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If heat transfers by convection and conduction and radiation, but there's nothing to convect or conduct in space, does that mean if you stuck a human in space (in a pressure suit so they don't decompress and die that way) they wouldn't immediately freeze? Does it mean would take a very long time for them to freeze? How quickly do things give off their entire energy in infrared heat radiation?


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in reply to @hillexed's post:

assume that a human is a black body1, and that the human in question is perpetually in shadow so they're not absorbing any energy from space other than the cosmic microwave background, which has a black body temperature of 2.726 K. the average surface area of the adult human body in the United States is about 1.9 square meters. by the Stefan-Boltzmann law, this means that a body at body temperature would radiate away about a kilowatt of power into space.

humans are mostly water, so let's assume they have the heat capacity of pure water; this means that the heat capacity of an 80kg human is about 300 kilojoules per kelvin. at a radiated power of about 1kW, this means that a human body would have to sit in space for about 5 minutes to cool down 1ºC; assuming that its temperature change was roughly linear (which would be true, since an ice cube is still much warmer than space) it would take about 3 1/2 hours to cool down to 0ºC. the latent heat of fusion of water at 0ºC is 334 J/g, so the body would then have to radiate 26.7MJ more heat before freezing solid, which would take another 7 1/2 hours.


  1. physics jargon for "an object that absorbs all of the radiation that hits it"

obvious caveats:

  • at least for the next few decades, a human dying in space would likely be bombarded with radiation from a nearby star, meaning that their net energy flux would vary radically based on whether they were in shadow or in light;
  • both the process of necrosis and the trillions of decomposing bacteria that live inside each of us would convert some of the chemical energy in the body to additional heat, but I don't know enough about forensic science to say how much.

Heck yeah thank you! Wow, a kilowatt is a crazy amount of power.

But humans make heat because get energy from food. One human consumes 2000 Calories a day. Assuming my hypothetical pressure suited human had lunch before being teleported into space, or had food in their pressure suit to keep consuming 2000 Calories a day, would that heat outweigh the radiation?

And "freezing" in the colloquial sense as applied to a human body hits a lot sooner than 0C. Hypothermia starts around 35C, so things start to get real bad for our hypothetical human in about 10-15 minutes.