Moon Day-/Nighttime Temperature Map; © by NASA
Roving on the moon can be a toasty experience, or a chilly one. The minimum temperature on the lunar surface is -183°C (90K) and the maximum +117°C (390K). This extreme environment can cause significant stresses to the technology used on the moon. One specific stress is due to exposure to the Sun's radiation. During the lunar day, the Sun is shining from above and is also being partially reflected back up from the lunar surface. Ideally, we would like to keep as much of this heat as possible outside of the rover. In addition, there is heat being generated by the electronic components inside the rover. This heat must be transported to the rover's surface where it can be radiated away. Thus, the rover should be designed so that the radiation from outside is reflected away while the heat from inside is brought to the surface and then radiated to open space. To understand how this is accomplished, we need to have a closer look at how most materials respond to heat and radiation.
In this picture you can see the spectrum of the sun and the amount of energy it emits depending on frequency of radiation. The sun itself is a so-called “Black body” and emits the most energy at 5777K, which is a nice bright blue color. The electronics on the other side emit most of its energy at 1000K and below. © by pts.com
When radiation strikes a surface, its energy is reflected, transmitted, or absorbed. In addition any surface will emit radiation. When radiation is reflected, the energy is immediately sent back out into the environment. Transmitted radiation will pass through an object with little or no modification, that means, you can see through it. Radiation energy, which is absorbed by a material, is typically converted into heat energy (or electricity in the case of solar panels). Any material that absorbs radiation is also capable
of emitting radiation. The emitted radiation is typically of the same spectral characteristics as the absorbed radiation. If an object is absorbing more energy than it emits, then its temperature will rise. If an object is emitting more energy than it absorbs, then its temperature will fall. Thermal equilibrium is obtained when the amount of energy emitted by an object equals the amount of energy absorbed by an object. For example, the lunar surface achieves thermal equilibrium at 117°C (390K) when exposed to sunlight, and -183°C (90K) in the shade. The temperature at which thermal equilibrium occurs is dependent on the properties of the material and the spectrum of radiation to which the material is exposed. Thus, the temperature of the surface of the moon is dependent on the material properties of lunar regolith and the spectrum of radiation from the Sun.
of emitting radiation. The emitted radiation is typically of the same spectral characteristics as the absorbed radiation. If an object is absorbing more energy than it emits, then its temperature will rise. If an object is emitting more energy than it absorbs, then its temperature will fall. Thermal equilibrium is obtained when the amount of energy emitted by an object equals the amount of energy absorbed by an object. For example, the lunar surface achieves thermal equilibrium at 117°C (390K) when exposed to sunlight, and -183°C (90K) in the shade. The temperature at which thermal equilibrium occurs is dependent on the properties of the material and the spectrum of radiation to which the material is exposed. Thus, the temperature of the surface of the moon is dependent on the material properties of lunar regolith and the spectrum of radiation from the Sun.
A rover on the surface of the moon must be constructed out of materials which behave properly when exposed to the radiation environment of the lunar surface. Ideally, we would like to build the rover out of a material that would be able to reflect the most of the spectrum of the sun on the outside while absorbing very little. On the inside, however, we would like there to be very little reflection or absorption of the infrared radiation generated by the heat of the rover's electronics. Unfortunately, we cannot have it both ways. A material which reflects infrared light from the outside will usually reflect on the inside as well. A rover constructed of this kind of material would turn into a wonderful oven, heated from the inside by its own electronics.
The challenge is to find materials that reflect the spectrum of radiation from the sun, which is predominantly in the visible range, while also absorbing and re-emitting the spectrum of radiation generated by the electronics, which is primarily in the infrared range. We then rely on the ability of the material to reach thermal equilibrium between the infrared radiation being absorbed from the Sun and the electronics with the radiation being emitted by the material back into empty space, which has a temperature of about -271°C (2K).
Surveyor 1 send to moon prior to Apollo missions; © by NASA
Previous NASA missions often used white or silver materials on the parts of the probes that were exposed to light. Other candidate materials include anodized aluminum and gold. Aluminum will typically reflect most of the visible spectrum, but will absorb and emit infrared light. Gold on the other hand reflects infrared through red and orange light and absorbs blue through violet and ultraviolet light. In summary that means: Gold keeps you form getting an cold and Aluminum prevents you from heating up in the sunlight. So for a rover you would use anodized aluminum on top of the rover to reflect the sunlight, absorb the heat from inside and emit it to space and on the bottom gold evaporated foil would be perfect as there’s almost no direct sunlight but infrared light.
Historically, the spacecraft which most closely resemble the rovers being designed for the Google Lunar X-Prize are the Surveyor probes launched by the United States in preparation for the Apollo missions. For more information into the topics discussed in this article, we recommend the following paper available from the NASA archives (page 181ff):
Surveyor Program Results.
Surveyor Program Results.
Authors: Arne Reiners & Daniel Ziegenberg
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