I was just reading an article on the Space Shuttle Columbia tragedy and I began to wonder, why do they need to re-enter the atmosphere at roughly 17,000mph? Why not slow down to a less “burn up on re-entry” speed?
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Because when they are up in orbit they move at such very high speeds.
An orbit like the one of the ISS, where many space craft go, means moving so fast that down here on earth it would be something like 22 times the speed of sound.
You can of course slow down before you enter the atmosphere.
However slowing down would require extra fuel.
To slow down you need to sue just as much fuel as you needed to speed up in the first place.
But you can’t just carry twice the amount of fuel in your rocket.
Every bit of extra fuel weight means you need even more fuel to burn more fuel to move it.
In the end you need to carry the fuel to carry the fuel to carry the fuel and so on.
This is why rockets are mostly fuel.
Most of the fuel rockets carry gets used to carry other fuel. Only a small fraction of it is used to carry the actual payload.
This makes carrying extra fuel to slow down when you de-orbit a thing to avoid.
Instead they cleverly use the atmosphere to slow down their craft. This is called aerobraking.
If you could supply energy or fuel to a craft while it is up in orbit, slowing it down before it hits the atmosphere would be something you would want to do. but right now slowing down with the air is the best we have.
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