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HP3 – the Heat Flow and Physical Properties Package – is an experiment package on-board the upcoming NASA Mars Mission InSight (Interior Exploration Using Seismic Investigation, Geodesy, and Heat Transport).The InSight Mission will investigate the interior structure of Mars using seismoligical and geodetical measurements and quantify the planetary heat budget by measuring the surface planetary heat flow. InSight is scheduled to be launched in May 2018 and to land on Mars at end of November 2018. The main payloads of the InSight lander are a seismometer (SEIS), the HP3 heat flow probe, as well as the Rotation and Interior Structure Experiment (RISE). An ancillary sensor package consisting of atmospheric pressure and temperature sensors (APSS) as well as a magnetometer complement the payload. After landing on Mars the seismometer and HP3 will be deployed onto the Martian surface by the robotic arm of the lander. HP3 is the contribution of DLR (Deutsches zentrum für Luft und Raumfahrt, Germany) to the InSight mission. It is designed to determine the geothermal heat flux by penetrating down into the Martian surface to at least 3m, with the goal of reaching 5m depth. HP3 measures the thermal conductivity as function of depth during the penetration phase, and the thermal profile of the subsurface will be monitored for a full Martian year after reaching the final depth. HP3 is composed of the following subsystems: • A set of thermal sensors to determine thermal conductivity and subsurface temperature (TEM) • A self-penetrating probe (termed the mole) to emplace sensors in the subsurface • Two measurements suites to determine the depth of the thermal sensors (TLM & STATIL) • A radiometer to determine the surface temperature forcing (RAD) • The instrument main (backend) electronics (BEE) The HP3 deployable elements are housed inside a support structure, and electrical connections to the lander and BEE are provided by the HP3 supply tethers. The support structure also guides the mole during its initial penetration into the surface.The mole is a mechanically actuated force hammering device for soil penetration. It pulls a tether with thermal sensors and supply lines behind. By penetrating into the Martian subsurface the thermal sensors (TEM) will be deployed. The mole is equipped in its back part with the subsystem STATIL (STATIC Tilt Measurement Suite) which determines the mole inclination during the penetration. The length of paid-out tether is measured by the TLM subsystem (Tether Length Monitor). The path of the mole and the depth of the thermal sensors will be deterimed from the TLM and STATIL data. The radiometer is mounted