The space probes which flew through the envrionment of Comet Halley have been bombarded by a flow of molecules and dust particles which impacted their surface with a relative velocity of the order of 70--80 km s-1. The emission of secondary electrons and sputtered ions caused by these impacts was a potential source of interference for the experiments which analyzed the gas and plasma environment of the comet. The impact plasma detector is a simple device which measured the saturation current of the secondary electrons emitted from a gold target mounted on Vega-1 and Vega-2. The effects of the gas and dust impacts can be easily separated; the total gas production rate of the nucleus is estimated to be of the order of 1030 molecules/s at 0.8 AU.