Comment by PeterHolzwarth
2 days ago
"space is cold"
I've always enjoyed thinking about this. Temperature is a characteristic of matter. There is vanishingly little matter in space. Due to that, one could perhaps say that space, in a way of looking at it, has no temperature. This helps give some insight into what you mention of the difficulties in dealing with heat in space - radiative cooling is all you get.
I once read that, while the image we have in our mind of being ejected out of an airlock from a space station in orbit around Earth results in instant ice-cube, the reality is that, due to our distance from the sun, that situation - ignoring the lack of oxygen etc that would kill you - is such that we would in fact die from heat exhaustion: our bodies would be unable to radiate enough heat vs what we would receive from the sun.
In contrast, were one to experience the same unceremonious orbital defenestration around Mars, the distance from the sun is sufficient that we would die from hypothermia (ceteris paribus, of course).
A perfect vacuum might have no temperature, but space is not a perfect vacuum, and has a well-defined temperature. More insight would be found in thinking about what temperature precisely means, and the difference between it and heat capacity.
I think your second sentence is what they were referencing. Space has a temperature. But because the matter is so sparse and there’s so little thermal mass to carry heat around as a result, we don’t have an intuitive grasp on what the temperature numbers mean.
To rephrase it slightly. It's not a perfect vacuum, but compared to terrestrial conditions it's much closer to the former than the latter. The physics naturally reflects that fact.
To illustrate the point with a concrete example. You can heat something with the thermal transfer rate of aerogel to an absurdly high temperature and it will still be safe to pick up with your bare hand. Physics says it has a temperature but our intuition says something is wrong with the physics.
I think otherwise.
I think the better argument to be made here is "space has a temperature, and in the thermosphere the temperature can get up to thousands of degrees. Space near Earth is not cold."
Assuming merely attitude control, sure only radiative cooling is available, but its very easy to design for arbitrary cooling rates assuming any given operating temperature:
Budget the solar panel area as a function of the maximum computational load.
The rest of the satellite must be within the shade of the solar panel, so it basically only sees cold space, so we need a convex body shape, to insure that every surface of the satellite (ignoring the solar panels) is radiatively cooling over its full hemisphere.
So pretend the sun is "below", the solar panels are facing down, then select an extra point above the solar panel base to form a pyramid. The area of the slanted top sides of the pyramid are the cooling surfaces, no matter how close or far above the solar panels we place this apex point, the sides will never see the sun because they are shielded by the solar panel base. Given a target operating temperature, each unit surface area (emissivity 1) will radiate at a specific rate, and we can choose the total cooling rate by making the pyramid arbitrarily long and sharp, thus increasing the cooling area. We can set the satellite temperature to be arbitrarily low.
Forget the armchair "autodidact" computer nerds for a minute
Making the pyramid arbitrarily long and sharp will arbitrarily diminish the heat conductance through the pyramid, so the farther from the pyramid base, the colder it will be and the less it will radiate.
So no, you cannot increase too much the height of the pyramid, there will be some optimum value at which the pyramid will certainly not be sharp. The optimum height will depend on how much of the pyramid is solid and which is the heat conductance of the material. Circulating liquid through the pyramid will also have limited benefits, as the power required for that will generate additional heat that must be dissipated.
A practical radiation panel will be covered with cones or some other such shapes in order to increase its radiating surface, but the ratio in which the surface can be increased in comparison with a flat panel is limited.
we are not discussing a schoolbook exercise, we are not calculating passive heat conduction of a pyramid heated to a base, since it's not a schoolbook exercise we can decide on the condition, we could put in heat pipes etc.
its CPU/GPU clusters, so we don't have 0 control on where to locate what heat generators, but even if we had 0 control over it, the shape and height of the pyramid does not preclude heat pipes (not solid bars of metal, but having a hot side where latent heat of a gas condensing to a liquid on the cold side and then evaporating on the hot side).
heat pipes have enormous thermal conductivities
> The rest of the satellite must be within the shade of the solar panel,
Problem is with solar panels themselves. When you get 1.3kW of energy per square meter and use 325w of that for electricity (25% efficiency) that means you have to get rid of almost 1kW of energy for each meter of your panel. You can do it radiatively with back surface of panels, but your panels might reach equilibrium at over 120°C, which means they stop actually producing energy. If you want to do it purely radiatively, you would need to increase temperature of some surface pointing away from sun to much more than 120°C and pump heat from your panels with some heatpump.
When the cost of the solar panels does not matter you can reach an efficiency close to 50% (with multi-junction solar cells) and the panels will also be able to work at higher temperatures.
Nevertheless, the problem described by you remains, the panels must dissipate an amount of heat at least equal with the amount of useful power that is generated. Therefore they cannot have other heat radiators on their backside, except those for their own heat.
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no matter how inefficient the solar panels, even with 1% efficiency, you could make the pyramid sharp enough to dissipate the heat stabilizing at any arbitrary low temperature (well, must still be above the temperature of CMB)
The sun is not the only radiative body in the solar system.
> Temperature is a characteristic of matter. There is vanishingly little matter in space. Due to that, one could perhaps say that space, in a way of looking at it, has no temperature.
Temperature: NaN °C
Temperature is a property of systems in thermal equilibrium. One such system is blackbody radiation, basically a gas of photons that is in thermal equilibrium.
The universe is filled with such a bath of radiation, so it makes sense to say the temperature of space is the temperature of this bath. Of course, in galaxies, or even more so near stars, there's additional radiation that is not in thermal equilibrium.
Related: what color is space?
It's "Cosmic latte". https://en.wikipedia.org/wiki/Cosmic_latte
I saw that too but wonder if it's different for the sparse matter in the interstellar medium, excluding the visible objects.