Calibration of the Galileo/Ulysses dust detectors with different projectile materials and at varying impact angles

Abstract

A dust detector for the two forthcoming deep-space probes GALILEO and ULYSSES (the former ISPM) was developed at the Max-Planck-Institut für Kernphysik in Heidelberg, F.R.G. This paper gives a description of the detector, describes its scientific purposes and reports the results of calibration tests. The detection probability for particles coming from different directions was calculated in a computer simulation. This calculation led to the following values. (1) The maximum sensitive area is 0.1 m 2. (2) Under the assumption of an isotropic flux 50% of all impacts occur at angles lower than 32° (relative to sensor axis). (3) The effective viewing cone of the instrument is 1.4 sr. Its actual viewing is larger (70°), but the detection probability decreases strongly for larger incidence angles. For the calibration tests we chose particles (speed range from 1 to 70 km s −1, mass range from 10 −15 to 10 −10 g) of different compositions (silicate, carbon, iron) and shot them onto the detector under different impact angles. These materials should represent the compositional range of interplanetary dust particles. Since the detector neither determines the chemical composition nor the impact angle of an impinging particle, we determined the calibration by averaging over impact angle and an assumed compositional distribution. This averaging leads to increased uncertainties of factors 1.6 for impact speed and 6 for mass. The speed dependent sensitivity threshold is 1.2 × 10 −13 g at 5 km s −1, 1.5 × 10 −14 g at 10 km s −1, 2.0 × 10 −15 g at 20 km s −1 and 1.3 × 10 −16 g at 40 km s −1. Masses are determined in a range of 10 6 above the sensitivity threshold.