The Cometary Acoustic Surface Sounding Experiment (CASSE) is an experiment to investigate the surface and subsurface of comet 67P. A set of sensors have been built into the feet of the lander which will have two functions:
To listen for noise produced within the nucleus of the comet caused by various sources (expansion and contraction from heating and cooling, impacts or seismic events). In this way it operated in a similar way to a seismometer on Earth, used to characterise earthquakes.
To generate sound and use the reflections of that sound to provide information about the material that it has passed through, like a sonar. This could provide information on any layering within the comet, as well as holes and other features.
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