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'Cometary Acoustic Sounding Surface Experiment'
Primary Investigator: D. Möhlmann, DLR Institute of Space Simulation, Cologne Germany
The The CASSE group at DLR, Cologne.
The Cometary Acoustic Sounding Surface
Experiment (CASSE) instrument onboard the Rosetta Lander
consists mainly of several piezo-electric acoustic transmitters and receivers
(accelerometers) placed on the Lander's feet.The transmitters make use of the piezo-electric effect: When a piezo-active material, such as Quartz, Turmalin or special ceramics, gets applied to a electric voltage, it reacts with a deformation. Vice versa, mechanical stress produces an electric charge (voltage) on the surface of the crystall. 'Piezo' is derived from the greek word for 'to press', piezein. The piezo-electric effect was discovered 1880 by Pierre and Jacques Curie.
Providing an AC voltage, this effect produces mechanical vibrations of the material and therefore generates sound waves (=loud speaker principle). Each of the lander's foot is equipped with such an actuator to acoustically excite the foot. The foot couples to the ground and passes the acoustic wave to the cometary material, where it propagates along the surface, but also penetrates the soil.
The conversion from acoustic waves into electrical signals (microphone principle) is performed by piezo-electric accelerometers: A small 'seismic' mass is mounted on top of a piece of piezo-ceramics. When accelerated, this mass applies, due to its momentum of inertia, a force to the ceramic, which produces an electric charge proportional to the acceleration. This allows to measure the acceleration and, after integrating two times, the displacement of the sensor.
The piezo-transmitters and receivers offer two operation modes of CASSE:
- passive mode, where the receivers just listen into the cometary nucleus, to monitor the seismic and acoustic activity, and
- active mode, where sound is used to measure some mechanical properties of the surface material and the structure of the subsurface layers, like an acoustic Radar.
First preparatory experiments at DLR's Institute of Space Simulation, Cologne, proof the feasibility of CASSE, i.e. we will be able to determine the elastic properties of the cometary surface layer, and also to see structures beneath the surface, especially the transition zone from surface regolith to pristine material, which is expected to be in the range of some meters.
More about the scientific goals of CASSE...
[ Deutsch | Planetary physics | SESAME | Foot Design | Experiments | Sci. Goals ]