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Been reading about early space exploration again, especially Sputnik 2 (1957) and Venera 1 (1961). Both of these suffered from overheating.

It strikes me that a metal object will easily "absorb" the Sun's rays, which are not mitigated by any atmosphere. A deep space probe will not have the protection of Earth's magnetic field either.

So it seems to me that a metal object, if warmed by the Sun, will cook anything inside of it. So how is a spacecraft engineered to not overheat? Especially for something like a solar sensor or solar panel, which cannot be covered by a reflective sheet.

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The key points in thermal design for satellites, which envelopes "to prevent overheating" are:

  1. Have a design that specifically has a heat loss path, this by definition has to be a high conductance path rather than an insulator. Consider that if an item has two different paths it may be very difficult to know which will dominate in any situation.

  2. The high conductance path has to apply at each step from the heat source to the vacuum of space:

    • components bonded to circuit board with thermally conducting adhesive,
    • short lateral paths in circuit boards,
    • thickened floor of electronic boxes (allows spreading of heat for greater access to radiator space)
    • interface filler between equipment box and panel,
    • direct panel view of space with low solar incidence AND / OR heat pipe network in panel AND / OR mirror finish on outside of panel if solar incidence cannot be avoided.
  3. Methods for the above steps are often chosen to minimise the uncertainty as much as for their maximum performance: it is much less of a headache to have a slightly sub-optimal design that you know will work rather than to be worrying about metal-metal interfaces.

  4. Thereafter consider insulating direct solar input paths. Again do this both a) to reduce the heat load and b) to improve certainty in the controlled space.

  5. Clearly there will be minimum temperature requirements too. There will be compromises in terms of performance and uncertainty to meet both the hot and cold limits in all seasons.

    • from a reliability perspective consider heaters before heatpipes, and passive heatpipes before loop heat pipes or pumps,
    • clearly there is another compromise on keeping heater power within the available range,
    • given the interest in keeping uncertainties down, steer away from relying on the thermal mass of components too much. Clearly for sudden changes like eclipse this is important but it means you are relying now on a much longer chain of uncertainties.
  6. Assurance
    • make a thermal mathematical model, get someone else to review it
    • test it in a chamber with lots of thermocouples and compare the test with the model, then use this to improve the model links
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There are many ways to control the temperature of a satellite and the distribution of heat within a satellite. Often they are wrapped in multi layer insulation, which gives them their typical golden shine, or other surface treatments like white paint are applied. Other measures include retractable blinds, orienting the satellite in a specific way to the sun, heat pipes/heat bridges and radiators.

These are just some tools that professional thermal analysts use in the design of satellites to keep the critical components within their respective operating temperature envelopes. Usually most importantly, the batteries must be kept within a few tens of degrees (C) at all times.

You also touched briefly on magnetic particles, which are deflected by the earth's magnetic field, but that is another matter altogether, because particles do not cause a significant amount of extra heat flux on spacecraft.

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