Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion

TL;DR

This study used SOHO/SWAN observations to measure water production rates of interstellar comet 3I/ATLAS after perihelion, revealing a decrease from 3.17 x 10^29 to 1-2 x 10^28 molecules per second over 40 days.

Contribution

First application of SOHO/SWAN data to quantify water production of interstellar comet 3I/ATLAS post-perihelion, demonstrating a method for such measurements.

Findings

Water production rate was 3.17 x 10^29 s^-1 at 1.40 au from the Sun.

Water production decreased to 1-2 x 10^28 s^-1 after 40 days.

Method validated for interstellar comet observations.

Abstract

The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the Solar and Heliosphere Observatory (SOHO) observed the hydrogen coma of interstellar comet 3I/ATLAS, also called C/2025 N1 (ATLAS), beginning on November 6, 2025, 9 days after perihelion. Water production rates were calculated from each image of 3I/ATLAS using the methodology of Makinen & Combi (2005, Icarus 177, 217) and fluorescence rates, g-factors, calculated using the daily solar Lyman-alpha fluxes from the LASP database (https://lasp.colorado.edu/lisird/data) corrected for solar rotation and for the comet heliocentric velocity. The method has been used for over 90 comet apparitions (Combi 2022; Combi et al. 2019). A water production rate of 3.17 x 10^29 s^-1 was found on November 6 when the comet was at a heliocentric distance of 1.40 au and at a sufficient solar elongation angle. It decreased over time after that, down to 1-2 x 10^28 s^-1 around 40 days post-perihelion (December 9).