[HPsquared]

The thing about space is, heat transfer is all radiative. The "dark" side of the probe, which is in its own shadow, faces the darkness of space (which is very cold indeed). Therefore you can put insulation on the side facing the sun, and a radiator on the dark side to remove any heat that gets through the insulation.

[criddell]

But space is a vacuum which is a pretty good thermal insulator. How are they dumping the heat?

[haimez]

The physical mechanism is radiative cooling: https://en.m.wikipedia.org/wiki/Radiative_cooling

More details about the specific mechanism on the Parker Solar Probe’s cooling system: https://blogs.nasa.gov/parkersolarprobe/2018/09/14/parker-so...

[DecoPerson]

Pick a material for the surface of the radiator which has high emissivity.

That is, when the material gets warm, it will convert the thermal energy to electromagnetic energy (light). Some of this light will escape, leading to a cooling effect.

This is known as “incandescence” or “black body radiation”, and it’s why metals glow red hot when they’re heated to a certain temperature. (Though it’s not always red, it can be white, blue, or even UV. Animals are warm enough to glow in infrared, which is what thermal cameras detect).

[moffkalast]

IR radiation, in a nutshell.