Surface Mechanical Properties of Comet 67P

Kurzfassung

In August 2014 the ESA spacecraft Rosetta encountered the comet 67P/ChuryumovGerasimenko. The overall objective of the Rosetta mission was to determine physical and chemical properties of comet 67P. The orbiter of Rosetta was itself an instrument platform. It also carried the lander Philae that landed on the comet’s nucleus on November 12th 2014. Philae had ten different instruments on board including the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME) comprising the Comet Acoustic Surface Sounding Experiment (CASSE), the Dust Impact Monitor (DIM), and the electric impedance probe (PP). The mission ended on September 30th 2016 with the deorbiting of Rosetta onto comet 67P. The Comet Acoustic Surface Sounding Experiment (CASSE) was housed in the soles of Philae’s landing-gear feet, see Fig. 1a. The acceleration signals at the first landing site Agilkia which occurred in the first seconds of the touchdown at an impact velocity of approx. 1 m/s were recorded by CASSE (Fig. 1b) 1. The inversion of the data based on the transfer function of the landing gear and the foot soles yielded the compression strength and the elastic modulus of the cometary soil. Both the compression strength and the elastic modulus of the surface of comet 67P are very low compared to commonly used engineering materials 2. The data support the concept that the elastic (E) and strength properties (σ) of the comet material correspond to very porous solids constituted by the regolith particles with porosities up to 80%. The σ/E ratio ≈ 1/1000 agrees well with relations known in material science 3. The data also allow an estimate of the fracture toughness K1c which is compared with data for crack growth observed in the neck of the comet 4. Furthermore, Short-term Fourier-Transforms of the signals give additional hints how the lander comet contact evolved in time. The Multi-purpose Sensors for Surface and Sub-Surface Science (MUPUS) listening experiment in joint operation with CASSE. The main Goal of MUPUS was to measure the thermal properties of the surface comet material5 . To this end an instrumented rod was hammered into the ground.