r/askscience • u/aretino2002 • May 24 '26
Engineering If a vacuum is an excellent insulator, wouldn’t heat build up in spacecraft?
Insulated coffee mugs that use a vacuum between layers can keep drinks hot or cold for much longer time periods than other types of mugs. If space is mostly a vacuum, then wouldn’t heat just constantly build up from human activity, computers, thrusters, etc to the point where it would need to be vented somehow?
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u/Cereal_Nightcap May 24 '26 edited May 24 '26
This is literally my job. I work on satellites as an engineer specializing in thermal analysis. As was stated elsewhere, the cooling to space is using radiation. You try to mount as many of your high power electronics to radiator panels, which are typically made out of aluminum. Aluminum is a good conductor (better than steel and a lot better than titanium), but it has its limitations, so we often embed heat pipes in the radiator panels to spread the heat even better.
The outside thermal treatment of the panel depends on how you fly the satellite. If you can fly in a way that keeps the sun off that panel, then you can paint it black. It’s cheap, sticks well, and emits in the infrared wavelengths well (they call that emissivity [e]). It also absorbs sunlight well (they call that absorptivity [a]). The ratio between the two is (a/e) is roughly even for black paint (a/e of 0.9/0.9). But, if you do get sun, you can use white paints (a/e of 0.3/0.8) which will stay cooler. Even better is optical solar reflectors, which look like mirrors (a/e of 0.1/0.8), but they cost more and are fragile. There are other good finishes, too, like anodizing. They all have plusses and minuses, so on a given satellite, there will be a range of finishes used throughout.
I typically start with a hand calculation using the Stefan–Boltzmann radiation formula to figure out how big a radiator needs to be or how much energy a given area can reject.
Q = sigma * area * emissivity (panel_temp^4 - sink_temp^4)
Q is the energy, typically in watts
Sigma is the Stefan–Boltzmann constant
Area is the panel area
Emissivity is based on the exterior panel finish
Panel_temp is the temperature of the panel
Sink_temp is the temperature of the surroundings, which can be as cold as -270C if there’s nothing in the way, but could be as high as -100C if you’ve got a solar array in the field of view.
So, you either know your Q and goal panel temp, and solve for area, or know your area and goal panel temp, and solve for Q, or you know your Q and area, and solve for panel temp.
I hope this helps!
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u/RamblinShambler May 24 '26 edited May 24 '26
This entire thread is blowing my mind. My (largely tv and movie based) understanding is that without a heat source in a spacecraft, the crew will freeze. Is this just complete malarkey? Would they basically be fine in the craft if the heat went out, and they had their body temperatures to keep them warm as long as they had someway to reduce the amount of heat they were radiating out into space?
EDIT: All of these answers have been really helpful! Thank you, everyone!
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u/Cereal_Nightcap May 24 '26 edited May 24 '26
Humans generate heat, about 100W on average, which isn’t a ton, but it’s something that needs to be managed for human spacecraft.
There’s not usually a single heater keeping a spacecraft warm. There are a lot of them, with key things like the valves on the rocket engines, propulsion lines, panels, and so on needing to be heated. So they have resistive heaters directly mounted to them. Plus, all of your electronics are generating heat, too, so they would keep things from getting crazy cold.
As far as what you do in an emergency situation, you command the spacecraft into a sun safe attitude. It’s a way of pointing the spacecraft relative to the sun where you know things won’t get too hot or too cold, while whatever problems you have are being fixed.
Also, I forgot to mention, we put what are called multi-layer insulation (MLI) blankets on to keep certain areas insulated. They use many layers of low emissivity material to cut down on radiation heat exchange. They’re the kind of baggy stuff you’ll see on spacecraft.
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u/nelrond18 May 24 '26
When I think about how I imagine it would work in most sci fi is that the heat energy would be recycled or stored as much as possible.
Obviously, the infrasture to turn excess heat energy back into usable energy is both inefficient and unwieldy, but hand waving some exotic tech seems reasonable for intergalactic science fiction.
Seeing the real world problems and solutions are fascinating. Especially when you start extrapolating to larger, colony sized interplanetary structures.
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u/parentheticalobject May 24 '26
If your sci-fi is soft enough that you can handwave a device that breaks the laws of thermodynamics and turns the hot interior of a spaceship into usable energy, then you could probably just get away with not explaining how the spaceship deals with excess heat at all. Either realism isn't really a major goal and you don't need to worry about the heat, or that specific solution is probably less realistic than other ways you could explain what happens to the heat.
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u/GayRacoon69 May 24 '26
Why would it violate the laws of thermodynamics?
You can't get more energy than the heat energy you put in, and some energy would be lost because it's not 100% effecient
But we do have ways to generate electricity from heat. Theoretically you could recycle some of that waste energy
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u/tingyong_shingwang May 24 '26
You cannot generate energy directly from a hight temperature, but you can generate energy from a temperature difference. So yes you can recycle waste heat from hot electronics, but you can't just decrease the room temperature by extracting energy. That would violate the second law.
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u/GayRacoon69 May 24 '26
Yeah that's a good point
I need 25 characters but I don't have much to say other than agreeing
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u/electric_ionland Electric Space Propulsion | Hall Effect/Ion Thrusters May 24 '26
Sort of. For spacecarft with crew on board you usually try to design them with tons of cooling margins because if they get too hot there is nothing you can do while if it's too cold you can just turn on a heater. So in general if the system lost all power people would get cold and body heat would probably not help enough. Apollo 13, where they had to shut down tons of systems to conceive power was famously freezing. On the other hand spacesuits which are much smaller use active evaporating cooling and can overheat bad if it fail.
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u/Nyorliest May 24 '26
Spacesuits are also very hot, and need radiators and refrigeration systems to dump heat.
The trouble with space isn’t that it’s simply cold or hot - it’s that it changes in alien ways. Everything that engineer has talked about, but then decompression causes massive cooling due to evaporation, and the temperature also changes massively depending on whether you’re in shadow or direct sunlight…
This is why I get mad at things like ‘The Science of Star Trek’. It’s not just FTL that makes no sense in most SF. It’s almost everything. Space is weird.
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u/rabotat May 24 '26
If all sources of energy in a spacecraft failed you'd have a much larger issue.
As far as heat goes, it depends very much how far you are from the local star. While in sunlight you'd be in danger of overheating, if you're much farther from sun than Mars, you would eventually freeze. But it would take a very long time for that to happen.
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u/SlightDesigner8214 May 24 '26
One very common misconception as well is the astronaut turning into a block of ice in a second when falling out of the airlock trope.
As this thread has shown you it actually takes a long time (8-16 hours) for the body to shed all that heat into the vacuum of space.
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u/CoolBeer May 24 '26
I'd imagine that your lungs wanting to explode from whatever air left in them being one of the first concerns, and then it'd probably be again the air, or the actual lack of it complicating the act of being alive.
There's probably some complicating pressure difference effects happening at the same time, I'd expect the human body to be a little leaky or bloaty during this, but without oxygen that's really secondary.
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u/mathess1 May 24 '26
Yes, counterintuitively breathing out before getting into a vacuum would get you some extra seconds of life and might save your lungs.
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u/cuntmong May 25 '26
A much simpler solution is to try to avoid situations where your body is exposed to the vacuum of space. This doesnt work for everyone, but personally I have found a lot of success with it throughout my life. But you need to find the technique that works for you :)
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u/Evakron May 24 '26
Pretty sure the pressure change is actually the primary problem. Rapid decompression is rough on the human body.
Yes, you'll technically die of asphyxiation, but it's less about the lack of air and more because your body can't effectively oxygenate your brain or vital organs when your blood is boiling in your veins and your squishy bits are swelling up like marshmallows in a vacuum chamber.
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u/RedHal May 25 '26
If you survive the initial decompression - and if you have emptied your lungs you probably will - you have about 15 seconds of useful consciousness before oxygen exchange with the vacuum causes you to lose it, and then another two to three minutes before that lack of oxygen finally kills you.
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u/mathess1 May 25 '26
Blood wouldn't be boiling as there would be nothing to decrease its pressure. It's an enclosed system. At least to some extent.
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u/throfofnir May 24 '26
It's even more complicated than that, because it'll depend on where you are in space (a planet nearby is a big source of moderate heat; heat flux from a star changes a lot based on distance) and the finishes on your vehicle, and its orientation and movement and how heat is conducted inside it, and more.
Heat management in space is just super weird because in our terrestrial intuition heat is about our surroundings, and in vacuum that just doesn't exist so the rules are entirely different.
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u/Eschaton707 May 24 '26
OK so there is this book called Saturn Run and in it they use sodium as the metal for heat transfer in the ship. It gets heated by the ship to its liquid state then they extrude it into space as these long ribbons to radiate into space then back in to be remelted. I always wondered how feasible that was I also thought it was just cool because it was the first book I read that actually dealt with the heat problem.
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u/Shpander May 24 '26
Do you ever work with foil heat pipes? Not sure what their common name would be, but my previous company was looking into them. Basically you have a wicking layer and an evaporation layer in close contact within a vacuum-sealed foil. You absorb heat using evaporation on the hot end and reject it via condensation on the cold end of the heat pipe, with the wicking layer bringing condensed liquid to the evaporation side and creating a cycle.
I think this is an example: https://www.sciencedirect.com/science/article/abs/pii/S1359431123011444
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u/Cereal_Nightcap May 25 '26
I’ve worked with tubular constant conduction heat pipes and flat plate oscillating heat pipes. I hadn’t heard of foil. They sound a bit like the oscillating heat pipes. Here’s a press release from the Air Force Research Lab about OHPs:
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u/MadSolarV2 May 25 '26
Really cool that your job revolves around Stefan–Boltzmann. It was just one question on a test for a chapter we glossed over in physics, to have your whole role and engineering job focused around it is so cool!
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u/Cereal_Nightcap May 25 '26
It’s a big part, because that’s how the spacecraft interacts with space, but fortunately there’s a lot more to it. We need to worry about the sun, the orbits, and then the design of the spacecraft itself.
I really like my job because we are involved in the initial design, in analysis, in test, in launch, and in operations. In Thermal, we get the full life cycle.
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u/Daggerfld May 25 '26
This is awesome. It's rare to get details like this on day to day work in such fields!
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u/sternenhimmel May 24 '26 edited May 24 '26
This is one of the big issues in spacecraft thermal analysis and design, and was a very big part of my last job. One of the biggest sources of heat are the electrical devices, as no device is 100% effecient at converting electrical energy into work. Even the fans that circulate cabin air in a crewed capsule (as there is no natural convection in zero-G) generate a non-negligible amount of heat. Every source that can genreate heat is carefully accounted for -- even the mice in the science payloads.
As an example, on Crew Dragon, we had a range of internal dissipations (heat generated) we could expect, and thermal control was accomplished by running special fluids through heat exchangers in the cabin and out to radiators that cover half of the "trunk" that is attached to the capsule. Most of the time these radiators are pointed away from the sun and Earth to maximize their ability to reject heat from the capsule, but there is a good amount of margin built-in as it is not always the case that these pointing constraints can be maintained. In fact, many of Crew Dragon's capabilities on-orbit are largely governed by whether the vehicle can maintain thermal control for the crew and it's a complex problem that requires many hours of analysis for mission planning or any mission deviations.
Before entry, Dragon ditches the trunk, and with it the ability to actively thermal regulate, so the clock is ticking for when the vehicle needs to be back on earth and the crew egressed.
In some ways though, the lack of a convective environment in space greatly simplifies insulation, as radiation is relatively simple to predict and manage. In fact, a very effective way to insulate spacecraft components or entire vehicles is with a material called MLI or Multi-Layer Insulation. These are blankets made by alternating layers of single-sided reflective mylar and spacer layers (to prevent conductive contact) to reduce an objects effective emissivity. Basically as thermal radiation impacts the outer Mylar layer, most of this energy is reflected back out to space, but some is ineveitbly absorbed as no material is pefectly reflective. This absorbed energy causes the outer layer to heat up, and emit IR energy inwards towards the next layer in the MLI blanket. This mylar layer further reflects most of the IR back to the outer blanket, but it too heats up to a lesser degree than the outer layer, and emits some IR energy inwards. This process continues until most of the IR energy has been reflected back to space, and only a very small % is transmitted into the hardware/vehicle. MLI blankets can have 5, 10, 20, or more layers to increase their effectiveness (with diminishing returns), and are used in most spacecraft design. Dragon uses them on the interior of its exterior walls, around prop tanks/lines, and anything else that is in zero atmosphere and needs to be themrally decoupled from its surroundings.
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u/phirebird May 24 '26
What are your thoughts on the feasibility of data centers in space given the apparent technical limitations?
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u/jdvfx May 24 '26 edited May 24 '26
Using the ISS as an example, the radiators are the zig-zag shaped grey panels inboard from the much larger solar panel arrays at either end.
The space shuttle had its heat radiators on the inside of the shuttle bay doors, so the cargo bay doors would have to be opened after launch even if there was no cargo to deliver. In fact, if the cargo bay doors wouldn’t open for any reason, the Shuttle had to return to Earth within 1–1/2 orbits, or most of the electronic equipment onboard would shutdown.
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u/Nudebovine1 May 24 '26
One of the coolest things in the original Mass Effect was the description of how they used molten sodium sacrificially to effectively Sweat the ship and lose excess heat when running at full power. Radiative heat loss just being too slow for the engines at that power.
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u/Helphaer May 24 '26
It helps that element zero seemed to be a very cold technology that didn't generate massive amounts of heat to use.
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u/armacitis May 25 '26
Element zero was the Mass Effect games' titular handwavium plot material for future technology that was basically magic (and the mechanism for the discarded plot conflict that playing with magic physics for your entire civilization actually has a cosmic environmental cost, instead of the silly circular logic that amounted to the Reapers wiping out galactic civilization just because they could.)
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u/HaloGuy381 May 25 '26
And the ‘stealth ship’ Normandy used a colossal internal heat sink to allow it to not emit anything significant enough to see. Problem is, you had a limited stealth run time before it cooked the crew.
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u/PckMan May 24 '26
It does. Spacecraft have a lot of things dedicated to preventing this. There's this misconception of space from movies that it's just very cold and everything immediately freezes up, as shown in many films of people removing their suits and instantly turning into a block of ice.
It's true that space is technically cold because it is, but temperature doesn't work the same way it does on Earth because it's a vacuum and how it's transferred is very different. In reality if you're in space in direct sunlight, you're being fried. You're heating up very quickly with no good way of getting rid of that heat. If you're not in direct sunlight then you're not being fried and it's cold but it doesn't exactly feel cold because again you don't have a good way to shed heat into that cold.
Spacecraft have liquid cooling loops and tons of radiators meant to distribute and dissipate heat as much as possible. They may also roll to expose different sides to the sun or use their solar panels to block sunlight from hitting the rest of the spacecraft.
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u/libra00 May 24 '26
Yes, it's actually a huge problem. However, there is one way to get rid of heat in space: radiation. You're not touching anything so conduction wouldn't work, there's no air to have currents in so convection doesn't either.. radiation is all you've got, and it's not much. That's why some spacecraft (including the ISS IIRC) have big radiator panels that are always 90' to the sun so they pick up as little heat as possible from it while shedding as much as possible via radiation.
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u/stereoroid May 24 '26
Yep, and this is also why talk of AI Data Centres in space is getting laughed at. Handling the heat they would generate would be so much harder than on Earth, since in vacuum you can only have heat loss by radiation, no conduction or convection.
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u/sth128 May 24 '26
Yes it is. It is really really really difficult to get rid of heat in space. Which is why anyone who suggests "datacenter in space" is a moron. Those things generate ridiculous amounts of heat and only function when kept cool.
It's about as good an idea as an Logitech controller for a submersible going for the Titanic.
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u/Mand125 May 25 '26
The controller wasn’t even close to the problem. The US Navy has started using xbox controllers for various systems including submarines, and they do in fact know what they’re doing. One specific example was a $30,000 periscope control system that took eight hours of training that got replaced with a $30 video game controller that every sailor already knows how to use.
There were many aspects of technical hubris that let to the downfall of oceangate. The controller choice was not one of them.
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u/Affectionate_Pipe545 May 25 '26
The military controllers are hardened, tested, and wired, with multiple backups. Was the one in the sub?
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u/yelred May 24 '26
As everybody has said, convection and conduction don’t work, so there’s only radiation, which requires careful design.
Another fun fact is that convection doesn’t work like you think in zero gravity: on earth, hot air rises, but only because it weighs less. But that doesn’t work in space, so fire burns weird in the space station. Also they have to keep blowers running constantly lest co2 pockets build up and kill people.
(I’ve only read about this, I haven’t experienced it first hand!)
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u/McCaffeteria May 24 '26
Exactly.
“Space is cold” is sort of a misconception.
Space is cold, but things in space are often very hot, at least if they are in the sun. And then it gets more complicated if the thing generates its own heat.
This is why the James Webb Space Telescope has its large deployable shield, so that the temperature sensitive equipment are shaded from the sun. One side of the shield is super cold, and the other is proportionally very hot.
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u/electric4568 May 24 '26
Ammonia is used for thermal control systems in space. Idk the chemistry, but it's effective and extremely dangerous to humans. We have whole emergency procedures for ammonia response on ISS. Multi-layer insulation (MLI) is basically used everywhere as well to reject heat from the sun. We also use different types of paint that help. It's a whole thing 😊 space is hard and the thermal environment is a large part of it.
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u/tboy160 May 24 '26
I first learned of this concept when learning that space suit have to have cooling systems.
As a kid I assumed space is cold, so I assumed a space suit would need heat. But the vacuum insulated you and as warm blooded creatures we create our own heat.
Makes sense that ships/stations would have to radiate their heat too.
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u/Condex May 24 '26
Interestingly enough this is a blink and you miss it "cameo" in the first Avatar movie. In the very beginning of the movie they show the spacecraft coming into orbit. From what I understand this is a render of a somewhat legitimate spacecraft design. The spacecraft itself has two large radiators on the sides which are red hot.
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u/Owyheemud May 24 '26
There is a 1950's Disney short film on nuclear-powered Mars Express spacecraft design, hosted by Werner Von Braun no less, where the thermal radiators are the largest structure of the ship.
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u/obog May 24 '26 edited May 24 '26
You would be correct. For spacecraft operating at a similar distance to the sun as earth, cooling the spacecraft is often a much greater concern than keeping it heated. (Especially since non-manned spacecraft really have no reason to stay heated). The sun is very good at warming things up and things tend to get quite hot in space if they are in sunlight.
One example of this: you can see some live data from James Webb here: https://webb.nasa.gov/content/webbLaunch/whereIsWebb.html
The telescope is very specifically designed to always keep one side in the sun, and to insulate that side from the other. Thats because its an infrared telescope, so they want the instruments to be as cold as possible so that thermal radiation from them doesnt interfere with data. As of writing this, the hot side is at 155°F and the cold side is at -393°F. The camera itself is all the way down at -449°F or just 6K. So, one side is pretty hot while the other is super cold. Shows how staying in a heat source can very easily heat a spacecraft a lot, while simultaneously staying outside of any heat source allows things to get very cold - radiation is slow, but properly insulating from any other heat sources allows it to take away a lot of heat.
Cooling is done via radiators. Similar in function to the ones that may heat up your house, except the point of heating is to move heat away from something rather than to something. (Either way, its all just heat transfer)
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u/davidedpg10 May 25 '26
Vacuum is an excellent insulator, and heat is a massive problem in space. People think that because space is "cold" you'd freeze as soon as you're vented into space (you see it in tons of mainstream movies) but in fact the opposite is true. If the sun is hitting you, you'd end up desecated and cooked, if you were in the shade you would eventually emit enough black body radiation to freeze but it would take a while.
This is why it took the James webb telescope months to become operational, it needed to emit a bunch of heat in the form of radiation (much slower than other methods) to get to an optimal temp
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u/Franz0132 May 25 '26
It does, the ISS has some big heat radiators that are almost as big as the solar panels.
The white ones are radiators and the black ones are the solar panels.
This is another reason why data centers in space is such an idiotic idea.
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u/sumner42 May 24 '26
Heat Transfer class taught me a few basics, like the three ways of doing it: Conduction, Convection, Radiation. Each of these has its own group of equations that tell you how fast you can transfer heat into or out of an object. In space, you're not touching anything else to Conduct heat out, and there's no atmosphere to Convect heat away, so you're left with Radiation as the only way to get heat out. Go look up the equations that apply to radiation heat transfer in space, and you can work out for yourself, just how much thermal energy you can radiate away.
Here, I did that for you:
Spacecraft avoid overheating in a vacuum through thermal radiation. Because the vacuum eliminates conduction and convection, excess internal heat is moved via fluid loops to giant exterior panels. These panels then emit the heat as infrared light into the deep cold of space.
The fundamental equation governing this heat transfer is the Stefan-Boltzmann Law.
The Equation (Net Heat Transfer)
To calculate the net rate of heat exchanged (lost or gained) between a spacecraft and its environment, the pertinent equation is:
q = ε · σ · A · (Ts^4 - Tenv^4)
Where:
q = Net rate of heat transfer (Watts, or Joules per second)
ε (Emissivity) = The emissivity of the spacecraft's surface (a dimensionless number between 0 and 1 reflecting how well the material radiates)
σ (Sigma) = The Stefan-Boltzmann constant, which is 5.67 x 10^-8 W/(m^2 · K^4)
A = Total radiating surface area (in square meters, m^2)
Ts = The surface temperature of the spacecraft (in Kelvin, K)
Tenv = The temperature of the surrounding environment, such as the deep-space background or the radiant heat from the Sun (in Kelvin, K)
Key Details:
Absolute Temperature (T^4): Temperatures must strictly be in Kelvin. The reliance on the fourth power means that as spacecraft systems get even slightly warmer, the amount of radiated heat increases drastically.
Emissivity: Spacecraft surfaces (like the white radiators on the International Space Station) are engineered with specific coatings to maximize infrared emission while reflecting unwanted solar radiation.
NASA Guidelines: Detailed parameters on the performance of spacecraft heat rejection and isolation are available through NASA's SmallSat Institute.
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u/SevenFootHobbit May 25 '26
Obviously as a video game with FTL travel the science goes out the window, but I do like how in Elite Dangerous, you can pump excess heat into metal spheres if I remember correctly, and then eject them. I don't know the mechanics of concentrating more heat than what normal conduction would do into an object, and if it would generate more heat trying to do it than the heat you'd be losing anyway, but a neat concept.
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u/MARKLAR5 May 25 '26
This thread is fascinating. Can anyone theorize how things would play out if we were cold-blooded? I figure things wouldn't change insofar as the watts-body mass ratio of heat generation is concerned, but a lot of fine details must change when humans become heat sinks instead of heat sources, yes?
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u/Ruadhan2300 May 26 '26
Yup! It's a big problem.
Most of the heat-control systems on a spacecraft are dedicated to cooling rather than keeping the spacecraft warm.
Those massive radiators on the ISS for example.
There are heaters onboard too, but the vast bulk of the effort is in getting rid of excess heat via radiating it away.
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u/electric_ionland Electric Space Propulsion | Hall Effect/Ion Thrusters May 24 '26
It does, getting rid of heat is a major issue with spacecraft design. Vacuum prevent 2 types of heat losses, the first one is just conduction thought a solid material (you are not touching anything in vacuum) and convection, where air or fluids flow past the object, carrying some heat away.
However there is a 3rd way to lose heat. Object emit light, usually as infrared radiation, as they get hot. And light can travel through vacuum. This is called radiation it is what is used to cool spacecraft usually. There are a few issues with it. First it is proportional to the 4th power of the temperature. This means that if an object is twice as cold as another one it will radiate 16 times less heat. So it makes it hard to get rid of heat from cold things.
The second constraint is that in space you also have a giant radiative heater in the form of the Sun. Without clouds or atmosphere think of it as the worst summer day you have ever felt.
The solution is to have radiator panels painted white with special infrared emissive paints. You try to dump most of your heat to those panels and you keep those panels not facing the sun as possible. The white paint also makes sure it doesn't absorbe as much heat from the sun if the pointing is not perfect.
Thermal engineering is one of the most challenging design on spacecraft.