Survival of the fittest - follow up #2
At the beginning of the year we introduced to you one of the problems that we face during our mission to the moon. How is it possible to keep a rover warm enough in the ice cold lunar night, so that the damage due to the cold is kept to a minimum? You have sent us a number of interesting ideas and we promised to analyze and evaluate them.
Some weeks ago we described all the aspects of the problem, and what challenges can be expected in the vacuum. This week we present the first of your ideas, and analyze it.
"Dig a trench to avoid heat dissipation"
This solution is based on the fact that regolith is a good insulator, which helps to avoid heat dissipation through radiation. So it should be possible to simply dig a trench to keep the rover warm.
The idea sounds quite simple. Let's dig a hole, drive the rover inside and thus avoid losing heat that the rover radiates. The first rule to keep in mind on the moon: Don't dig holes that are already there! The surface of the moon is pockmarked – full of holes in all shapes, you just have to look for them.
So let's go out and find ourselves a hole that's about 1 meter (3 feet) deep, big enough for the rover to drive in and out. However, that would only solve one part of the problem. At night the rover would lose heat to its surroundings in the form of electromagnetic radiation, especially as infrared light. The rover is not covered from above, which means that energy would still be radiated into the void of interstellar space. That heat is lost and gone forever. Towards the sides and the ground, the rover would first warm up the regolith until a radiative equilibrium is reached. Then the regolith radiates as much heat back to the rover as the rover radiates toward the regolith. The issue with that is, that the rover can only store a certain amount of heat energy in its structure, depending on the material. In any case the stored heat in the rover is much too little to sufficiently warm up the surrounding regolith.
The only remaining choice now is to break the first rule and dig our own trench. The advantage of this is, that at a depth of 1m the regolith has a constant temperature of -20°C, instead of -170°C on the surface. Thus there would be a much higher initial temperature around the rover, and the radiative equilibrium would be reached much faster.
But one problem remains that this approach doesn't solve, either. The lunar night lasts for 14 days, and by then the rover has lost all its heat by dissipation upward into space. And we would actually face another problem; when driving the rover into a hole or trench, we need to stop work earlier in the evening in order to get there and we miss the first rays of morning sun. This would needlessly cut short our mission time of less than 14 (earth) days. And if we had to dig the hole ourselves, another part of the mission time would be lost for that. But time is precious, and we'd rather use it for some more expeditions in the surrounding area, instead.
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- "Rover Phoenix" © by NASA - Public Domain
- "Martian Rover" © by NASA - Public Domain
- "Dark Lunar Crater" © by NASA - Public Domain