PZT Dust Impact Monitor (DIM) Onboard Rosetta/Philae: Experimental Results and Theoretical Background

Abstract

The Dust Impact Monitor (DIM) experiment on board the Rosetta spacecraft's lander, Philae, is a cube with three sides covered with piezo-electric sensors (total sensitive area: ~ 70 ccm), aimed at measuring the physical properties of millimetric and sub-millimetric dust particles that move near the surface of comet 67P/Churyumov-Gerasimenko. After being launched in March 2004, the Rosetta spacecraft encounters after a 10.5 years flight the comet 67P in mid 2014 and a landing of Philae is planned for mid-November 2014. In this work we study the performance of DIM based on impact experiments and compare the measurements with the sensor's expected theoretical behavior as derived from Hertz' theory of elastic impact. We present the results of impact experiments performed with spherical particles of different densities and elastic properties. We performed three types of experiment: (a) we analyze the performance of the different sensor sides under identical impacts, (b) we investigate the performance of DIM under impacts of different materials and different impact speeds, and (c) we report on the behavior of the DIM sensor with varying impact angle. We discuss the influence of the microstructure of the PZT sensor on the signal strength and its variation with position of the impacting particles. Our results show that the signal strength and the contact times measured with the DIM PZT sensors can be well approximated by Hertz' contact mechanics.