What the gold blanket actually does

There is a little square of metallised film pinned above my desk. It is packaging, not flight hardware. Visitors ask about it more often than they ask about the rather good circuit diagram beside it. Shiny things have a considerable advantage.

The gold-coloured blankets in spacecraft photographs are usually part of a multilayer insulation system. Thin reflective layers, separated to limit contact, reduce radiative heat exchange. The visible outer sheet is not the whole blanket, and its colour alone will not tell you the complete material stack. ^[1]

On Earth, a wool blanket traps air and changes how heat moves through and around it. In space there is no surrounding atmosphere to carry heat away by ordinary convection. Radiation still works, and solid supports still conduct. Multilayer insulation addresses the radiative part; it cannot wish away a metal bracket connecting the warm box to the cold structure.

I find it useful to draw the bracket first. Otherwise the drawing becomes a beautiful insulated enclosure with a conveniently invisible thermal short through its mounting feet.

More layers are not an unlimited improvement. The spacing and contacts matter. NASA measurements of cryogenic insulation discuss the trade between reducing radiation with additional layers and increasing conduction through the material between them. Packing everything tighter can spoil the assumption you started with. ^[2]

A blanket also has edges, seams and penetrations. There must be places for connectors and instruments, and somewhere for unwanted heat to go. Cover a radiator because it looks like an untidy patch in a photograph and you have improved the appearance of a failing spacecraft.

So the next time a spacecraft looks as if somebody wrapped it in Christmas paper, look for the bits left uncovered. They may tell you more about the thermal design than the gold does.