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Moon and Mars rovers go through a lot of trouble to keep themselves heated to avoid "freezing to death".

My question is why exactly are all these measures necessary? As in, which components of the robots freeze? What alternatives do we have to design robots that just don't need to worry? And what would the trade offs be?

One common answer I found on my research was the liquid on lithium ion batteries. One proposed solution was solid state batteries. But are the batteries really the only component at risk? Would Moon rovers have zero concern about low temperature if they had a SSB? Or is there something more to it?

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    $\begingroup$ Solid state batteries that have energy densities competitive with liquid-based lithium ion batteries are a very recent technology development, with much of the research and development happening after the design and development of the Mars 2020 rover was complete. $\endgroup$ – David Hammen Mar 29 at 12:26
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    $\begingroup$ Off-the-lot cars from the lower 48 have major issues dealing with winter in Nome, Alaska. It isn't just a space thing... $\endgroup$ – Jon Custer Mar 30 at 14:57
  • $\begingroup$ When one common answer in your research was about the liquid on lithium ion batteries, what else was there? In any case, which element of your robot would not be susceptible to cold, let alone freezing? $\endgroup$ – Robbie Goodwin Mar 30 at 21:06
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There are many things that change at low temperatures. You already mentioned batteries, which are problematic for two reasons: First, liquid electrolytes get frozen at some point. Second, chemical reactions tend to happen a lot more slowly at low temperatures.

Next issue comes from moving parts: Every joint needs some kind of lubricant - and these may start freezing as well. It won't help to join parts without lubricant by making them very smooth - while this seems like a good idea on Earth, in a vacuum cold welding will happen and make the parts stick together. Some of these issues could be worked around using magnetic bearings, but this is usually a lot more complicated to build.

An yet unrelated issue comes from electronics: Semiconductors change their behavior dramatically the lower the temperature is. For example the voltage drop in diodes rises at low temperatures, which ultimately leads to any chip not working any more. Circuits can be designed to operate at a wide range of temperatures, but anything below -40°C can't be handled by commercial chips. (@FluffyFlareon elaborated on this issue in their answer).

And the last point to address here: many materials change properties with temperature. Most notably, thermal expansion is a major problem when a machine composed of various materials is subject to temperature changes. As different materials expand and shrink at different rates, mechanical stress and cracks will occur. In addition, some materials start to get brittle at low temperatures. Most noticeably any plastic compounds, but metals as well.

Many aspects can be overcome with careful design, but usually batteries are the first thing to fail as temperatures get lower.

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    $\begingroup$ You came close to mentioning this, but another major issue is that many components include multiple materials with different thermal expansion coefficients. Differences in thermal contraction can build up stresses that crack circuit boards and solder joints, pull contacts and mechanisms out of alignment, etc. The brittle behavior of many materials at low temperatures just makes this even worse. $\endgroup$ – Christopher James Huff Mar 28 at 20:10
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    $\begingroup$ Another part of the answer is that components like batteries and chips are developed for the conditions we expect on Earth. (And even then, don't work all that well in cold winter temperatures, which is why I keep my portable electronics in inside pockets when cross-country skiing.) To get ones that work at Martian low temps, you'd have to do a lot of special purpose R&D, which would add a lot of expense. $\endgroup$ – jamesqf Mar 29 at 3:20
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    $\begingroup$ @jamesqf Drop the condtional tense from your last sentence: To get electronics that work at Martian low temps, you have to do a lot of special purpose R&D, which adds a lot of expense. (They don't use off-the-shelf components on interplanetary spacecraft...) $\endgroup$ – Oscar Bravo Mar 29 at 10:45
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    $\begingroup$ @oscar-bravo they actually do. Some of the cameras on board the latest rover are GoPro's. $\endgroup$ – Sam Mar 29 at 15:03
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    $\begingroup$ @Oscar Bravo: Off the shelf, no. Using the same basic principles & designs, though. E.g. the Perserverence CPUs are radiation-hardened PowerPC chips, basically the same as you might have found in Earthly (non ISA) computers & embedded application circa 2005: mars.nasa.gov/mars2020/spacecraft/rover/brains $\endgroup$ – jamesqf Mar 29 at 17:45
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Purely based on an operational standpoint of semiconductive devices, equivalent resistances are considerably higher at low temperatures, and thus any calibrated circuit or component, including Rcomps, DLLs, PLLs, and clock compensators, will operate unexpectedly or not at all. These kinds of circuits are specifically developed with PVT (process, voltage, temperature) conditions and ranges built into the core design, so operating ranges not considered, accounted for, or tested, will (likely) cause the circuits to fail or work poorly. The wider the temperature ranges needed, the harder the design, because there are power, timing, noise, variation, and EM tradeoffs that occur across low and high temps.

Analog circuits are often very sensitive to small differences in voltage and temperature, so various drivers, amplifiers, oscillators, and LDOs will be affected considerably by low temperatures compared to when operating at RT. RF transmitters and receivers are particularly prickly.

Note that surface temperatures of these bodies can range massively. The moon has 13-day periods of sunshine, followed by 13-day nights; temperatures range from 100 K to 400 K. The range on Mars at any given location (say, on the equator) is less pronounced, but still sizeable; for instance, Viking measured a temperature range of 160 K to 260 K at its landing site. And Spirit measured temperatures in excess of 300 K. That's a large temperature range, and the electronics and material thermal properties of expansion, etc., need to deliver at either extreme of the spectrum.

At very low temperatures, the linearity that exists between carrier density and temperature is completely disrupted, and depending on the base semiconductive material used (Si vs Ge, for instance), the ionized mobility of circuits can deprecate to the point that there's insufficient carriers for the circuits to operate at all. This phenomenon is known as "freeze-out", but it generally occurs at temperatures below 100 K, becoming particularly prominent below 77 K, LN temperatures.

Due to low temperature effects on carrier mobility, the speed that signals propagate across channels and through digital gates and electronics will be affected. Although there are several attractive qualities introduced into CMOS devices at low and ultra-low temperatures, such as virtually eliminating transistor latchup, designing electronics to respond to wide temperature ranges presents problems with certifying timing arcs across all temperatures, and timing convergence of all paths can be mission-critical.

So yes, batteries might be the first thing to fail, but in the event that they don't, there are a myriad of other 1st-order electronics issues that could occur rendering a rover unusable for its intended purpose of measuring and transmitting data and moving around the surface of a moon or planet. I haven't even gotten into condensation, thermal expansion, mechanical defects, or failing sensors, as those aren't really my area of knowledge.

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  • $\begingroup$ The synodic day on the moon is actually 15 days of night and 15 of daylight. And welcome to Space Exploration $\endgroup$ – kim holder Mar 29 at 15:10
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The main obstacle is the existence of liquids. Liquid phase is quite "fragile" and in contrast with gases and solids, exist in pretty narrow conditions.

Without a liquid phase, a lot of complex chemistry and mechanics become impossible. Solids are reluctant to react to each other chemically (atoms and molecules are locked down and can't travel to meet each other). Result: no batteries, no electrolytic capacitors, no various types of sensors, etc...

Any friction becomes a destructive process so no much of a movement either. No rubber seals (rubber is a type of liquid, actually)...

We happen to live in conditions where liquids are abundant (that's why we exist in the first place), but Mars (and most of the universe in general) is different.

We don't have the technology to "engineer around" the absence of liquids - other than, you see, heating the important parts, using whatever energy sources are available.

(These problems are not limited to the technology world. That's how warm-bodied animals evolved.)

p.s. Venus offers the opposite engineering task: way too hot for most liquids. Cooling something is harder than heating, that's how thermodynamics works.

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  • $\begingroup$ Venus also melts our semiconductors. $\endgroup$ – Joshua Mar 29 at 20:33
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    $\begingroup$ @Joshua Venus surface temperature is like 400C. Silicon (melts at ~1400C) and Germanium (~1000C) are somewhat safe in this regard. I am not saying that they will work, but they will not melt. $\endgroup$ – fraxinus Mar 29 at 21:04
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    $\begingroup$ It's actually the soldier joints that melt. $\endgroup$ – Joshua Mar 29 at 21:10
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    $\begingroup$ Tin solder joints also suffer at low (but still livable for humans) temperatures - "tin pest". Not the best material for extended temperature range, is it?. Tin sold-i-er is another thing, see here en.wikipedia.org/wiki/The_Steadfast_Tin_Soldier $\endgroup$ – fraxinus Mar 29 at 21:55
  • $\begingroup$ But can't we "engineer around the absence of liquids" by replacing materials with new ones that stay liquid at lower temperatures? Or use alternative solutions like in the example of solid state batteries that need no liquids? $\endgroup$ – VIBrunazo Apr 19 at 17:27

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