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The first — and possibly deepest — in situ science measurements on the 46P/Wirtanen nucleus will be made by two sensors of the Rosetta Lander's MUPUS experiment. A piezoelectric shock accelerometer (ANC-M) and a resistance temperature sensor (ANC-T) will be mounted in the Lander's harpoon anchor. This will be shot into the surface at about 60 m s −1 on touchdown, reaching a final depth of between a few centimetres and about 2.5 m, depending on the hardness of the ground and the maximum available cable length. Early indications of the strength of the surface material and any distinct layers should prove valuable to subsequent depth-sensitive investigations, including the MUPUS thermal probe, seismic sounding experiments, the sampling drill and composition analyses of the extracted material. Interpretation of the ANC-M data will help to constrain models of the formation and evolution of the material found at the landing site and document the mechanical and structural context of nearby sampled material. We report on the results of recent test shots performed with a prototype anchor into several porous materials: two types of glass foam, H2O ice and CO2 ice. With the help of data from direct shear tests and quasi-static penetration tests, we interpret the processed deceleration data using a cavity-expansion penetration model. Layers of distinctly different strengths can be detected and located, and the deceleration profiles are in reasonable agreement with the profiles obtained by quasi-static tests. The anchor projectile's long sharp tip tends to smear out the boundaries, however. In applying the penetration model we found that the coefficient of sliding friction and the target's volumetric strain have a much stronger influence on the deceleration profile than the initial target density and angle of internal friction. Very small values of volumetric strain (corresponding to high ‘drag coefficient’) were required to fit deceleration profiles to the measured data for the glass foam, contrary to what we initially expected by inspecting the thin layer of crushed material around the walls of the penetrated channel. We interpret this to mean that such brittle, porous materials as the glass foam (and perhaps highly porous, brittle, cryogenic ice) do not exhibit plastic deformation before failing completely by the crushing of cell walls. The decelerating forces are thus thought to be dominated by momentum transfer to the crushed material and by the crushing strength of the cellular microstructure, rather than by the force required to deform the target plastically. The cavity-expansion model seems to be well-suited to the ice shots, but for the brittle, cellular glass foam, alternative approaches, taking into account the material's microstructure, are needed. As a first step in this direction, a microstructural model linking textural properties of the material (pore and grain size, and relative contact area between grains) is applied to the glass foam data, obtained from quasi-static penetration tests and from direct shear strength tests. It is demonstrated that the dependence of strength on porosity can be well represented by the model suggested. A microstructural model for sintered ices, relating strength properties to porosity and thermal properties, would be useful for interpretation of MUPUS ANC-M data in the context of other physical properties measurements. The work presented here may also have some relevance to the design of future comet landers or penetrators. The harpoon anchor/penetrometer approach could be employed on other minor body landing missions, while the modelling technique is similar in many ways to that appropriate for other penetrometers/penetrators.

Owing to the low surface gravity of the Rosetta target comet 46P/Wirtanen, a means of anchoring the Rosetta Lander to the cometary surface will be necessary. This task can be accomplished by firing an anchor into the cometary soil immediately after touchdown to prevent a rebound of the spacecraft from the surface or subsequent ejection by other forces, and to allow for mechanical activities (drilling, etc.) at the landing site. The rationale for anchoring is examined, based on estimates of the main forces likely to act on the spacecraft after landing. We report on the development of an anchoring device using a pyrotechnic gas generator as a power source and an instrumented anchor. In addition to the anchoring function, which is the primary purpose of this system, the integration of acceleration and temperature sensors into the tip offers the possibility to determine some important material properties of the cometary surface layer. The accelerometer is designed to measure the deceleration history of the projectile and is thus expected to give information on how the material properties (in particular strength) change within the penetrated layer(s), while the temperature sensor will measure temperature variations at the depth at which the anchor finally comes to rest. As the mechanical properties of the material are not known, it is difficult to predict the final depth of the anchor with any great certainty, but it may well be greater than that reached by any other of the lander's instruments. The instrumented anchor will be part of the MUPUS experiment, selected to form part of the Rosetta Lander payload. We report on results of laboratory simulations of anchor penetration performed at the Institut fur Weltraumforschung, Graz, and compare these with models of projectile penetration. The value of the results expected from the penetrometry experiment in the context of an improved understanding of cometary processes is discussed.