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On earth, blankets serve to primarily buffer temperature changes / heat transfer that would occur via convection (e.g. a blanket traps air near you so that after your body warms it, it doesn't just get blown away and replaced with cooler air), and conduction (e.g. if you're lying on ice, a blanket between you and the ice with poor thermal conductivity will slow the rate of conductive heat transfer).

In the case of a free-floating spacecraft in the vacuum of space, convection and conduction don't exist, and temperature / heat transfer are dominated by incoming radiation (from the sun) and outgoing radiation (radiative black-body cooling from the spacecraft, if I'm using that term correctly). (To be clear, conduction does exist in space between objects that are in physical contact, but I'm talking about heat transfer through the surface of free-floating objects for the purpose of this question).

How are space blankets designed to buffer radiation? In the simplest form, I imagine a space blanket would be a thin material that's separated from the spacecraft skin by a thin layer of vacuum, i.e. it would only exchange heat with the outside environment and with the spacecraft via radiation, and so the goal would be to minimize the radiation accepted from the spacecraft and emitted towards the spacecraft. Do space blanket materials simply have a poor capacity to absorb and emit radiative heat, i.e. they're super reflective? Or is there more that factors into their design?

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    $\begingroup$ Why would conduction not exist in a vacuum? $\endgroup$
    – fred_dot_u
    Commented Jun 29, 2020 at 14:14
  • $\begingroup$ Heat could be transferred by conduction in satellites in space. $\endgroup$
    – Star Man
    Commented Jun 29, 2020 at 21:45
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    $\begingroup$ @fred_dot_u conduction does not exist in a vacuum because there's nothing to conduct heat. Conduction is the transfer of heat within a material or through immediate contact. If you're touching something, then conduction works; if you're not touching something directly but there's (for example) air between you, then there will be some conduction through that air, but if you're not touching anything at all then there's nothing that could conduct heat to you. $\endgroup$
    – Peteris
    Commented Jun 30, 2020 at 0:23
  • $\begingroup$ The blanket statement, "... and conduction don't exist" is ambiguous and on the surface, false. Conduction exists in a vacuum when two objects of different thermal characteristics are in contact with each other. $\endgroup$
    – fred_dot_u
    Commented Jun 30, 2020 at 9:25
  • $\begingroup$ Correct - I was ambiguous in my phrasing. My question refers to heat transfer in and out of a free-floating object in space, which is in physical contact with no other objects. In this case, conduction is not a factor, because heat does not conduct through a vacuum. Made a slight edit to the question to clarify what I meant - sorry for the confusion! $\endgroup$
    – Kasper Kubica
    Commented Jun 30, 2020 at 14:56

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In addition to what @PearsonArtPhoto said about the reflectivity, the large number of layers plays an important role in being effective radiative insulation.

To oversimplify somewhat, besides just straight reflection, the absorption and reradiation of heat helps to a large extent. In general, a large amount of heat energy comes in from one side. That energy heats up the first layer, which radiates heat itself according to its own temperature, but it does so in both directions more or less equally, which, give or take, cuts in half the amount of heat going toward the cold side. Repeat this over 40 layers, and it adds up substantially. Add that on top of the reflectivity of the individual layers as well, and you have remarkably effective insulation.

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They tend to be a combination of a highly reflective material, aluminized mylar usually, with a thin mesh in between the layers. The goal is to reflect as much light as possible (specifically those wavelengths emitted by objects at human-body temperature), and keep direct contact between elements to a minimum. Wikipedia has a great picture, shown below.

enter image description here

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    $\begingroup$ @oldtechaa what could be more explicit than "reflect as much light as possible" ? All radiated thermal energy is photons. $\endgroup$
    – Carl Witthoft
    Commented Jun 30, 2020 at 12:15
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    $\begingroup$ That makes sense! This may warrant a separate question, but if reflectivity and minimal contact are the two central goals, why is the outside of the Soyuz thermal blanket black rather than silver? For that matter, why aren't all spacecraft shiny and highly reflective? $\endgroup$
    – Kasper Kubica
    Commented Jun 30, 2020 at 15:02
  • $\begingroup$ @CarlWitthoft, that's a fair point. Many people unfamiliar with the topic would consider light to only be visible light, however, so clarification could help them. $\endgroup$
    – oldtechaa
    Commented Jun 30, 2020 at 16:06

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