Physical properties of cometary dust: Relation of DIDSY data to grain properties

The Dust Impact Detection System (DIDSY) for the Giotto Halley Mission consists of two types of sensors for the detection of cometary dust particles: two impact plasma sensors and five piezo-electric momentum sensors. One sensor of each type is covered by a penetration film. A 1 μm thick aluminum film covers an impact plasma sensor. One momentum sensor is mounted onto the rear shield behind the 1 mm front shield made from aluminum. The parameters measured are the total charge released upon impact and the amplitude of the acoustic signal generated by the impact. Both quantities depend on the mass and speed of the impacting particles. At the impact speed of 68 km/sec the mass of cometary dust particles can be determined in the mass range from 10^?17 g to 10^?3 g. From the difference in the countrates measured by the sensors with and without penetration film the average bulk density of dust particles of masses 10^?14 g and 10^?6 g can be determined. With appropriate calibration an accuracy of a factor of 2 for both the mass and density determination can be obtained.