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Citation

Hagermann A, Nadalini R, Spohn T, Richter L, Ambrosi R, Ball AJ, Hall C, Knollenberg J, Lichopoj A, Messina G, Mucklow M, Romstedt J, Sohl F & Zarnecki J (2007) HP3: An instrument suite to explore the subsurface of terrestrial planets: Prototype development and test campaign results. In: International Astronautical Federation - 58th International Astronautical Congress 2007. 2. International Astronautical Federation - 58th International Astronautical Congress 2007, 24.09.2007-28.09.2007. Hyderabad India: International Astronautical Federation, pp. 892-904. https://www.scopus.com/inward/record.uri?eid=2-s2.0-54949144340&partnerID=40&md5=d79e551602b629286a6a950a2d79f5a7

Abstract
The terrestrial planets, and many of the major moons in the solar system, have considerable portions of their surfaces covered by regolith layers storing in their physical properties a record of the processes having taken place during and after its deposition. The study of these properties below the immediate surface allows a glimpse in the bodies' past for periods of up to billions of years. Due to the general scarcity of lander missions and the lack on them of instruments able to reach the subsurface, access to depths beyond a few tens of centimetres still remains to be accomplished. The results from Viking and MER missions show that already investigations of the first layers of soil are extremely interesting. In the last decade, thanks to continuous financial support from ESA and DLR, a new class of lightweight and very efficient tools has been developed to explore the regolith up to depths of a few meters: the Moles (i.e. PLUTO, the sampling Mole on board Beagle 2). Combining the efficiency of Moles with the most advanced in situ measurement techniques, our team has developed and built a fully functional prototype of HP3, the Heat flow and Physical Properties Package, a suite of sensors able to measure up to six parameters of the soil investigated. HP3 is an integrated instrument suite, deployed in the soil by a dedicated Mole, up to depths of 5m. While descending and once in place, it performs in-situ measurements of the regolith status and properties. Three different sensors compose HP3: thermometers and heaters measures the thermal status and the thermal properties; accelerometers and inclinometers derives the mechanical characteristics and the precise position of the tool, while a γ-ray Compton backscattering densitometer measures the density of the surrounding material. By combining the data, HP3 can derive the surface heat flow, thus providing data on the internal structure and thermal evolution of the target planet. In the chosen configuration, HP3, housed in a cylindrical container, is pulled into the soil by the deployment Mole (the "tractor"). A flat Copper-Kapton tether connects the ensemble to the lander element and houses the thermal sensors. In the course of the study, a prototype model of the system has been built and extensively tested for system functionality and performance of the various sensors. Multiple tests have been performed in vacuum, at normal and at Martian atmospheric pressure to verify that the design is compatible with a Martian mission and that HP3 is able to measure the required parameters even under the temperature and pressure conditions expected on Mars. The prototype and all the results of the test campaigns are presented in the present article, together with the expected near future developments. Copyright IAF/IAA. All rights reserved.