SESAME Observations during the Philae SDL and FSS Phases

Abstract

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.