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Let us say I have a theoretical super thin spacesuit which has zero thermal insulation. As if person would be naked in space, but all other variables - pressure, oxygen, food and other life support are taken care of, by some miniaturized futuristic super-tech.

Would the person freeze or overheat?

On the one hand he is losing heat due to phenomenon called 'black body radiation'. Heat just radiates out and there is nothing to stop this heat loss because this spacesuit has no thermal insulation.

On the other hand, vacuum itself is a perfect insulator. That is why to keep your coffee hot inside a thermos flask they put vacuum in it, and in space there is no air or water or other medium to cool it off.

Yes, some piece of metal or any other material would lose all its heat sooner or later and reach a temperature of absolute zero, but a person's body is producing heat continuously.

Would he overheat or freeze?

If you ask the Apollo 13 crew, after their craft went bust, they were freezing. One long chilly ride home. Very chilly.

Eugene from Gemini 9 was sweating profusely and felt very badly hot, and Alexei (the first man ever to go for a space walk,) would probably agree.

How would my almost naked man in space feel?

EDIT: To clarify things: my spaceman is in far far away deep space where he is not exposed to sunlight.

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    $\begingroup$ Is the person illuminated by the sun? $\endgroup$
    – Organic Marble
    Dec 16, 2022 at 23:34
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    $\begingroup$ Heat production of an astronaut may be low about 70 to 100 W at rest but up to 1600 to 2000 W at very hard work. $\endgroup$
    – Uwe
    Dec 17, 2022 at 0:01
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    $\begingroup$ @ShoulO The heat production of a body at rest is dissipated over the whole body surface. 100 W is not that much wearing underwear in air below 20 °C. $\endgroup$
    – Uwe
    Dec 17, 2022 at 13:23
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    $\begingroup$ At blood temperature, a perfect black body will emit a little over 500 W per square meter, and an adult human has around 2 m^2 of surface area, but a complicated shape that will reduce the effective area for radiation, and surface temperatures will be lower (with emission scaling with the fourth power of temperature). So, it depends on the emissivity of the suit, the person's posture, and their level of activity, but 100 W is really not much at all. $\endgroup$
    – Christopher James Huff
    Dec 17, 2022 at 14:48
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    $\begingroup$ "Yes, some piece of metal or any other material would lose all its heat sooner or later and reach a temperature of absolute zero" That would be a very long time. The cosmic microwave background comes from everywhere in space so it would be difficult to get below 2.7 Kelvin. $\endgroup$
    – Jason Goemaat
    Dec 17, 2022 at 18:28

3 Answers 3

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Answer to a similar question on physics

Say the average surface area for an adult man is 1.9 m2. According to the Stefan-Boltzmann law, the black body radiation emitted is a constant proportional to the surface area and temperature: $5.67*10^{-8}\ Wm^{-2}K^{-4}$. With 1.9 m2 as the surface area and 310 as body temperature, that gives $(5.67*10^{-8})*1.9*310^4$. Close to 995 watts. That's enough to lower an 80 kg man's temperature by 1 degree Celsius in about 5-6 minutes. According to this answer a 2500 calorie diet would mean they produce about 120W of heat on average. According to this article.), the maximum heat produced from shivering is about 500 watts. Cyclists can hit 1500 W in short bursts, but only maintain about 500 watts over time, so eventually you would freeze.

Also, it depends where in space you are. If you're at the Earth's distance from the sun, the solar radiance is about $1360\frac{W}{m^2}$. If you paint your space suit black, you could just about absorb enough energy from the sun to keep from freezing. Just keep rotating while trying to optimize the area facing the sun and don't go into a shadow, and you would be shivering.

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  • $\begingroup$ So basically to stay alive in deep space he would need heating, not cooling. He would need to carry about 875 w heater. Basically a standard house heater (1kw) would suffice. According tour numbers. $\endgroup$
    – ShoulO
    Dec 17, 2022 at 20:47
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    $\begingroup$ Keep in mind that black-body radiation calculations are based on, well, a black body. If your super-thin spacesuit is painted white, or metallic silver, the rate of temperature loss is vastly lower. (This is why the inside of the vacuum chamber in a thermos is polished stainless steel.) $\endgroup$
    – Skyler
    Dec 17, 2022 at 20:49
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    $\begingroup$ This is also why mylar survival blankets work despite being similarly thin. With a reflective enough surface, your radiative losses can be arbitrarily low. $\endgroup$
    – Skyler
    Dec 17, 2022 at 20:50
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    $\begingroup$ @Skyler I don't think the color of the material matters much for emission of black body radiation. I think it comes more from the fact that it is refers to a body that is opaque and non-reflective so you don't have to account for radiation coming through or being reflected from a body. Good point about mylar survival blankets which would reflect a lot of the emissions back at the body, but also reflect much of the solar radiation away. I discounted them putting them in the same category as 'insulation' $\endgroup$
    – Jason Goemaat
    Dec 17, 2022 at 22:00
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    $\begingroup$ I think "maximum meat" has a typo. $\endgroup$
    – user2357112
    Dec 19, 2022 at 0:49
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Answer: It depends on the suit and the physical activity, but for real astronauts in real suits, overheating is a much bigger problem than freezing.

If your Mechanical Counterpressure Suit is breathable like the Bio-Suit, sweat would evaporate into space to provide cooling,. https://en.wikipedia.org/wiki/Mechanical_counterpressure_suit

enter image description here

However, these are not available at Amazon.com yet.

Space suits used in practice are all pressurized, therefore air-tight and sweat-proof. Body heat needs to be removed by a refrigeration system.

The Apollo suits used a very robust refrigeration system which relied on water evaporating into a low pressure vessel. The pressure inside the vessel was regulated between suit pressure and vacuum. Water would evaporate from a 2.7 liter reservoir until its vapor pressure matched the regulator pressure. Therefore the temperature could be regulated by adjusting the vessel pressure. I wish it were so easy when I’m camping. https://en.wikipedia.org/wiki/Liquid_cooling_and_ventilation_garment

enter image description here

Cooled fluid was then pumped through a Liquid Cooling and Ventilation Garment (LCVG)

enter image description here

http://cargocollective.com/designisempty/Puma-Process-1

As a rule, disposing of heat is a much bigger problem than keeping warm. However, in your post you describe a naked astronaut in a thin suit. If they are exposed to the sun in LEO, and not rotating rotisserie-style, the sunny side would get very hot and the shaded side very cold. If the space suit had the same albedo as the Earth, and the astronaut turning slowly, the average temperature would be roughly the same as Earth's average temperature of 15*C

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    $\begingroup$ Regarding the last sentence, "... the average temperature would be roughly the same as Earth's average temperature of 15°C", that's going to depends on the suit's reflectivity in the visible / near infrared range, on the suits emissivity in the thermal infrared, and the astronaut's orientation. The Earth's mean temperature results from the Earth's somewhat high albedo (about 30%) and from greenhouse gases such as water vapor, carbon dioxide, and methane. I don't think the Earth mean temperature vs a suited astronaut's temperature are comparable. $\endgroup$
    – David Hammen
    Dec 17, 2022 at 13:46
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    $\begingroup$ @DavidHammen ... all good points. That's why I sprinkled weasel words "same albedo; turning slowly; average; roughly". Using Effective Temperature calculations will at least let you find the ballpark, +/- 50*C en.wikipedia.org/wiki/Effective_temperature $\endgroup$
    – Woody
    Dec 17, 2022 at 18:20
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    $\begingroup$ @Woody I am confused. You are saying "overheating is a much bigger problem than freezing", then "shaded side very cold". I do not understand. Please help me to. This is exactly my question - would he freeze or would he overheat? I edited my post stating that my spaceman is in galaxy far far away and is not exposed to sunlight to remove this variable. $\endgroup$
    – ShoulO
    Dec 17, 2022 at 20:16
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    $\begingroup$ @ShoulO ... If there is no sunlight, your astronaut would freeze. Since there is no convection or conduction of heat, we are only dealing with radiation. Equilibrium temperature would be about the same as the cold side of JWST. Not compatible with life. $\endgroup$
    – Woody
    Dec 17, 2022 at 22:45
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    $\begingroup$ In the answer you say overheating is the bigger problem. Then in the recent comment you say "your astronaut would freeze". What am I missing that makes it seem to me like those statements contradict each other? $\endgroup$
    – Todd Wilcox
    Dec 18, 2022 at 3:09
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Overheat

Under normal earth conditions, a human body only emits about 2/3 of its thermal energy as radiation. That means we dump a full 1/3 of our metabolic waste heat via convection + conduction.

Now, you said that vacuum is "a perfect insulator". In fact, it is no such thing. Its insulating effects are restricted to blocking convective and conductive heat transfers. It does absolutely nothing to stop radiative transfers, which is why good thermos bottles are mirrored on the inside.

Insulators

Here, we have a problem. Because you said that the suit has "zero thermal insulation". But given that almost 1/3 of human heat loss occurs via the convection/conduction routes, and the suit maintains life support, I have to assume that the suit does not allow the astronaut to sweat away all their water to the vacuum of space. If the suit is indeed watertight, then it is insulating 1/3 of normal heat loss! So, the question, as stated, is not well-posed.

On earth, when humans need to operate in very cold environments, they usually layer clothes with different fabrics which serve different purposes. The innermost base layer is usually designed to wick water away from the wearer so that sweat can serve its normal function of removing heat from the body instead of building up on the skin.

There is also the ambiguity of the suit's material construction. To say that it has "zero thermal insulation" implies that it is a perfect thermal conductor. In that case, the suit should reach equilibrium with the wearer very quickly. Even so, unless the suit is also water-permeable, the suit will also be limited to the radiative throughput of the wearer's surface area.

Graybody

Complicating the matter is the fact that a human is not a very good blackbody. A blackbody must be in internal thermal equilibrium. However, the body actively maintains more heat in the core than the periphery, and will shunt blood flow to the limbs in order to preserve core heat. There are thermal gradients all over the body, which is why medical thermometers are internal, and external readings are considerably lower (easily 4-5 C).

Finally, you have the fact that while the body can survive high heat-loss environments by shunting blood to the core, it has very little recourse for surviving pathologically low heat-loss environments. It cannot create a cold sink out of nowhere, in violation of thermodynamics (I mean, it could theoretically concentrate excess heat in a specialized organ for a finite time, but this just delays the inevitable).

Human Limits

So, while the lowest recorded body temperature is below 12 C, the highest is only 46 C. Even 41 C is associated with organ failure. So while the body can survive a loss of 25 C on the low end, on the high end, there is only about 4 C headroom for heat accumulation before you are looking at likely long-term injury.

Now, if you take away 1/3 of heat dissipation and force the body to dump that heat via radiation, you need to be able to do so without increasing core temp by more than 4 C. A blackbody certainly couldn't increase radiative output by 50% with just a 4 C increase in temperature. What your astronaut needs is a 50% increase in surface area: i.e., a decent-sized heatsink.

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  • $\begingroup$ aaa yes, the sweating issue. Well spotted. I should have removed this variable by stating that my astronaut is given some super-pill which stops sweating. $\endgroup$
    – ShoulO
    Dec 23, 2022 at 18:31

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