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 × 1028 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 × 105 to 3 × 106 s during the 42 days preceding the observation.