How do space-bound heaters work?

Question

A brief follow-up to Could Rosetta have been awakened from Earth if necessary?

The Rosetta FAQ writes to say

Q. Was Rosetta completely shut down?

Almost. Only the computer and several heaters remained active. These have been automatically controlled to ensure that the entire satellite doesn't freeze ...

The same source further writes to say (italics mine!)

Q. What has to happen before Rosetta can transmit its first signal?

The spacecraft has been programmed to only transmit its first signal once the on-board wake-up procedure has reached a certain stage. The internal alarm clock will go off at 10:00 UTC on 20 January, triggering the exit from hibernation. This process that includes switching on the star trackers, slowing the spacecraft's hibernation spin and switching on and warming up certain systems...

Having a heater to keep the systems from entering a deep-freeze makes sense. If anything such heating would be mandatory given contemporary technology, and the cold of space. Heat transfer, however, requires a medium; something that space has little of.

Closed body craft, such as ISS, possess an atmosphere which aids air-conditioning.

• How do heaters on Rosetta work?
  • Are the electronics embedded in some thermally conductive medium?
  • How are mechanical systems heated in such space-bound systems? Hydraulics heating?

Answer 1

The spacecraft's own truss and elements are thermally conductive, so there's no need for hot air heat distribution. Air (or gas) is actually not such a good heat conductor due to its low density, unless it's highly compressed, at which point you risk mechanical damage to parts it would be heating up. It would also unnecessarily add mass to the spacecraft and unless it was a hermetically tight system, these gases would be just another consumable, limiting your spacecraft's longevity. High-pressure containers also tend to be rather heavy.

Most commonly, various shape heating elements are attached to parts of the spacecraft that are thermally sensitive and require specific temperature to function, and the heat they generate is conducted through the spacecraft's own body, or better thermally conductive materials (e.g. Kapton), piping with heat conductive liquids, etc. are used to reach places that might heat up slower or require too much heat to be applied at one point to be later transferred to another. So we'd usually talk of a combination of multiple heating (heat generating) and heat distributing (thermally conductive) elements, applied wherever they're needed.

Specifically with Rosetta, the supplier of flexible heaters used was RICA that is a part of Zoppas Industries (here's their catalog for flexible heating elements and heat distributors). They supply heating elements for the space industry using silicone, polyimide, polyester, mica,... constructions in all kinds of shapes and sizes, including, e.g., band heaters for nozzles. Some other flexible heaters and heat distributors might look like this:

                          

^{Polyimide (Kapton) thermofoil heaters are for use in applications with space and weight limitations, or where the heater will be exposed to vacuum, oil, or chemicals, such as in medical devices, aerospace, aviation,... (source: MachineDesign.com)}