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CASSE - The ROSETTA Lander Comet Acoustic Surface Sounding Experiment - Status of some Aspects, the Technical Realisation and Laboratory Simulations

: Kochan, H.; Feibig, W.; Konopka, U.; Kretschmer, M.; Möhlmann, D.; Seidensticker, K.; Arnold, W.; Gebhardt, W.; Licht, R.


Planetary and Space Science 48 (2000), No.5, pp.385-399
ISSN: 0032-0633
Journal Article
Fraunhofer IZFP ()
surface characterization; surface inspection; acoustic wave; actuator

In 2003 the ROSETTA space mission to Comet 46P/Wirtanen will be launched by the European Space Agency (ESA). On board of the spacecraft there will be a Lander platform, which opens for the first time the possibility to carry out "in situ" measurements on a comet surface. The ROSETTA Lander experiment CASSE aims to investigate the outermost surface of the comet by means of transmitting, receiving and even passively monitoring acoustic waves in a frequency range from a few hundred to several kilohertz. In particular, by transmitting and recording compression and shear waves, the mechanical properties of the comentary surface will be investigated. To quarantee sufficient ground contact in any foreseeable surface topography, and in any composition of the material to be encountered, e.g. dust, sand, ice and mixtures of these materials, the acoustic transmitters (actuators), stacked piezoceramics, and triaxial commercial piezoelectric accelerometers as receivers, will be integrated in each of the three feet of the ROSETTA Lander. The Lander feet thus act as antennas for the transmitters and receivers. By switching different actuators and accelerometers, an analysis of the cometary surface material will become possible by direct foot to foot transmission of the acoustic signals. Further, an in-depth sounding will allow the detection of layered stuctures or embedded local inhomogeneities. The ground contact of the Lander will be strengthened by a harpoon providing a fixation force up to at least 5 N per foot.
This paper describes the first design of the Lander feet incorporating piezoceramic composite stacks as sound transmiters and receivers, and the experiments to test their efficiency. In the first part of this paper, the results of acoustic wave propagation experiments in ice, sand and olivine dust, performed in the laboratory are reported. These experiments verify that a signal strength of at least 20 dB above noise can be expected for aacoustic wave propagation in cometary analogue materials. The second part of the paper deals with the outdoor experiments, performed with sensors integrated into the Lander feet prototypes and placed in a sand filled construction container. The experiments serve to estimate the minimal range of detectability of the sounding signals.