Economics of Orbital vs. Terrestrial Data Centers • Hacker News Board

Re: Economics of Orbital vs. Terrestrial Data Centers

#241

Post by yetihehe » Tue, Dec 16, 2025, 5:08 PM UTC

torginus wrote: post #226

TheOtherHobbes wrote: post #194

Earlier quoted context omitted.

σ is such a small number in Stefan-Boltzman that it makes no difference at all until your radiators get hot enough to start melting. You not only need absolute huge radiators for a space data centre, you need an active cooling/pumping system to make sure the heat is evenly distributed across them. I'm fairly sure no one has built a kilometer-sized fridge radiator before, especially not in space. You can't just stick…

Out of curiosity, I plugged in the numbers - I have solar at home, and a 2 m2 panel makes about 500w - i assume the one in orbit will be a bit more efficient without atmosphere and a bit more fancy, making it generate 750w. If we run the radiators at 80C (a reasonable temp for silicon), that's about 350K, assuming the outside is 0K which makes the radiator be able to radiate away about 1500W, so roughly double. Depen…

> 2 m2 panel makes about 500w

It receives around 2.5kW[0] of energy (in orbit), of which it converts 500W to electric energy, some small amount is reflected and the rest ends up as heat, so use 1kW/m^2 as your input value.

> If we run the radiators at 80C (a reasonable temp for silicon), that's about 350K, assuming the outside is 0K which makes the radiator be able to radiate away about 1500W, so roughly double.

1500W for 2m^2 is less than 2000kW, so your panel will heat up.

[0] https://www.sciencedirect.com/topics/engineering/solar-radia...

Re: Economics of Orbital vs. Terrestrial Data Centers

#242

Post by BobaFloutist » Tue, Dec 16, 2025, 5:12 PM UTC

vessenes wrote: post #169

jofer wrote: post #39

What really worries me is that I keep hearing "cooling is cheap and easy in space!" in a lot of these conversations, and it couldn't be farther from the truth. Cooling is _really_ hard and can't use efficient (i.e. advection-based air or water cooling) approaches and are limited to dramatically less efficient radiative cooling. It doesn't matter that space is cold because cooling is damned hard in a vacuum. The artic…

Jusssst had this conversation two nights ago with a smart drunk friend. To his credit when I asked "what's heat?" and he said "molecules moving fast" and I said "how many molecules are there in space to bump against?" He immediately got it. I'm always curious what ideas someone that isn't familiar with a problem space comes up with for solutions, so I canvased him for thoughts -- nothing novel, unfortunately, but if…

I got really annoyed when I first realized that heat and sound (and kinetic energy) are both "molecules moving," because they behave so dramatically differently on a human scale.

And yes, obviously they aren't moving in the same way, but it's still kind of weird to think about.

Re: Economics of Orbital vs. Terrestrial Data Centers

#243

Post by jdhwosnhw » Tue, Dec 16, 2025, 5:20 PM UTC

torginus wrote: post #226

TheOtherHobbes wrote: post #194

Earlier quoted context omitted.

σ is such a small number in Stefan-Boltzman that it makes no difference at all until your radiators get hot enough to start melting. You not only need absolute huge radiators for a space data centre, you need an active cooling/pumping system to make sure the heat is evenly distributed across them. I'm fairly sure no one has built a kilometer-sized fridge radiator before, especially not in space. You can't just stick…

Out of curiosity, I plugged in the numbers - I have solar at home, and a 2 m2 panel makes about 500w - i assume the one in orbit will be a bit more efficient without atmosphere and a bit more fancy, making it generate 750w. If we run the radiators at 80C (a reasonable temp for silicon), that's about 350K, assuming the outside is 0K which makes the radiator be able to radiate away about 1500W, so roughly double. Depen…

This argument assumes that you only need to radiate away the energy that the solar actively turns into electricity, but you also need to dissipate all the excess heat that wasn’t converted. The solar bolometric flux at the earth is 1300 w/m2, or 2600 for 2 sq m. That works out to an efficiency of ~20% for your home solar, and your assumed value of 750 w yields an efficiency of ~30%, which is reasonable for space-rated solar. But assuming an overall albedo of ~5% that means that you were only accounting for a third of the total energy that needs to be radiated.

Put another way, 2 sq m intercepts 2600 w of solar power but only radiates ~1700 w at 350 k, which means it needs to be run at a higher temperature of nearly 125 celsius to achieve equilibrium.

Re: Economics of Orbital vs. Terrestrial Data Centers

#244

Post by yetihehe » Tue, Dec 16, 2025, 5:21 PM UTC

DoctorOetker wrote: post #235

PeterHolzwarth wrote: post #132

Earlier quoted context omitted.

"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…

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…

> 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.

Re: Economics of Orbital vs. Terrestrial Data Centers

#245

Post by yetihehe » Tue, Dec 16, 2025, 5:23 PM UTC

fulafel wrote: post #233

PeterHolzwarth wrote: post #132

Earlier quoted context omitted.

"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…

Related: what color is space?

It's "Cosmic latte". https://en.wikipedia.org/wiki/Cosmic_latte

Re: Economics of Orbital vs. Terrestrial Data Centers

#246

Post by ygouzerh » Tue, Dec 16, 2025, 5:31 PM UTC

rswail wrote: post #134

Aside from the economics, the question is why do it in orbit vs on land (or sea)? What are the regulatory/legal gains? Lack of jurisdiction means open slather? What are the national security gains? Redundancy and resiliency by each satellite being a "micro-compute" connected by high speed laser links? So more resilient to attack? Why do it at all?

I think the only reason is for legal purpose.

If data is downloaded illegally from space, stored in space and model trained on it... it will be a mess juridically if someone complain.

Same for model inference, it will be hard for a government to put controls on the model output.

Re: Economics of Orbital vs. Terrestrial Data Centers

#247

Post by Yizahi » Tue, Dec 16, 2025, 5:46 PM UTC

torginus wrote: post #227

Yizahi wrote: post #187

Earlier quoted context omitted.

Author also forgot batteries for the solar shade transition period and then additional solar panels to charge these batteries during the solar "day" period. then insulation for batteries. Then power converters and pumps for radiators and additional radiators to cool the cooling infrastructure. Overall not a great model. But on the other hand, even an amateur can use this model and imagine that additional parts and co…

I'd say int makes much more sense to just shut off in the sunshade. The advantage of orbital solar, comes not so much from the lack of atmosphere, but the fact that depending on your orbit, you can be in sunlight for 60-100% of the time.

That proposal I've seen a few times too, basically put up a constellation up there, linked with laser comms and then transfer data to the illuminated sats in a loop. That sounds possible, but I have doubts. First of all if we take 400 km orbit, the "online" time would be something like 50 minutes. We need to boot up the system fully, run comm apps, locate a peer satellite and download data from it (which needs to be prepared in a portable form), write it locally and start calculations, then by the end of the 50 min repeat. All these operations are slow, especially boot time of the servers (which could be optimized of course). It would be great if some expert could tell us if it is feasible or not.

Re: Economics of Orbital vs. Terrestrial Data Centers

#248

Post by modeless » Tue, Dec 16, 2025, 5:50 PM UTC

ithkuil wrote: post #167

modeless wrote: post #114

Earlier quoted context omitted.

The temperature that you raise to the fourth power is not Celsius, it's Kelvin. Otherwise things at -200 C would radiate more heat than things at 100 C. Also the temperature of space is ~3 K (cosmic microwave background), not 10 C.

There is a large region of the upper atmosphere called the thermosphere where there is still a little bit of air. The pressure is extremely low but the few molecules that are there are bombarded by intense radiation and thus reach pretty high temperatures, even 2000 C! But since there are so few such molecules in any cubic meter, there isn't much energy in them. So if you put an object in such a rarefied atmosphere.…

These satellites will certainly be above the themosphere. The temperature of the sparse molecules in space is not relevant for cooling because there are too few of them. We're talking about radiative cooling here.

Re: Economics of Orbital vs. Terrestrial Data Centers

#249

Post by uplifter » Tue, Dec 16, 2025, 6:08 PM UTC

torginus wrote: post #226

TheOtherHobbes wrote: post #194

Earlier quoted context omitted.

σ is such a small number in Stefan-Boltzman that it makes no difference at all until your radiators get hot enough to start melting. You not only need absolute huge radiators for a space data centre, you need an active cooling/pumping system to make sure the heat is evenly distributed across them. I'm fairly sure no one has built a kilometer-sized fridge radiator before, especially not in space. You can't just stick…

Out of curiosity, I plugged in the numbers - I have solar at home, and a 2 m2 panel makes about 500w - i assume the one in orbit will be a bit more efficient without atmosphere and a bit more fancy, making it generate 750w. If we run the radiators at 80C (a reasonable temp for silicon), that's about 350K, assuming the outside is 0K which makes the radiator be able to radiate away about 1500W, so roughly double. Depen…

>Depending on what percentage of time we spend in sunlight (depends on orbit, but the number's between 50%-100%, with a 66% a good estimate for LEO), we can reduce the radiator surface area by that amount.

You need enough radiators for peak capacity, not just for the average. It's analogous to how you can't put a smaller heat sink on your home PC just because you only run it 66% of the time.

Re: Economics of Orbital vs. Terrestrial Data Centers

#250

Post by uplifter » Tue, Dec 16, 2025, 6:37 PM UTC

terminalshort wrote: post #220

uplifter wrote: post #160

Earlier quoted context omitted.

It's actually only about 3x. As you intimated, the radiated heat Energy output of an object is described by the Stefan-Boltzmann Law, which is E = [Object Temp ]^4 * [Stefan-Boltzmann Constant] However, Temp must be in units of an absolute temperature scale, typically Kelvin. So the relative heat output of a 90C vs 20C objects will be (translating to K): 383^4 / 293^4 = 2.919x Plugging in the constant (5.67 * 10^-8 W…

You forgot about the background. The background temp at Earths distance from the sun is around 283K. Room temperature is around 293K, and a computer can operate at 363K. So for an object at 283K the radiation will be (293^4 - 283^4) = , and a computer will be (363^4 - 283^4) (293^4 - 283^4) = 9.55e8 (363^4 - 283^4) = 1.09e10 So about 10x I have no problem with your other numbers which I left out as I was just making…

The background temp at Earth's orbit is due to the incidence of solar flux, which I took account of.

I'm assuming the radiators are shaded from that flux by the rest of the satellite, for efficiency reasons, so we don't need to account for solar flux directly heating up the radiators themselves and reducing their efficiency.

In the shade, the radiators emission is relative to the background temp of empty space, which is only 2.7 K[0]. I did neglect to account for that temperature, that's true, but it should be negligible in its effects (for our rough estimate purposes).

[0] https://sciencenotes.org/how-cold-is-space-what-is-its-tempe...