Dust Impact Monitor DIM Onboard Rosetta / Philae: Comparison of experimental Results and the Theory behind the Experiment

The Rosetta lander spacecraft Philae will land on the nucleus surface of comet 67P/Churyumov-Gerasimenko in November 2014. Philae is equipped with the Dust Impact Monitor (DIM). DIM is part of the SESAME instrument package onboard Philae [Seidensticker et al., 2007] and employs piezoelectric PZT sensors to detect dust particle impacts. The sensors are 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 cm**2. 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

SESAME Observations during the Philae SDL and FSS Phases

SESAME (Surface Electric Sounding and Acoustic Monitoring Experiment) that comprises the three instruments CASSE (Comet Acoustic Surface Sounding Experiment), DIM (Dust Impact Monitor) and PP (Permittivity Probe) was operated at various instances during the Philae SDL (Separation, Descent, Landing) and FSS (First Science Sequence) mission phases. During SDL all three instruments conducted calibration as well as science measurements and operated until after the first touchdown of Philae. At the final landing site of Philae several measurements were executed by all three instruments. We will present a short overview of the SESAME instruments and the measurements conducted during the Philae SDL and FSS mission phases. Finally, the first results are presented. CASSE sensors are housed in the six soles of Philae’s landing feet, thus being the first part of the spacecraft that has mechanical contact with the comet. CASSE registered the touchdown at 15:34:04 (hh:mm:ss) UTC. The accelerations measured by the CASSE indicate a non-vertical touchdown. The surface at the intended landing site "J" appears to be covered by a dust layer of several centimeters thickness over a hard matter layer with a reduced Young's modulus exceeding 1 MPa. At the final landing site CASSE was able to record the MUPUS-PEN hammering. We hope to able to determine from these data sound velocities and from these elastic properties at the final landing site. The DIM instrument was operated during descent as well as five times during the FSS phase. During descent DIM registered only one, most likely fluffy particle at near 2.85 km from the comet. From a preliminary analysis we deduce a size from 1.15 to 2.28 mm and an impact velocity in the range from 0.14 to 0.96 m/s (assuming a reduced Young's modulus between 50 to 500 MPa and a bulk density range between 0.35 and 0.40 g cm-3). No particles were detected at the final landing site during 5 h of observations. This might indicate a low activity there, what is not astonishing, as the final landing site is close to the edge of the illuminated cometary surface. PP calibration measurements shortly after separation with an unfolded landing gear are partially strongly disturbed. At the final landing site, PP conducted four measurements using the feet electrodes (two as receivers, one as transmitter). The use of the transmitter electrodes on the MUPUS-PEN and APXS for the quadrupole measurement was not possible due to the lack of operation time of Philae. Using a preliminary calibration there are strong indications that at least under part of Philae there is a large amount of water ice

Cosmic ray dose monitoring using RadFET sensors of the Rosetta instruments SESAME and COSIMA

On its more than 10 years journey to comet 67P/Churyumov-Gerasimenko, Rosetta carried RadFET ionising dose monitors in the central electronics of the orbiter instrument COSIMA and the lander instrument SESAME. The readings of the dosimeters were corrected for the temperature of the devices during measurements. Because the sensitivity of RadFETs depends on the energy of impinging charged particles, a mean efficiency factor for the prevalent proton radiation was determined by applying nine efficiency models to proton energy spectra of Rosetta's radiation environment. The resulting dose profiles show linear increases of the accumulated dose with time, mainly caused by galactic cosmic radiation, and the arrival of two solar particle events in 2005. The accumulated dose (in Silicon) during 3909 days in space from 2004-03-02 to 2014-11-14 was 3.2 +/- 0.6 Gy in case of COSIMA and 1.9 +/- 0.4 Gy for SESAME. The deviation of the two measurements is mainly due to the solar particle event in September 2005, which had a 5.3 +/- 1.0 times stronger impact on the COSIMA RadFET. Measured dose levels are one order of magnitude lower than those expected before launch for not being exceeded on the 90% confidence level, which is mainly due to the low solar activity during the mission so far. (C) 2016 IAA. Published by Elsevier Ltd. All rights reserved

Dust Impact Monitor (SESAME-DIM) Measurements at Comet 67P/Churyumov-Gerasimenko

Context. The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae carries the Dust Impact Monitor (DIM) on board, which is part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME). DIM employs piezoelectric PZT sensors to detect impacts by submillimeter- and millimeter-sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma. Aims. The DIM sensor measures dynamical data such as flux and the directionality of the impacting particles. Mass and speed of the particles can be constrained assuming density and elastic particle properties. Methods. DIM was operated during three mission phases of Philae at the comet: (1) Before the separation of Philae from Rosetta at distances of about 9.6 km, 11.8 km, and 25.3 km from the nucleus barycenter. In this mission phase particles released from the nucleus on radial trajectories remained undetectable because of significant obscuration by the structures of Rosetta, and no dust particles were indeed detected; (2) during Philae’s descent to its nominal landing site Agilkia, DIM detected one approximately millimeter-sized particle at a distance of 5.0 km from the nucleus’ barycenter, corresponding to an altitude of 2.4 km from the surface. This is the closest ever dust detection at a cometary nucleus by a dedicated in situ dust detector; and (3) at Philae’s final landing site, Abydos, DIM detected no dust impact which may be due to low cometary activity in the vicinity of Philae or due to shading by obstacles close to Philae, or both. Results. Laboratory calibration experiments showed that the material properties of the detected particle are compatible with a porous particle having a bulk density of approximately 250 kg

First results of on-comet observations by SESAME/PP

The Permittivity Probe (PP) as part of the SESAME instrument group performed measurements directly before Philae's separation from Rosetta, during its descent and on the first day on the comet surface at Philae's final landing position. The purpose of PP is the monitoring of the electrical properties of the comet's subsurface material down to a depth of about 2 m and their variation with temperature and distance from the Sun. The used implementation is especially sensitive to the presence and properties of water ice inside the observed volume. As a secondary objective the instrument tries to observe low-frequency plasma waves below 20 kHz as generated by interaction between the comet and the solar wind. The descent measurements were mainly intended to calibrate the instrument in its final configuration with deployed landing gear and without being influenced by the vicinity of the Rosetta structure, while the environmental properties like permittivity of vacuum were known. Possible deviations from this assumption were monitored by RPC and will be taken into account during the detailed data analysis. Unfortunately these measurements were heavily distorted by saturation of the sensitive pre-amplifiers in the landing gear feet, which were for the first time confronted with CONSERT radio pulses operating with deployed antenna. While PPís own transmitter for injecting currents into the medium worked nominally and the measured data can be used for calibrating the injected currents on the comet surface, probably only a small sub-set of the receiver data can be used for the intended stray capacitance calibration after post processing to extract the few time intervals where the receivers were not in saturation. The on-surface measurements were all performed during the initial safe-mode interval while the illuminated period changed into comet night, allowing the observation of the temperature dependence of the measurement parameters. Though the second transmitter electrode attached to the MUPUS PEN and the APXS sensor, not-yet deployed at that time, could not be utilized, the first interpretation of data collected in this reduced configuration indicate the presence of cold water ice at least underneath the -Y foot of Philae. The distance between the +Y foot and the surface is either too large to see a similar effect or the subsurface material in the partly sun-lit area of the current location is depleted of ice. Once the additional transmitter electrodes at the deployed MUPUS PEN and APXS can be utilized during a later LTS mission, the initial measurements can be re-evaluated in more detail

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

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