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points by overfeed 3 days ago | 0 comments

> Well, it can’t use more than it can get from the sun that hits it. So it can’t need to radiate more than that away

You are correct that the first law of thermodynamics dictates the energy, but anyone who has parked a car in the sun will tell you, the temperature where the absorption/dissipation equilibrium is achieved is often incompatible with the survival of the cargo.

edit to your edit:

> Edit: ISS is about 90KW steady state, and the collector to radiator area ratio is about 5:1. Dunno about the mass ratio.

Cool; that's about half the wattage of one modern full-height Nvidia AI rack (172kW) - you need two ISSes to service just 1 rack; extrapolate that to a data center's worth of racks and see if the math is mathing. The ISS is about the size of a(n American) football field.

[–] robotresearcher 3 days ago

Again, just to remind you, I didn’t claim space datacenters are economically rational.

[–] overfeed 3 days ago

My original claim, in its entirety, was that on earth "You'll also get much better cooling for free via convection."

[–] robotresearcher 3 days ago

You sure will. My contribution is that you don’t need much better cooling.

[–] robotresearcher 3 days ago

The average temperature of a rock, car, or the ISS in low earth orbit is about -18C.

[–] Sammi 3 days ago

It's this huge misconception that space is super cold. A better mind model is that space doesn't have much of a temperature at all just cause there's nothing there. Vacuum doesn't have a temperature.

You won't immediately freeze if you enter space without a suit. You'll die almost immediately, but it will be because your blood will begin to boil because of the lack of pressure kinda like an extreme form of divers sickness - it's just not the cold that'll get you first.

[–] robotresearcher 2 days ago

That’s all true. But stuff in space has a temperature, and once I’d been dead a while in LEO, mine would be -18C.

[–] overfeed 2 days ago

> mine would be -18C

FYI; that's not how it works, you and the rock you mentioned in an earlier comment cannot have the same temperature of -18C. This is due to differences in material, and geometry to a lesser extent.

Even if you were rock-shaped, the rates of energy absorption and radiation is what determines your remains' temperature, this ratio differs for different materials, and thus difference fiilnal temperatures.

[–] robotresearcher 2 days ago

Sure it varies a bit. ISS average skin temp is 10C, for example.

The total available energy from the sun in LEO averages about 1.36KW/m^2, and more than this is radiated away by current deployed technology.

[–] overfeed 2 days ago

> Sure it varies a bit

No, not just a bit. It varies by hundreds of degrees depending on the materials α/ε ratio.

Your mental model is off: materials that good absorbers but poor emitters will get very hot, and the flip-side stay cool, and are used to make the cooling elements in the ISS . The temperature of ball of black nickel at 1 AU will settle at balmy 239C - not -18C. Polished Aluminum -> roughly 145C.

[–] robotresearcher 2 days ago

I’ve made a note not to make my space data center a black nickel sphere, and instead to make it like we generally make satellites.

My nickel guy is gonna be so disappointed.

I do understand the correction, so thank you. Doesn’t change my point at all, and I fear we are talking past each other so I’ll call it a day.