Calibration of the Dust Impact Monitor DIM onboard Rosetta/Philae

The Rosetta lander spacecraft Philae will land on the nucleus surface of comet 67P/Churyumov-Gerasimenko in late 2014. It is equipped with the Dust Impact Monitor instrument (DIM). DIM is part of the SESAME instrument package onboard Philae [Seidensticker et al., 2007] and consists of three piezoelectric PZT sensors. Each sensor is mounted on the outer side of a cube, facing in orthogonal directions, this way allowing for the detection of grains approaching normal to the nucleus surface and from two horizontal directions. DIM's total sensitive area is approximately 70 cm2. It will measure impacts of sub-millimeter and millimeter sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma by the escaping gas flow. A grain-size dependent fraction of the emitted grains is expected to fall back to the nucleus surface due to gravity. DIM will be able to detect both these components, the backfalling particles as well as the grains hitting the detector on direct trajectories from the surface. With DIM we will be able to measure fluxes, impact directions as well as the speed and size of the impacting cometary particles. We can simulate impacts onto the DIM sensor with particles of dierent materials (steel, glass, ruby, polyethylene), particle radii between 0.2 and 1mm and impact speeds up to 2m/s. Cometary grains on ballistic trajectories will have impact speeds below the escape speed from the nucleus surface (approximately 1.5m/s), thus the impact speeds achievable by our experiments cover the range expected at the comet. In this speed range the impact process can be described by Hertzian theory of elastic impact [Johnson, 1987], and in particular the voltage and contact time measured upon impact can be well described analytically [Seidensticker et al., 2007]. We present results of our impact experiments with two ight spare units of the DIM sensor, discuss the applicability of Hertzian theory for impact calibration of the sensor and discuss implications for our measurements at the comet