Development of Thermal Sensors and Drilling Systems for Application on Lunar Lander Missions

Abstract

The upcoming lunar lander missions, for example Chang'e 2 from CNSA and several mission proposals and studies currently under consideration at NASA (e.g. Neal et al., ROSES 2006 Proposal to NASA, 2006 Potsdam, Germany, 2007) and JAXA, Japan (Matsumoto et al., Acta Astronautica, 59:68-76, 2006) offer new possibilities to measure the thermal properties of the lunar regolith and to determine the global lunar heat flow more accurately than it is hitherto known. Both properties are of high importance for the understanding of the lunar structure and the evolution of the Moon-Earth system. In this paper we present some work on new thermal sensors to be used for in situ investigations of the lunar soil in combination with novel drilling techniques applicable for the lunar regolith. Such systems may preferably be mounted on mobile stations like the lunar rover currently built for the Chinese Chang'e 2 mission. A general description of a presently tested prototype of the lunar rover is given and mounting possibilities for a drilling system and thermal sensors are shown. Then we discuss some options for thermal sensors and drills and how they could be combined into one compact instrument. Subsequently a tube-like sensor suitable for measuring the thermal conductivity of the material surrounding a borehole 123Earth Moon Planet (2008) 103:119-141 DOI 10.1007 is described in more detail. Finally the performance of such a tube-shaped sensor when applied in a lunar borehole is investigated by thermal modelling and compared with the behaviour of a more conventional needle-shaped sensor.

Thermal conductivity measurements of coarse-grained gravel materials using a hollow cylindrical sensor

“…After adequate heating time, the temperature increase versus lnt becomes linear. For standard line heat sensors, the thermal conductivity k can then be calculated by using the following relation derived in [1] and explained in more detail in [6] and [7]:…”

The heated infinite cylinder with sheath and two thermal surface resistance layers

“…The main disadvantages of very thin wires are the lack of mechanical stiffness, the poor thermal contact with the medium, and the convection caused by moisture evaporation in the vicinity of the wire, the last effect being much less important in thick sensors [23]. Especially in instruments onboard space missions (landers or rovers) one is forced to use thicker cylinders to support remote mechanical insertion [13]. The heating of such probes is achieved by wire coils or metal layers deposited on carrier substrates like Kapton [20].…”

“…In certain applications, such as conductivity measurements with cylindrical probes, this resistance may play a significant role if the contact to the surrounding medium is poor. Such a situation can occur, for instance, when measuring the thermal conductivities of surface material on the Moon or comets [13,14,9]. The first application of this kind was made in the framework of the lunar conductivity measurements (Apollo 15 and 17), as described in [15][16][17][18].…”

Performance of thermal conductivity probes for planetary applications

“…Thus, the results of the numerical simulations can be used to analyze the measurement results for those glass beads. The value of contact conductance used for poor contact was taken from Kömle et al (2008) where it was derived for lunar conditions. As good contact a hundred times better contact conductance was considered.…”