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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The [tyranny of rocketry](https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation).
Slowing down requires fuel to provide thrust counter to your current direction. The craft would have to be launched with additional fuel to allow for this. Fuel is very heavy, which means you’d need even _more_ fuel to launch the additional fuel that you require for this maneuver. Fuel is also expensive, so the cost would go up dramatically.
Rather than doing that, you can just let the atmosphere slow you down instead. The friction created during reentry is a _great_ braking mechanism, so they just let the air do the work for them – no fuel required. Sure, this creates significant heat, but its easier to deal with that than the fuel.
The shuttles have limited fuel on board. The atmosphere is used as the braking mechanism to slow it down rather than carry enough fuel to do that.
Objects orbit the earth by going really really fast, and as they slow down they will orbit closer and closer to earth then eventually enter the atmosphere.
So the limitation on fuel is true, once the shuttle slow down enough, they will still enter the atmosphere at very high speed.
The Space Shuttle use up all the fuel in the big orange external fuel tank to accelerate up to their 17,000mph. In order to slow down they would have to use another full fuel tank as the energy requirements are the same. Getting the entire fuel tank into space would require an enormous rocket as the full fuel tank is extremely heavy. This is not practical to do. It is much easier to just install a heat shield and use the atmosphere to slow down. It is safer too as this eliminates any issues with the rocket, see the Challenger disaster.
Energy is king. It takes so much energy to change anything about an orbit. Creating an orbit is the most expensive part of space travel. That’s why they launch with giant fuel tanks and boosters nearly twice the size of the shuttle itself. But they detach during launch and they’re basically out of fuel by the time they get home.
You will find no better explanation not more entertaining than [this dry humor presentation ](https://youtu.be/Jb4prVsXkZU)
Imagine skipping stones across a lake. “Ohhh, I got a 5 skipper!”
Now imagine someone asking, why can’t the stone just slow down on the first touch of water and sink peacefully? Note that throwing the stone directly into the water without skipping creates such a shock that the little people inside that stone will turn to cream.
The space shuttle and other manned objects returning from space also kind of do a little dance when entering the atmosphere. Kind of like a skier doing S turns to control their speed and direction while going downhill.
Spacecraft need to be at that speed so they don’t fall back to Earth as they go around it. To reach this speed, spacecraft launch vehicles expend many, many times the payload’s mass in fuel.
To slow down again, all the kinetic energy in the vehicle’s velocity needs to be converted to something else. If the spacecraft was attached to another launch vehicle with the same amount of fuel as the one that boosted it to orbit, then it could just de-orbit itself and land. But as I said, the fuel needed for that is many times the spacecraft’s mass.
Instead, the reentry vehicle manoeuvres so that its orbit grazes the upper atmosphere. Friction between the atmosphere and the spacecraft converts energy from its velocity into heat — lots of heat! In fact, the amount of heat generated during reentry is the same amount as was generated by the launch vehicle’s rockets during launch, only spread out over the much longer reentry phase.
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.
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.
With space being a vacuum there are very few molecules for the shuttle to pass by and therefore virtually no friction, meaning the shuttle can’t slow down on its own the way a car would if you lift off the gas or an aeroplane would if its engines cut out (Newton’s First Law). The only way to actually adjust its speed is through its engines (either forward or reverse) pushing out exhaust gas (Newton’s Third Law).
Because the shuttle uses almost all its fuel to escape Earth’s atmosphere in the first place, it doesn’t have any way of slowing itself down, instead it allows friction to do the job for it on re-entry to the atmosphere. This means, though, that it will still have roughly the same speed with which it left the atmosphere in the first place.
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