The CONSERT Instrument during Philae's Descent onto 67P/C-G’s surface: Insights on Philae’s Attitude and the Surface Permittivity Measurements at the Agilkia-Landing-Site

International audienceThe main scientific objective of the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) aboard ESA spacecraft Rosetta is the dielectric characterization of comet 67P/Churyumov–Gerasimenko’s nucleus.This was done by means of bi-static radio propagation measurements of the CONSERT instrument between the lander Philae launched onto the comet’s surface and the orbiter Rosetta. The CONSERT unit aboard the lander was receiving and processing the radio signal emitted by the orbiter counterpart of the instrument. The lander unit was then retransmitting a signal back to the orbiter. This happened at a time scale of milliseconds.In addition to the operation at the first science sequence, CONSERT was operated during the separation and descent of Philae onto the comet's surface.During the descent phase of Philae the received CONSERT signal was a superposition of the direct propagation path between Rosetta and Philae and indirect paths caused by reflections of 67P/C-G’s surface.From peak power measurements of the dominant direct path between Rosetta and Philae during the descent we were able to reconstruct the lander’s attitude and estimate the spin rate of the lander along the descent trajectory. Certain operations and manoeuvres of orbiter and lander, e.g. the deployment of the lander legs and CONSERT antennas or the orbiter change of attitude in order to orient the science towards the assumed lander position, are also visible in the CONSERT data.The information gained on the landers attitude is used in the reconstruction of the dielectric properties of 67P/C-G’s surface and near subsurface (metric to decametric scale).During roughly the last third of the descent, the comet’s surface is visible for the CONSERT instrument enabling a mean permittivity estimation of the surface and near subsurface covered by the instruments footprint along the descent path. The comparatively large timespan with surface signatures exhibits a good spatial diversity necessary for the mapping of dominant signatures and the estimation of the dielectric properties of prominent features. From this data it is possible to create a contrast and permittivity mapping of the comet’s surface in the vicinity of the Agilkia landing site