How come ISS space capsules for re-entry to earth don’t burn up in the atmosphere the same way comets and meteors do?
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They have a very controlled re-entry attitude, they go slower and they have an ablative heat shield.
They’re made totally out of extreme heat resistant metals/materials, unlike meteor/meteorites that are made of chunks of rock that have iron and nickel in them.
For something coming from the ISS, the energy to be scrubbed off is proportional to its orbital speed squared, or 64 (km/s)^(2), while for something coming from far from the Earth, it’s proportional to something like escape speed squared, or 121 (km/s)^(2) — about *twice* as much per kg.
The capsules aren’t dense — they float in the ocean afterwards — and come in with a broad heat shield facing forward, so they can lose a lot of that energy in the thin upper atmosphere.
Meteors come in much faster and uncontrolled. They are also unevenly made which creates added turbulence, friction, and pockets of extreme heat.
Re-entry capsules have heat shields that insulate the capsule. Some portions even burn off to carry the heat away. They are also smooth to transfer the air around as much as possible.
The modules are falling in from near Earth orbit. They come in carefully to bleed of there orbital velocity in the upper atmosphere.
Basically, because we put a lot of care and effort into ensuring this is the case.
We make sure not to hit the atmosphere head-on, instead using a very shallow angle that takes us through the thinner top layers of the atmosphere for as long as possible to gradually slow down before dropping down into the denser atmosphere.
We also fit them with parachutes to help us slow down further, and a heat shield designed specifically to protect against the heat (it is of course made of heat-resistent materials, but it’s also shaped to create a sort of air bubble in front of it which absorbs most of the heat)
If either of those factors go wrong (wrong reentry angle, or if the parachutes or heatshield get damaged or fail), then the whole thing may well burn up too.
For spacecraft coming from the ISS specifically, reentry speed is also quite a lot lower than for something coming from the moon or from outer space. That helps too 🙂
Multiple people have mentioned the ceramic heat shielding and how quickly it dissapates heat. This is actually way cooler than it sounds. Here’s a demonstration using one of the tiles from the NASA space shuttle program. [https://www.youtube.com/watch?v=Pp9Yax8UNoM](https://www.youtube.com/watch?v=Pp9Yax8UNoM)
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