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Comment by gregschlom | Hacker News Reader

The video at the end of the article is absolutely fascinating.

Also, it's the first time I hear that rejecting heat is a big deal in space. If anything, I would have assumed that heating the station would have been a problem, rather than keeping it cold.

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Most people assume that, and most sci-fi reinforces that opinion. But a generic Thermos is a vacuum flask.

Space is an incredible insulator. The only way to cool things is to radiate heat.

Thank you. TIL, actually not learned realized, space is actually a good insulator out of earth conditions too. Prejudices. We can never know what if something we thing is a reality or prejudice. Science and skepticism is that important.

Also of note is that while a vacuum doesn't necessarily have a temperature (it's just a medium), space does.

Even if you go far away from anything else, you will still reach thermal equilibrium at the Cosmic microwave background (CMB) temperature, which is 2.7K -- it's essentially the "temperature of the universe", radiation which has been emitted as far back as the first moments of the universe.

And of course, if you create a vacuum chamber on Earth, any object inside will always reach equilibrium at ambient temperature (in the lack of any heat engines on the body -- even then the fuel would deplete some time).

The sci-fi game series Mass Effect had a good explaination for this, and tied it in with the notion of ships having recognizable "signatures":

http://masseffect.wikia.com/wiki/Codex/Ships_and_Vehicles#St...

It also works similarly in Eve Online, btw.

Indeed, whenever corp-mates would ask why there are brightly lit "windows" on their ships I would point out that they were actually heat dissipation panels.

A major mechanic in Elite Dangerous is managing your ship's internal heat and heat signature.

Technically interplanetary space is a hot plasma too. The particles emitted by the Sun are very hot according to kinetic theory (which only depends on the mean velocity of particles) since they have a lot of energy. It's a plasma because the particles are charged and accelerated by the Sun's magnetic field.

Of course the density of these particles is very low, so if your definition of heat factors those in then space is cold.

Evaporative cooling ought to work really well?

It would with an unlimited supply of coolant, but bringing mass is expensive. Unless you start grabbing asteroids and using your waste heat to vaporize them or something....

Bolt habitat to asteroid heatsink, profit.

That's what we have Planetary Resources for ;).

But that would use up material, all of which has to be trucked up to orbit at great expense.

Why did it get so cold inside of Apollo 13 then?

My understanding is that the spacecraft was designed to radiate large amounts of heat, with the expectation that systems inside the spacecraft would be producing a lot of it. Apollo 13 shut down much of the equipment inside the spacecraft, which meant that it was no longer producing as much heat as was expected.

you can have further insight here http://www.projectrho.com/public_html/rocket/misconceptions.... and then lose countless hours on that website if the topic fascinates you

your not wrong with losing hours...

And then go and play Kerbal Space Program to get these concepts become intuitive. Warning: another countless hours sink.

so my stupid question is, cannot it be used for power? Surely the same system which radiates the heat can be exploited to generate power with something akin to a sterling engine.

A heat engine is great when your cold sink is 'free', but here, the cold sink would be the radiator fins, and extracting energy from the temperature difference between main ISS and radiator fins would effectively just make the radiator fins worse at cooling the ISS.

Which is fine if you have lots of excess radiator fins. But they almost certainly don't -- shipping up extra radiator fins in order to get power from the excess cooling capacity would be less efficient than just adding solar cells (efficiency of a heat engine maxes out at 1 - (T_cold / T_hot)).

These radiators exist specifically to deal with power which has already been used. As stated in the article, the station needs a few kW of electrical power. Eventually all this power ends up as a waste heat energy. There is nothing that can be done with it, so it has to be dumped into space.

There's also solar heating of the station itself to worry about. Although to some extent you can make the station reflective (white) to deal with that.

Space is pretty much a perfect storm. As if relying only on radiating heat were not enough, you need gravity (or some acceleration) for convection to work. And then simply pumping a fluid through tubes is an interesting issue when the fluid does not know which way is down.

> it's the first time I hear that rejecting heat is a big deal in space

Play some Mass Effect :)

Same exposure to the sun, but no air convection.

The barbecue roll. The Apollo spacecraft would slowly rotate along it's longitudinal axis while it traveled to the moon to help dissipate heat.

Hmm. That made me wonder about Apollo 13, where the "crew endured temperatures at or below freezing for the bulk of the return flight" [1], apparently [2] because they had to turn off heaters and electronic gear to conserve power.

The fact they had heaters suggests that the roll wasn't so much to dump heat as to spread it around, to reduce the risk of structural damage from differential heating [3]. That's presumably less viable on a big structure like the ISS.

[1] http://www.spaceline.org/flightchron/apollo13.html

[2] https://answers.yahoo.com/question/index?qid=20121015080310A...

[3] https://books.google.com/books?id=31GqndM3fk8C&pg=PA419&dq=d...

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