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Consider the following set-up: a spacecraft radiator is placed inside a vacuum chamber with transparent walls, on the surface of the Earth. The radiator acts as a heatsink touching one of the walls of the vacuum chamber.

Can that wall of the vacuum chamber then passively cool down below ambient temperature of the surrounding air?

The idea is that air does not interact with the insulated spacecraft radiator, which emits energy into the surroundings. Since the vacuum chamber is transparent, radiation does not heat its walls, but leaves through the atmosphere. In fact, the radiator could emit energy through the atmospheric transparency window.

If possible, this would be rather a useful device. This article sounds like it could be done.

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    $\begingroup$ Consider that the air and the chamber are black body radiators. That said, the atmosphere is far from a black body absorber, so one can cool below local air temperature as has been known for centuries to make ice in the desert... $\endgroup$
    – Jon Custer
    Aug 25, 2022 at 13:59
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    $\begingroup$ Since the walls of the vacuum chamber are transparent, won't the surrounding air heat the spacecraft radiator by radiation ? There is a reason why thermos flasks are not transparent. $\endgroup$
    – gandalf61
    Aug 25, 2022 at 14:00
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    $\begingroup$ FYI: A recent question here asked about this: skycoolsystems.com/technology The company claims that their panels can passively cool a working fluid to a temperature that is as much as 15F lower than the surrounding air temperature (exact number depends on weather conditions, I think.) $\endgroup$
    – Solomon Slow
    Aug 25, 2022 at 19:42

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No need for a vacuum or a fancy radiator. Take a clean aluminum can with the lid removed. Suspend a thermistor inside it. Attach a second thermistor to the can. Point it at the sky on a clear, dry night. Compare the measured temperatures. The thermistor attached to the can basically measures the air temperature, while the suspended thermistor radiates to the colder sky, and will be cooler.

I learned of this from an infrared astronomer who had built such a device to monitor observing conditions. Hot sky due to humidity or clouds interferes with infrared observations.

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  • $\begingroup$ Alright, but contact with air sets a limit to the achievable temperature difference. Wouldn't the radiator in a vacuum enable greater cooling? For example, a temperature difference of 100° C $\endgroup$
    – MrMartin
    Aug 26, 2022 at 6:35
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    $\begingroup$ @MrMartin It gets tricky. For a vacuum, you need a window: its emission loads the radiator. The atmosphere is opaque (and thus hot) in parts of the spectrum. I doubt anything close to 100C is possible. $\endgroup$
    – John Doty
    Aug 26, 2022 at 13:32

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