Pioneer Venus lyman‐α observations of comet P/Giacobini‐Zinner and the life expectancy of cometary hydrogen

Pioneer Venus observed the hydrogen coma of Comet P/Giacobini‐Zinner on 11 September 1985 during the ICE fly‐by. Analysis of these data with a time‐dependent 3‐D particle‐trajectory model implies a water production rate of 2.2 × 10 28 s −1 on that day. The model includes the irregular variations in the H lifetime and the H Lyman‐α fluorescence rate determined from simultaneous measurements of the solar wind by ICE and of the solar UV by SME, respectively. The H lifetime varied from 2 × 10 5 to 3 × 10 6 s during the 42 days preceding the observation.