Modeling Halley before and after the encounters

Numerical models developed prior to the 1986 spacecraft flybys at comet P/Halley described anticipated dust and gas environments. Predicted values for gas production, for dynamic ranges of gas and dust experiments, and for likely spacecraft effects were matched during the flybys well within expected margins; in particular the dust-impact-generated attitude disturbance experienced by Giotto several seconds before closest approach was within the envelope of expectations from the models. Three major areas are identified in which 1986 results for Halley might be used to improve the models, namely (1) gas production values from earth-based and in situ observations, (2) dust flux and fluence values in the coma from three spacecraft, and (3) dust size distributions from the in situ data which require the presence of numerous particles at masses less than 10 to the -17th kg.