Space exploration comes with boundless fears and unknowns – malfunctions that could take lives or leave astronauts stranded in space. Who would guess that something as familiar as dust would cause dangerous risks such as overheating spacecraft? Well, that’s exactly what has happened. During several Apollo missions, deposits of lunar dust accumulated on the battery cooling radiator of the Lunar Roving Vehicles, causing overheating of the battery.
Lunar dust is fine and sharp – a severe combination. The thin particles are easily kicked up by minimal disturbances and then cling onto surfaces – accumulating into a thicker layer that weathers and overheats the mechanics. To understand how this layer of lunar dust causes overheating, one must first understand how heat transfer works on the Moon – an area with minimal atmosphere. In space, direct heat transfer from contact with another material (conduction) or wind (convection) are small in comparison to radiative heat transfer. Radiative heat transfer describes the process by which heat moves from one object to another through electromagnetic waves, like the warmth you feel from sunlight, without needing direct contact or a medium like air.
In the space industry, the radiative heat transfer of an object is mainly described by two values, the heat entering the system by absorbing sunlight (𝛼) and the heat emitted by the surface (𝜀). This ratio 𝛼/𝜀 is carefully designed and implemented into the exterior coating of man-made spacecraft and satellites for thermal management, with 𝛼 minimized and 𝜀 maximized. Adding even a little dust to this controlled surface can drastically alter the thermal properties, especially when radiative heat is the primary source of heat transfer in space.
How much dust can accumulate before there is risk of danger? Researchers from the European Space Agency, led by Alice Suarez-Kahan, attempt to answer this question by characterizing the thermal conductivity of simulated lunar and Martian dust to develop mitigation methods. Specifically, they analyzed how the thermal-optical properties of a surface are impacted by the size of dust particles and the coverage following dust deposition.
The dust was applied to quartz window with different levels of coverage, confirmed and quantified with optical microscopy. The portable reflectometers from Surface Optics Corp, the SOC 410-Solar (335 nm to 2500 nm) and SOC ET-100 (1.9 μm to 21 μm) were used to determine the ratio of solar absorption to thermal emission as a function 𝛼s / 𝜀IR of dust coverage (Figure 1a). Experimental values of absorptance slightly differed from theoretical values, with the experimental absorptance values higher, particularly with higher coverage (Figure 1b). This is likely due to the non-linear deposition of the dust. Overlapping areas and a non-uniform coating led to a complex reflectance profile as illustrated in Figure 2.
The discrepancy demonstrates the necessity to properly understand how deposition, size of dust particles, and placement affect optical properties and thermal conductivity in order to mitigate the risks posed by dust during missions on the Moon and Mars. This work is ongoing with the ESA planning to improve their simulated dust samples and experimental set up, with the goal of fully understanding the process of dust deposition and its effect on thermal conductivity in a vacuum environment.
For more information about our solar absorption and infrared emittance measurements, reach out to djacobsen@surfaceoptics.com
Published Research
Kahan, A & Delacourt, B & Dias, N & Hołyńska, Małgorzata & Tighe, A. (2023). Characterisation of lunar and Martian dust simulants and impacts on thermo-optical properties of space materials. IOP Conference Series: Materials Science and Engineering. 1287. 012036. 10.1088/1757-899X/1287/1/012036. https://iopscience.iop.org/article/10.1088/1757-899X/1287/1/012036/pdf
