Start with the heat leak

I have spent a surprising amount of time drawing boxes with arrows coming out of them. This seemed as good a place as any to start a notebook.

Take a surface of area 0.1 m², held at 250 K, looking into very cold surroundings. Give it an emissivity of 0.8. Ignore sunlight, nearby warm objects and solid conductive paths for the moment. Its net radiative loss is approximately:

That is a sizeable heater for a small box. If an idealised surface in the same geometry had an effective emissivity of 0.03, the corresponding loss would be about 0.66 W. These are chosen numbers to expose the sensitivity, not measured performance for a blanket you can buy.

The geometry assumption matters. A real surface sees some mixture of sky, ground and other hardware. Those surfaces have temperatures and occupy different fractions of its view. You cannot copy the 17.7 W into a lander budget without replacing my conveniently empty surroundings with the actual scene.

Now put back the supports and cables I removed. Each can conduct heat. Add electronics that dissipate power inside the box. Add changing external conditions, thermal capacity and allowable temperatures. The sketch starts to resemble a thermal model instead of an exercise.

ESA’s account of thermal-control methods makes a useful distinction between passive measures, such as coatings and insulation, and active equipment. I keep returning to the passive side first because reducing a heat leak can reduce the power system needed to make up for it. ^[1]

I am not proposing that every thermal problem can be solved with another layer of shiny film. I am proposing that we find out where the heat is going before arguing about where to get more of it.

That is the remit of this site, more or less. Small calculations, actual hardware, and a little room in the margin for things I have missed.