HP³ is a so-called instrumented, electromechanical mole equipped with an active as well as passive Thermal Measurement Suite (TEM-A and TEM-P) and an accelerometer and tiltmeter (ACTIL), which penetrates through the soil via an internal hammering mechanism. The thermal sensors are placed on a 5 meters long ‘Science Tether’, which shall measure temperature and its gradient until this depth. The instrument will address a variety of fundamental questions on planetary development, habitability and geological stratification through weathering. HP3 has been proposed as part of the Humboldt payload of the ExoMars mission before the mission’s re-definition in early 2009 resulting in cancellation of the geophysics package. During the course of the downselection of the Discovery program of NASA in 2012, the mole was selected as a payload of the mission InSight, which will be launched to Mars in early 2016. Furthermore, other possible applications of the instrument such as on the Moon (International Lunar Network) are envisaged.
The Mole is based on the PLUTO Mole (PLanetary Underground TOol) developed by DLR for the Beagle2 mission. The Mole penetrates into the soil pulling the tether behind it. The forward motion is realized by means of a hammering mechanism. The Mole contains the active temperature sensors TEM-A as well as ACTIL and the necessary front-end electronics (FEE), which is isolated from high shock levels by a damping mechanism. Energy and data is exchanged via the science tether between the Mole and the support structure, an Engineering Tether brings data towards the lander. During the mole hammering phase into the soil, the depth profile of physical properties such as the heat conductivity and temperature distribution below the surface is obtained. After the mole has reached its final depth, the package will monitor the temperature gradient on a long-term basis along the depth of the borehole to determine, the planetary heat flow together with the physical soil properties measurements.