Eli5: why can spacecrafts not be slowed down enough to red-enter the atmosphere without burning up?

Question

Eli5: why can spacecrafts not be slowed down enough to red-enter the atmosphere without burning up?

In: 228

Answer 1

In principle, they can, but it would require nearly as much fuel as it took to get them into orbit in the first place. And because they’d have to carry that fuel, the requirement to get them into orbit would be higher still (by a *lot*, because fuel requirements grow exponentially with the amount of total thrust [technically: delta-v] you need).

It’s easier both from an engineering and cost perspective to just brake them against the atmosphere, rather than try to slow them down via rocket, because aerobraking only requires a heat shield (which doesn’t add that much weight).