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CASSE - Cometary Acoustic Surface Sounding Experiment

Measuring modes of CASSE. Only one sole per Lander foot is shown.

The CASSE instrument shall investigate the cometary surface by acoustic sounding methods. To this end the soles of the Lander feet, which can induce and receive sound waves into and from the surface matter, are used both as transmitters and receivers. The landing gear of the cometary lander Philae has a three-leg structure, where each foot has two soles that can move in height against each other. This ensures a good contact with the rough cometary surface.

The acoustic sounding and receiving system of CASSE has two principal operation modes:

• Listen passively, like a seismometer, into the comet nucleus and locate noise sources, caused by thermal stress, seismic effects, particle impacts or Lander instruments.

• Perform actively, like a sonar, a sounding of the surface layers. One can calculate the elastic constants of the cometary surface from the measured sound velocities c_P and c_S. It is intended to conduct a deep sounding in order to detect layering and other inhomogeneities, like holes. This requires a complex analysis of the acoustic signals.

Transmitter (left, fixed between brownish glass fiber mountings at the sole edge). In order to avoid electrical disturbances for PP, the transmitter are wrapped in gold-colored, conducting Kapton foil. On the inner side of the cover (right) an impact protection device is mounted. It should limit the bending of the sole during landing thereby avoiding any break.

Three soles (one for each foot) contain piezo-electric transducers (transmitters, see  figure on the right) that excite acoustic waves within the sole. These signals will be registered by means of tri-axial piezo-electric accelerometers (see figure on the lower right) in the other three soles of the Lander Philae. Thermistors (so-called Pt 1000) on each piezo-electric sensor will measure the temperature.

CASSE has been developed at the DLR Institute of Space Simulation and was built in collaboration with the engineering company von Hoerner & Sulger (electronics). The transmitters were developed, designed for the expected cometary conditions, and built at the FraunhoferInstitute for Nondestructive Testing (IZFP) in Saarbrücken, Germany.

Properties of CASSE
Sounding frequency	0.1 to 6 kHz
Receiving frequency	< 100 kHz (at most 12 channels simultaneously)
Temperature range	-160 to +100 °C
Mass (without soles)	550 g
Power	≤ 1250 mW

Accelerometer (right between brownish glass fiber mountings) in the sole of one foot of Philae. On the inner side of the cover (left) one can see an impact protection device. It should limit the bending of the sole during landing thereby avoiding any break.