Why do spacecrafts like the decommissioned Space Shuttles re-enter the atmosphere at such high speeds?

Question

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?

In: 2

Answer 1

All the other answers are correct, but to add on to them:

You can’t apply the common sense of a planetary environment to objects outside of that environment. Living on the surface of a planet, we have many convenient things that allow us to ‘cheat’ the laws of physics and do things more efficiently than something in orbit can do them. And please note, when I say that we cheat the laws of physics, we’re not violating any of the laws – we’re just making use of environmental factors that do some of the work for us.

For example, slowing down. Remember Newton’s laws, specifically the first one: An object in motion tends to stay in motion. This is true for all objects – if put into motion, they will continue on that velocity indefinitely, unless another force intervenes. So if you’re traveling in your car at 60 mph and want to slow down, you need to exert a force to reduce your speed.

If you were in orbit at 60 mph (which isn’t nearly fast enough to be in orbit, but we’ll pretend it is) you would have to expend just as much fuel to decelerate as you did to accelerate. But if you’re in a planetary environment, rolling around the ground through an atmosphere, you have two forces working to slow you down for free. Atmospheric drag, and rolling friction with the ground (and also the internal mechanisms of your car). So even if you do nothing, your velocity will decrease.

Similar reasons explain why rockets are so much less efficient than jet engines. In an atmosphere, you can generate lift and thrust by essentially grabbing hold of air molecules and throwing them behind your plane and towards the ground (that’s basically what engines and wings do). But in a rocket, you can’t use the atmosphere to generate your lift and thrust. You have to generate it by violently throwing your fuel behind you at high velocity.