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

It takes an entire rocket’s worth fuel and tricks like dropping stages of the rocket to get a single small satellite up to orbital speed. While there are some losses to gravity and air-resistance on the way up, the majority of the effort is just getting the satellite up to orbital velocity 9 kilometers per sec – faster if you want to go beyond low orbit.

If you wanted to slow that satellite down to 0 meters per second in orbit (or something low) you would roughly need an entire rockets work of fuel up there with it, requiring a truly massive (if not impossible) vehicle. Thus if you carry a heatshield thats ~10% of spacecraft, you don’t need 100x its weight in fuel.

Space Dragon Capsule with Payload ~10,000 kg

Falcon 9 Booster: ~544,600 kg

PICA-X Heatshield mass: ~1000kg