelion water production rate of Q H 2 O ≈ 4 × 1 0 28 molecules s −1 , corresponding to a minimum active fraction of ∼30% (assuming a maximum nucleus radius of…