Here's a breakdown of the key themes discussed in the Hacker News thread, along with supporting quotes.
Crystal Spheres and Barriers in Sci-Fi
The discussion starts with a reference to the "Crystal Spheres" concept in science fiction, specifically relating to David Brin's book:
- bbarnett mentions: "https://en.wikipedia.org/wiki/The_Crystal_Spheres Made me think of this Brin book. The first ship to try to leave the solar system, crashes into an invisible crystal barrier. It's unbreakable."
Several users then discuss the specifics and apparent paradoxes within the plot of the book, primarily focusing on how to break the unbreakable sphere. * pulvinar: "It crashes into the sphere, but must have also broken it: 'the only way to break the crystal spheres is from the inside'." * throwawayffffas: "Yeah found it online, the ship that crashed on it broke it."
The (Counterintuitive) Heat of Space and the Voyager 2 Boundary
The conversation shifts to a real-world "boundary" as illuminated by Voyager 2 as linked in the OP, leading to a discussion about the heat and temperature involved, delving into the nature of heat transfer in the vacuum of space.
- mlhpdx: "It’s very odd to think of something extremely hot but with almost no density, and therefore very little heat transfer."
- jordanb: "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."
- 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."
- semi-extrinsic: "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."
Spacecraft Thermal Management: Radiators and Heat Rejection
A major theme revolves around how spacecraft, and particularly EVA suits, manage heat in space, including the necessity of large radiators.
- jordanb: "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."
- jordanb: "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."
- foxyv: "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."
- 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."
- mrguyorama: "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."
- 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."
- hwillis: "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."
Solar-Powered Cooling and Absorption Refrigeration
The discussion explores the possibility of using solar power for cooling in space, including the concept of absorption refrigerators.
- 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."
- eesmith: "[...] 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."
- mrguyorama: "Those people making ice with solar energy are still rejecting at minimum the ~334kj/kg to the environment."
Passive Radiative Cooling and Atmospheric Windows
The possibility of passively cooling using radiative heat transfer through atmospheric windows is brought up.
- 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."
- energ8: "It's a thing in from thousands of years ago https://en.m.wikipedia.org/wiki/Yakhch%C4%81l and today https://en.m.wikipedia.org/wiki/Passive_daytime_radiative_co..."
Space-Based Data Centers and Thermal Challenges
The discussion briefly touches upon the idea of locating data centers in space for cooling purposes, highlighting the radiator requirements.
- 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."
Sci-Fi Recommendations
A final, smaller theme is the recommendation of science fiction books with realistic depictions of heat management in space.
- 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."