tl;dr: "...will probably freeze to death...before it runs out of power..." If power keeps it from freezing to death and it hasn't run out of power, why would it freeze to death?

Space.com's NASA's Perseverance rover is the 1st spacecraft in years to carry fresh US plutonium. It won't be the last. includes the following passage:

So, if you want to send a spacecraft to our solar system's giant planets and beyond, or to other dark places like the permanently dark regions deep in craters near the moon's poles, you'll likely want nuclear power. That preference isn't just about sunlight either; nuclear power also helps spacecraft confront threats like low temperatures and high radiation.

"It allows us to explore where the sunlight doesn't get, but it also allows us to explore harsh environments, and that's because we can take our heat with us," June Zakrajsek, the Radioisotope Power Systems (RPS) program manager for NASA at the Glenn Research Center in Ohio, told Space.com. "It's the reliability and those kinds of factors that are really important for our missions, and we couldn't do some of the missions without it."

NASA's next plutonium-powered spacecraft will be the Dragonfly rotorcraft mission, launching in 2027 to Saturn's strange moon Titan, which NASA says receives about 1% of the sunlight that Earth does. Because of Dragonfly's nuclear power source, the spacecraft will probably freeze to death in the landscape of liquid methane and towering water-ice cliffs long before it runs out of power, Zakrajsek said.

Question: Why would Dragonfly freeze to death at all? All missions come to an end and for some reason, something breaks or solar panels get covered in dust or money runs out and administrators pull the plug, but coming from a NASA program manager this sounds so incredibly certain. Is there a plan to "freeze it to death" on purpose?

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    $\begingroup$ Like Russell's comment below: anytime the Power_In < Power_Out, the system will get colder despite the presence of "some" power. However, that's hardly because of the source being nuclear! I think he means that, were solar power to suffice, the system would never freeze. $\endgroup$ Commented Jun 4, 2021 at 14:02
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    $\begingroup$ Weird. As Wikipedia says, "Thermoelectric modules, though very reliable and long-lasting, are very inefficient; efficiencies above 10% have never been achieved and most RTGs have efficiencies between 3–7%". So you'd expect an old RTG to still work as a heater even when its electrical power output is no longer useful. OTOH, the coldness of Titan helps the thermocouple because of the big temperature difference, but it also puts a big demand on the heating functionality. $\endgroup$
    – PM 2Ring
    Commented Jun 4, 2021 at 16:38

2 Answers 2


The RTG makes heat, which is used in two ways in missions to cold places: Through thermocouples to generate electricity, and through waste heat. Some of the waste heat is cycled through coolant loops or heat pipes to keep things warm; the excess is radiated. The electricity can also be used to run heaters. As the RTG ages and the plutonium within it decays, it generates less heat, and therefore both less waste heat and less electricity. Eventually the RTG doesn't generate enough of the two to keep its critical components warm and something breaks.

Here's a picture of the RTG for Mars 2020 showing the heat pipes:

enter image description here

Essentially a nuclear battery, an MMRTG uses the heat from the natural radioactive decay of plutonium-238 to generate about 110 watts of electricity at the start of a mission. Besides generating useful electrical power, the MMRTG produces heat. Some of this heat can be used to maintain the rover's systems at the proper operating temperatures in the frigid cold of space and on the surface of Mars. Some of it is rejected into space via the rover's Heat Rejection System.

The gold-colored tubing on the heat exchangers form part of the cooling loops of that system. The tubes carry a fluid coolant called Trichlorofluoromethane (CFC-11) that helps dissipate the excess heat. The same tubes are used to pipe some of the heat back into the belly of the rover.

Source: Power for Mars 2020, July 24, 2019, NASA

  • $\begingroup$ Thanks for your answer! Can you focus a little more on the meaning of "will probably freeze to death...before it runs out of power". If power keeps it from freezing to death and it hasn't run out of power, why would it freeze to death? Thanks! $\endgroup$
    – uhoh
    Commented Jun 4, 2021 at 2:34
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    $\begingroup$ @uhoh I didn't comprehend the true meaning of your question when I answered. OK. I think someone at space.com is really misquoting Zakrajsek, or his attempt to dumb down something technical resulted in a gross misunderstanding. But I don't know. Sorry for the non-answer. $\endgroup$ Commented Jun 4, 2021 at 2:40
  • $\begingroup$ It's certainly a partial answer, thanks! I'll make a clarification tl;dr at the beginning of the question based on your feedback. $\endgroup$
    – uhoh
    Commented Jun 4, 2021 at 2:40
  • $\begingroup$ You can also consider just adding the key points in your comment back into your answer. It won't be the first time here that an answer contains a "the article is wrong" passage. :-) $\endgroup$
    – uhoh
    Commented Jun 4, 2021 at 2:43
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    $\begingroup$ I assume "before it runs out of power" means "before the amount of electrical power it generates drops permanently below its minimum electrical demand", not "before it generates zero power". $\endgroup$ Commented Jun 4, 2021 at 4:18

The problem here is that you are comparing a nuclear "battery" with a conventional battery. A conventional battery you draw power from it at whatever rate you want (up to some limit based on the battery) until it's gone, then it produces nothing.

However, nuclear "batteries" are a very different beast. They're actually generators, not batteries and simply deliver power at a fixed (although slowly declining) rate. You can't run them down, only time does that. Thus it's quite possible for a spacecraft to freeze to death (the cold gets something vital) while the generator is still working fine.

It's also possible for a spacecraft to "die" because the available power is too low to do anything. The Voyager probes aren't too far from meeting that fate.

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    $\begingroup$ I see, so if it needs 30 watts to keep it's flight batteries from freezing overnight and being permanently damaged, but only 20 watts to otherwise stay alive and trickle-charge the batteries for an occasional short flight, then it will freeze to death before running out of power to operate. While my numbers are arbitrary, such an inequality may be what the quote is referring to? $\endgroup$
    – uhoh
    Commented Jun 7, 2021 at 2:10
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    $\begingroup$ @uhoh That would be my thought. Spacecraft die when their available power drops below the worst case load. We saw that with Spirit and Opportunity--it was always touch and go whether they could survive the Martian winter, but when it warmed up again they woke up, until the time came when they couldn't get enough and were destroyed. A nuclear powered craft won't die from a lack of sunlight but the power issue is the same. $\endgroup$ Commented Jun 7, 2021 at 2:17

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