ESA turns to soil to keep lunar missions warm at night
ESA is developing a new heating system to help future missions to the Moon survive the freezing, fortnight-long lunar night. Currently, landers and rovers rely on nuclear or electric heating systems to keep their electronics warm, but the space agency is looking at ways to turn the lunar surface itself into a cheap, sustainable heat source that can be tapped after the Sun sets.
Because it's an airless desert exposed to the vacuum of space and the full fury of the Sun, the Moon is a world of extremes. During the 14-day-long nights, the surface temperature reaches –170° C (–274° F), and without special heating systems the electronics and other systems of rovers, experiments, spacecraft, and habitats would soon freeze into inoperability.
This has happened to more than one lunar mission, including the Soviet Lunokhod-2 when its nuclear heater died after four months in May 1973 and the unmanned rover failed to survive the long, cold night as its systems shut down for good. Even the manned missions were vulnerable to this, which is one reason why all six of the Apollo landings were made during the lunar day and left long before nightfall.
What ESA is looking at is a low-cost alternative to the expensive nuclear power units and large batteries, both with limited lives, now used by lunar landers and rovers to stay warm. The idea is to use the extremes of the lunar climate to heat spacecraft at night by using the Sun to heat the soil around landers or other equipment, then tapping that heat at night.
The surface of the Moon isn't just freezing cold at night, it's also boiling hot during the day with the thermometer topping out above 100° C (212° F) at the equator. So if the Sun can be concentrated on a patch of lunar soil, it could be turned into a heat sink like heating up a stoneware pot in a slow cooker to cook food.
Currently, the system ESA is working on comes in two versions. A simple one suitable for rovers and small spacecraft that uses reflectors to heat patches of the lunar soil to form "thermal wadis," or hot spots, that can then be tapped at night to keep the devices warm enough to function. The second one uses multiple reflectors and heat pipes to collect solar energy to run machinery directly during the day and to store excess heat to keep large installations, like a habitat, warm. In addition, schematics indicate that the heat pipes could be linked to cold spots, perhaps produced by shading the ground, to increase the efficiency of the heat engine.
"The principle has been worked out in detail," says ESA's Moritz Fontaine. "The next step, being undertaken through ESA's General Studies Programme, is to perform numerical and simulation studies to put values on the heat storage and electricity provision the system would enable. The results should then allow the construction of a small demonstrator to test the concept in practice."