Dust Impact Detector (DID)

Description

The investigation uses a system comprised of an array of dust impact sensors to answer some of the fundamental questions in cometary science, namely, the measure of the particulate mass efflux from a comet, its mass distribution, and the particulate grain density. Using the entire surface of the meteoroid shield of the spacecraft system, the array of sensors is able to detect masses over the range 1.E-17 to 1.E-3 g, and perhaps even larger. The upper limit is dependent on the maximum mass of the particulate matter intercepted by the probe during the fly-by. This mass range encompasses almost all the non-volatile comet nucleus component, currently believed to comprise 50% of the comet mass. The range also encompasses about 90% of the mass and scattering area distributions of the zodiacal cloud. The instrumentation is comprised of (1) an impact plasma micro-perforation and sensing array which determines the mass, penetration properties, density and ionization of the impacting particles in the range 1.E-17 to 1.E-10 g; (2) a penetration-initiated capacitor discharge array for determining the impacting flux above a precisely defined threshold of 1.E-9 g; and (3) a meteoroid shield array incorporating three transducers on the front shield and one on the rear shield. The latter array determines the impact position and momentum exchange of the entire probe from the mass range 1.E-10 to 1.E-3 g, or larger. The techniques have been selected with a special regard for Halley's comet environment and the anticipated high flux rates based on experience from previous missions. The comparison of different detection techniques and correlation of independent sensor outputs has guided the design. Reliability of event detection in the unexplored environment of the comet is high and the limiting accuracy of the measurements is believed to be better than 20% over a dynamic range of 1.E+14. A microprocessor is used to monitor each sensor status and to process each event as well as the cumulative data, which represents the total event amplitude distribution from each sensor.