Not friction
Why Do Spacecraft Get So Hot Coming Back?
A returning capsule is not rubbed hot by the air. It squashes the air, and squashed air gets hot.
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Step 1 of 5
Getting back down is the harder half.
All of that speed has to be turned into heat and left behind somewhere. Going up costs fuel. Coming down costs nothing at all — but a spacecraft in orbit is moving at about eight kilometres a second, and all of that has to be got rid of somehow.
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Step 2 of 5
The air is what takes the speed away.
There is no fuel to spare for braking, so the atmosphere does the job. Slamming into thickening air slows the craft down — and the energy has to go somewhere.
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Step 3 of 5
It is squashing, not rubbing.
Most people assume friction. It is not. The craft is moving faster than the air can get out of the way, so the air piles up and is violently compressed — and compressing gas heats it.
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Step 4 of 5
So the glowing part is a cushion, not the surface.
A pointed nose would hold that hot layer against itself The white-hot layer sits just ahead of the craft. A blunt, rounded shape pushes that shockwave further out in front, which is exactly why capsules are not pointed.
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Step 5 of 5
The shield is designed to be destroyed.
The heat shield chars and flakes away on purpose. Every piece that leaves carries heat off with it, so the craft is protected by steadily losing its own outer layer.
The Space Shuttle used tiles instead, which could be reused but had to be inspected one by one.
The short version
A returning spacecraft heats up because it compresses the air in front of it, not because the air rubs against it — so blunt shapes and sacrificial shields work best.
Try it yourself
Pump a bicycle tyre hard and feel the pump barrel. That warmth is compression too, just far gentler.