Computer modelling of a penetrator thermal sensor

Abstract

The Philae lander is part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko. It will use a harpoon like device to anchor itself onto the surface. The anchor will perhaps reach depths of 1-2 m. In the anchor is a temperature sensor that will measure the boundary temperature as part of the MUPUS experiment. As the anchor attains thermal equilibrium with the comet ice it may be possible to extract the thermal properties of the surrounding ice, such as the thermal diffusivity, by using the temperature sensor data. The anchor is not an optimal shape for a thermal probe and application of analytical solutions to the heat equation is inappropriate. We prepare a numerical model to fit temperature sensor data and extract the thermal diffusivity. Penetrator probes mechanically compact the material immediately surrounding them as they enter the target. If the thermal properties, composition and dimensions of the penetrator are known, then the thermal properties of this pristine material may be recovered although this will be a challenging measurement. We report on investigations, using a numerical thermal model, to simulate a variety of scenarios that the anchor may encounter and how they will affect the measurement.

Thermal and microstructural properties of fine-grained material at the Viking Lander 1 site

“…The structure of the lander will block direct sunlight, part of the longwave radiation from the sky, emit thermal radiation and reflect shortwave radiation. Similar considerations have to be made when making measurements with temperature sensors delivered on or inside subsurface probes ( Hagermann and Spohn, 1999;Paton et al, 2012Paton et al, , 2010. Conduction between the regolith and the probe is an important consideration but in our case the footpad sensors are far enough away from any subsurface structures that they will not be affected.…”

Thermal Stability of Water Ice in Ceres' Craters: The Case of Juling Crater

“…The Hertz factor is defined as the ratio between the grain‐to‐grain area of contact and the cross section of the grain. It can strongly affect the thermal history of a planetary body as well as a cometary type objects since it can substantially modify the thermal conductivity (and so the thermal inertia) of such objects (Kossacki et al, ; Paton et al, ). The range of variability of this parameter is wide: from 0.1 to 10 −4 , based on the KOSI (Kometensimulation) experiments (Huebner, ).…”

Investigating thermal properties of gas-filled planetary regoliths using a thermal probe

“…A numerical model can be used to invert the data (Hagermann and Spohn, 1999) if the thermal properties of the penetrator are known. If such a model is applied to the measured temperature to derive the thermal properties as in Paton et al (2010), and the heater-sensor is not thermally isolated from the penetrator, then an uncertainty in knowing the penetrator thermal properties will be transferred to the derived thermal properties of the target material.…”

“…For example the Deep Space 2 measurements used the rate of cooling to determine thermal properties of the surrounding Martian regolith. If the thermal properties of the penetrator are known then the thermal properties of the surrounding material can be determined (Urquhart and and Smrekar, 2000;Paton et al, 2010). Numerical models are required to solve the heat transfer equation as a penetrator will be far from the idealised geometry and the homogenous composition that is a requirement for the application of an analytical solution.…”