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What methods keep the internal liquids on lunar rovers from freezing? I understand that temps on the moon are extremely low, way past freezing, especially in places where the sun does not shine, yet fuels, pneumatic fluids and other liquids essential to rover operations stay functional. How is this done, besides the obvious heating coils and thermal wrappings that may be used? Are these liquids chemically endowed with certain "anti-freeze" components?

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3 Answers

If by rovers you mean the Lunakhod and Yutu rovers, they both use (used) nuclear radioisotope sources to assist in thermal control through the lunar nights. Applicable fluids would be battery electrolytes and gear lubricants. Batteries cannot be allowed to freeze, and so will depend on the radioisotope sources for survival. Gear lubricants can be allowed to freeze, so long as they are warmed up before use. That can be done with electrical heaters once the Sun comes up.

As noted in the comments, Yutu uses Li-ion batteries. They probably need to be kept above -20C.

Lunakhod had a big turtle-like lid that had the solar panels on the inside. During the lunar day, the lid was open for the solar panels to collect energy. During the lunar night, the lid was closed, providing thermal insulation to help retain the heat from the radioisotope source.

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I'm not aware of any liquids essential to rover operations. Motion can be performed by electric stepper motors and gears. Instead of fuel, electricity from solar panels or RTGs (radionuclide thermic generators) is used, stored in lithium ion batteries.

If liquids are needed for scientific purposes, they have to be kept at the proper temperature by heating/cooling and thermic isolation, as you state.

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One minor point: Li-Ion aren't necessarily the first choice, precisely because of their temperature dependence. –  Deer Hunter Jan 23 at 22:48
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There aren't many rovers on the moon, so here two cites: "Yutu's power supply subsystem consists of two solar panels: a set of lithium-ion batteries, hibernation awaking modules and a power control unit" (wiki.china.org.cn/wiki/index.php/Yutu_(Jade_Rabbit)); "Powered by Lithium-Ion Batteries, NASA Spacecraft Explore Mars and the Moon" (techbriefs.com/component/content/article/27-ntb/features/…). For what-ever reason, Li-ion batteries are taken at least in several cases. –  Gerald Jan 23 at 23:38
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We often think of the freezing point as being the temperature where water freezes (273.15 K). There are several scales for measuring temperature, for simplicity lets use Kelvin here. If you have watched a pot of water boil away you are aware the water has 3 states, vapor (steam), Liquid (water) & solid (ice). The point where a solid turns to a liquid as often called the melting point.

According to Wikipedia temperatures on the moon range from 26 K to 390 K. If we look at a list of melting points we see that element like Hydrogen & Neon have melting points below 26 K but they also boil around 20 - 27 K, so they would not be a good liquid for using on the moon. As they would vaporize in all but the coldest lunar winter day.

The lunar equator has temperatures ranging from 100 k to 390 K. As we look at the list of melting points there are some that are liquid around 100 k but they all have fairly short liquid ranges Oxygen has one of the bigger ranges of about 45 degrees but that is between 54 K and 90 K. It seem our biggest risk then is not in finding an element that will flow during lunar cold spells, but rather one that will not boil during the warm spells. Even water which boils at 373.16 K can boil on the moon.

There are also considerations for pressure, as it can modify the melting & boiling points. In short there is no single element that will work as liquid on the moon, so any fluid would either need to be compound or solution (antifreeze) to keep from freezing, some heat would likely be required for the coldest periods, and pressures maintained to prevent boiling from extremes of temperature or exposure to vacuum.

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Thank you, James, Gerald, and Mark. Your answers are extremely valuable to me, as my endeavors are in researching viable fluids for a lunar vehicle. The major difficulty right now is indeed the "freezing" risks of certain liquids, and so your input is critical. Again, thank you all for your generosity! –  user32016 Jan 25 at 18:48
    
I am intrigued by the idea of a lunar winter. For sure the Moon's axis has an inclination (though it is tiny in comparison to Earth's). But as there is no atmosphere or bodies of water I doubt there are any seasons of any sort. Or am I wrong? –  adrianmcmenamin Jan 28 at 13:19
    
@adrianmcmenamin that sounds like a question you should formally ask. –  James Jenkins Jan 28 at 13:42
    
@JamesJenkins Are there seasons on Luna? –  Michael Kjörling Jan 30 at 20:54
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