this post was submitted on 22 Jul 2025
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ELI5 please:
Apollo 13 is one of my favourite movies of all time, but one thing that always sticks out to me when I watch it is when they need to shutdown all systems to conserve power, the spacecraft becomes cold enough for the inside to freeze. I've looked this up and it is something that actually happened during the real emergency, but I've never been able to wrap my head around how a capsule with three people inside is able to freeze when the exterior is exposed to direct sunlight.
Space is hot because the average kinetic energy of particles is very high. Space feels cold because there are very few particles in a vacuum. It's like a tiny spark from machinery being very hot individually, but because the spark is so small, it doesn't do much energy to your body.
Having said that , there is about a thousand watts per square metre of insolation coming in from the Sun on the exterior of the craft, just like there is here on the ground on Earth.
I guess the Apollo designers figured it was easier to insulate and heat the cabin than absorb heat and then try and cool it.