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NASA's Voyager Found a 30k-50k Kelvin "Wall" at the Edge of Solar System

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246 points world2vec | 4 comments | 23 Jun 25 16:24 UTC | HN request time: 1.667s | source
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mlhpdx ◴[23 Jun 25 16:58 UTC] No.44357730[source]▶
It’s very odd to think of something extremely hot but with almost no density, and therefore very little heat transfer.
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jordanb ◴[23 Jun 25 17:21 UTC] No.44357945[source]▶
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.

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thom ◴[23 Jun 25 18:31 UTC] No.44358632[source]▶
See also: "let's build data centres in space, it's cold up there!"
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1. hwillis ◴[23 Jun 25 19:01 UTC] No.44358931[source]▶
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...

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2. perihelions ◴[23 Jun 25 19:11 UTC] No.44359019[source]▶
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).

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3. hwillis ◴[23 Jun 25 19:53 UTC] No.44359376[source]▶
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.

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4. perihelions ◴[23 Jun 25 20:20 UTC] No.44359654{3}[source]▶
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.