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jordanb parent
That's actually most of space. Space is a very hot environment, especially where we are so close to the sun. Think about it. When you stand outside in the sun you heat up. All that heat is coming from the sun. But a lot of it was filtered by the atmosphere, so if you're in space near earth it will be hotter than standing at the equator on a sunny day, in terms of radiation.

Then there's the fact that heat is very difficult to get rid of when in space. The ISS's radiators are much bigger than its solar panels. If you wanted to have a very-long eva spacesuit you'd have to have radiators much bigger than your body hanging off of it. Short evas are handled by starting the eva with cold liquids in the suit and letting them heat up.

All of the mockups of starships going to Mars mostly fail to represent where they're going to put the radiators to get rid of all the excess heat.

hwillis
> If you wanted to have a very-long eva spacesuit you'd have to have radiators much bigger than your body hanging off of it.

I was curious about this! The Extravehicular Mobility Units on the ISS have 8 hours of life support running on 1.42 kg of LiOH. That releases ~2 kJ per gram used, so .092 watts.

The 390 Wh battery puts out an average of 50 watts.

And the human is putting out at minimum 100 watts with bursts of 200+.

Long term it's probably reasonable to need at least 200 watts of heat rejection. That's about a square meter of most radiator, but it needs to be facing away from the station. You could put zones on the front/back and swap them depending on direction, as long as you aren't inside an enclosed but evacuated area, like between the Hubble and the Shuttle. The human body has a surface area of roughly 2 m^2 so its definitely not enough to handle it- half of that area is on your arms or between your legs and will just be radiating onto itself.

It's also not very feasible to have a sail-sized radiator floating around you. You'd definitely need a more effective radiator- something that absorbs all your heat and glows red hot to dump all that energy.

HPsquared
Or, evaporative cooling for spacewalks. Water heat of evaporation at 25°C is 678 Wh/kg, so 200W of heat is about 0.3 kg per hour. Quite manageable!

EDIT: Apparently the Apollo suits did this. An interesting detail is that they used sublimation (evaporating ice directly to vapor), because I suppose that's a lot more practical to exchange the heat.

pomian
Reminds me of the book Saturn Run, by John Sanford - which has a lot of effort put into the technology and radiation of heat in their space ship. Fun science fiction book.
seekup
I recall a good treatment of this issue in the early part of Joe Haldeman's classic The Forever War. Highly recommended.
bjelkeman-again
Started reading a preview of that. It starts really well. Thanks.
thom
See also: "let's build data centres in space, it's cold up there!"
hwillis
Per wiki: radiators reject 100-350 watts per m^2 and weigh ~12 kg per m^2. Not unlikely you would need 10x as much radiator as server. You need about as much area for radiators as you do for solar panels, but radiators are much heavier.

That also makes nuclear totally infeasible- since turbines are inefficient you'd need 2.5x as many radiators to reject waste heat. Solar would be much lighter.

https://en.wikipedia.org/wiki/Spacecraft_thermal_control#Rad...

perihelions
Nuclear power is very feasible in space. Perhaps you're overlooking that radiated power scales with the quartic of absolute temperature (T⁴); it's not difficult at all to radiate heat from a hot object, as it is for a room-temperature one.

(How hot? I won't quote a number, but space nuclear reactors are generally engineered around molten metals).

hwillis
Yeah, fair to say its feasible. ROSA on the ISS produces 240 W/m^2 and weighs 4 kg/m^2.

The S6W reactor in the seawolf submarines run at ~300 C and produce 177 MW waste heat for 43 MWe. If the radiators are 12 kg/m^2 and reject 16x as much heat (call it 3600 W/m^2) then you can produce 875 watts of electricity per m^2 and 290 watts at the same weight as the solar panels. Water coolant at 300 C also needs to be pressurized to 2000+ PSI, which would require a much heavier radiator, and the weight of the reactor, shielding, turbines and coolant makes it very hard to believe it could ever be better than solar panels, but it isn't infeasible.

Plus, liquid metal reactors can run at ~600 C and reject 5x as much heat per unit area. They have their own problems: it would be extremely difficult to re-liquify a lead-bismuth mix if the reactor is ever shut off. I'm also not particularly convinced that radiators running at higher temperatures wouldn't be far heavier, but for a sufficiently large station it would be an obvious choice.

perihelions
It goes up to 1,344 °C with Li, I think—it's a very different engineering space from the stuff on Earth.

The Soviet ones used K (or maybe NaK eutectic); there's a ring of potassium metal dust around the Earth people track by radar (highly reflective)—a remnant from one of them exploding.

ww520
The idea is not completely without merit. In gravity less environment, you can have much bigger and much thinner structure possible than on Earth.

Also the radiated heat from the Sun won't have much effect if the heat sink panels are facing perpendicular to the sun with two sides pointing sideway to deep space to radiate away the heat.

cma
But boiling water is just a few hundred Kelvin, this is tens of thousands. Would EVA spacesuits be able to radiate that much away if it was really that hot but for the atmosphere absorbing some?

I know it is much hotter, but that's way way hotter and they only find it at a "wall" way farther out.

This is more the temperature of the solar wind, dwarfing the steady state temperature you'd reach from the photonic solar radiation at any distance. The Sun's blackbody varies from like 5000K to 7000K, you won't see objects heated in the solar system heated higher than that even with full reflectors covering the field of view of the rear with more sun and being near the surface of the sun, other than a tiny amount higher from stellar wind, tidal friction, or nuclear radiation from the object's own material I don't think.

foxyv
> Would EVA spacesuits be able to radiate that much away if it was really that hot but for the atmosphere absorbing some?

Yes! The tiny number of particles are moving really fast, but there are very few of them. We are talking about vacuum that is less than 10^-17 torr. A thermos is about 10^-4 torr. The LHC only gets down to 10^-10 torr. At those pressures you can lower the temperature of a kilometer cube by 10 thousand kelvin by raising the temperature of a cubic centimeter of water by 1 kelvin. There is very little thermal mass in such a vacuum which is why temperature can swing to such wild levels.

This is also why spacecraft have to reject heat purely using radiation. Typically you heat up a panel with a lot of surface area using a heat pump and dump the energy into space as infrared. Some cooling paints on roofing do this at night which is kind of neat.

jamiek88
To add to this: Most of the heat the EVA suits deal with is generated by the human inside not the giant ball of nuclear fusion 8 light minutes away.
foxyv
Solar radiation is roughly 1 kilowatt per square meter. Human beings generate about 0.1 kilowatts. A good suit will try to reject as much of that kilowatt as possible. Also your dark side will radiate heat but the temperature differential is much lower.

Suits are insulating for a reason. You want to prevent heating on the sun side and prevent too much cooling on the space side. Your body is essentially encapsulated in a giant thermos.

Cooling is achieved using a recirculating cold water system that is good for a few hours of body heat. Water is initially cooled by the primary life support system of the spacecraft before an EVA. Pretty much it starts off pretty cold and slowly over time comes up to your body heat. Recent designs use evaporative cooling to re-cool the water.

Life support systems are so cool.

rtkwe
Absorbed light too but that's a bit easier to deal with and is why most things are white or reflective on the outside of anything in space that's not intentionally trying to absorb heat.
semi-extrinsic
At this low density, temperature is very different from what you are used to experiencing. You have to work through a heat flux balance to really get a grasp of it.

Temperature is just the heat of particles moving. In the extreme case of a handful of N2 molecules moving at 1% the speed of light, it has a temperature of something like 9 billion Kelvin. But it's not going to heat you up if it hits you.

cma
Even at low density, if it were a large volume, solid objects would heat up to that ambient temp. But this one is a minor volume and you would still be radiating it away much faster and not reach anywhere near the ambient temperature. In the middle of a large volume thoigh, you'd get too much incoming thermal radiation from particles within the volume and not be able to shed heat anywhere through radiation.
api
Lack of radiators is endemic in sci-fi. All those cool starships and torch rockets would bake their crews and then melt.

I didn't like the Avatar films except for the starships, which are among the more physically realistic in construction including massive radiators. They'd probably need to be even bigger though IRL if you're talking about something loony like an antimatter rocket.

PantaloonFlames
> Think about it. When you stand outside in the sun you heat up. All that heat is coming from the sun. But a lot of it was filtered by the atmosphere, so if you're in space near earth it will be hotter than standing at the equator on a sunny day, in terms of radiation.

I think you’re missing the key point - heat transfer. The reason we feel hot at the beach is not solely because of heat we absorb directly from solar energy. Some of the heat we feel is the lack of cooling because the surrounding air is warm, and our bodies cannot reject heat into it as easily as we can into air that is cool. And some is from heat reflecting up from the sand.

Theres a heat wave across much of the US right now. Even when the sun goes down it will still be hot. People will still be sweating , doing nothing, sitting on their porches. Because the air and the surrounding environment has absorbed the sun’s heat all day and is storing it.

That’s what you’re neglecting in your analysis of space.

im3w1l
Okay this may sound silly but what about a solar powered ac for cooling? Like solar radiation is 6000K right, so if you used that to pump your waste heat into say a 1000K radiator (aimed away from the sun obviously) I'm thinking it might give you plenty of negentropy but also radiate away heat at a decent pace.
itishappy
Skip the Sun! There's an "atmospheric window" in the IR. If you make a material that emits/absorbs (they're reversible) only in that region, and don't expose it to the Sun, then it will cool down to the temperature of space, roughly 3K or -270°C. In practice, it won't cool down anywhere near that much. It'll steal energy from it's surroundings due to conduction/convection, and the amount of energy that's actually radiated in this band by a slightly below room temperature material is pretty minimal. Still neat, entirely passive cooling by radiating to space!

https://en.wikipedia.org/wiki/Atmospheric_window

https://en.wikipedia.org/wiki/Passive_daytime_radiative_cool...

o11c
Note that for most of the United States, you must resurface your roof twice a year for this to be effective.
thatguy0900
Acs don't get rid of heat, they just move it around. At some point you need to put the heat somewhere and then your just back to giant radiators
hwillis
Radiative heat transfer is proportional to T^4. If your suit is 300 K(80F), bumping the temperature up by 100 C lets you radiate 3.16x as much heat from the same area.
eesmith
https://en.wikipedia.org/wiki/Absorption_refrigerator

> An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling process. Solar energy, burning a fossil fuel, waste heat from factories, and district heating systems are examples of heat sources that can be used. An absorption refrigerator uses two coolants: the first coolant performs evaporative cooling and then is absorbed into the second coolant; heat is needed to reset the two coolants to their initial states.

https://www.scientificamerican.com/article/solar-refrigerati...

> Fishermen in the village of Maruata, which is located on the Mexican Pacific coast 18 degrees north of the equator, have no electricity. But for the past 16 years they have been able to store their fish on ice: Seven ice makers, powered by nothing but the scorching sun, churn out a half ton of ice every day.

Sharlin
Yes? That's in the atmosphere where heat rejection is a vastly easier problem than in vacuum, thanks to convection.
mrguyorama
It literally doesn't matter what your refrigeration process is. You have to "reject" the heat energy at some point. In space, you can only do that with large radiators.

There is no physical process that turns energy into cold. All "cooling" processes are just a way of extracting heat from a closed space and rejecting it to a different space. You cannot destroy heat, only move it. That's fundamental to the universe. You cannot destroy energy, only transform it.

Neither link is a rebuttal of that. An absorption refrigerator still has to reject the pumped heat somewhere else. Those people making ice with solar energy are still rejecting at minimum the ~334kj/kg to the environment.

An absorption refrigerator does not absorb heat, it's called that because you are taking advantage of some energy configurations that occur when one fluid absorbs another. The action of pumping heat is the same.

eesmith
The question was 'what about a solar powered ac for cooling?', yes?

Giant radiators don't make ice.

The proposed method of pumping heat into someplace hot to make it hotter doesn't work. But there area definitely ways to do solar powered ac for cooling.

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