[close04]

Any reason why they're using water for cooling? There's no other liquid with lower density and perhaps better thermal properties?

I imagined they would try to save weight in some places if it allows them more freedom in others. Although I have no idea how much water is used in the first place so it might be a moot point.

[ComputerGuru]

> The coolant used for the system? About a gallon (3.7 liters) of deionized water. While plenty of chemical coolants exist, the range of temperatures the spacecraft will be exposed to varies between 50 F (10 C) and 257 F (125 C). Very few liquids can handle those ranges like water. To keep the water from boiling at the higher end of the temperatures, it will be pressurized so the boiling point is over 257 F (125 C).

[close04]

I just imagined that for an object sent to the Sun they'd want to... overdo it a little. Go for something synthetic that would behave even better or would save 1/2 Kg.

I'm just the inquisitive type. Explaining that something is used always makes me wonder "why not something else" :).

[tesseract]

Water has a higher specific heat than most substances, and probably the highest of anything that's a liquid across the operating temperature range for the electronics.

[perlgeek]

Not a spacecraft engineer, merely somebody who has studied physics a long time ago, so take my answer with a grain of salt.

Basically, there are two ways you can design a cooling system: one that involves phase change, and one that simply transports heat from A to B by moving a heat carrier, usually a liquid.

Phase change systems typically have a higher efficiency, because the phase change has larger relative energy than heat capacity. But it also has two disadvantages: it only works near the boiling point of the medium, and you need to deal with all the pressure changes that come with a phase change.

The Parker probe seems to use a "mere" transport, and there the heat capacity and the working range of the medium is very important, and water is a pretty good choice on both of these criteria.

Oh, and you also don't want something very corrosive to destroy your expensive space craft from the inside :-)

IIRC the ISS uses two cooling systems, one based on water that is used throughout the station, and then an ammonium-based system that takes the heat off the water and transports it to the heat radiators. But on the ISS, maintenance is possible.