why is it such a big deal for spaceships to reenter the earth's atmosphere?

Question

why do objects that enter at the wrong angle burn up? why can't a spaceship just come down slowly at any angle? if they ever built a lift up into space (as suggested by arthur c clarke) would it need a heat shield to protect the floor?

I know that spaceships travel fast to get out of our atmosphere, and they can't slow down without using huge amounts of energy, but what if they match their direction of travel to that of the rotating planet? wouldn't that make them relatively stationary so they could just drop down at any old angle?

Answer 1

The key point is the idea of coming down slowly. It is simply not practical.

An orbit is simply a free-fall in which your horizontal velocity is sufficient that as gravity pulls your path towards Earth the surfaceof Earth curves away from you, so you never hit the ground. To get to orbit you need to travel at at least 17,500mph. Take the shuttle as an example. Getting it to 17,500mph takes all the fuel in the solid rocket boosters (the white tubes on the sides that separate after a couple of minutes) and in the external tank (the large orange cylinder the orbiter is bolted to). As long as it is in space it will continue to travel at 17,500mph, because there is nothing there to slow it down. The moment it drops below 17,500mph it will return to Earth because Earth's gravity will now be pulling it down more than its horizontal velocity can counter. This is how re-entry is initiated: small rocket motors fire to slow the shuttle down.

However, and here's the critical part, because of the laws of conservation of energy and momentum, if it took all that fuel used at liftoff to accelerate the shuttle to 17,500mph, it will take the same amount to slow it from 17,500mph to a stop. It just doesn't have that capability. To give it that capability you'd need to find a way to launch that amount of fuel into orbit so the shuttle could meet up, dock and refuel. Not only that, but once it had slowed down by using the fuel it would then accelerate due to gravity.

From an economic point of view it is far easier to let the friction of the atmosphere do the slowing down, so that's what has been done on all spacecraft.

As for a space lift, that would be a different proposition. The lift isn't burning fuel and will not need to travel so fast to get up and down the shaft, therefore it will not require a heat shield.

[Edited to add]

To deal with your additional detail, orbiting spacecraft are going at 17,500mph, while the Earth's surface moves at about 1000mph at the equator. Most rockets take off in an easterly direction already, in order to take advantage of the 1000mph velocity already imparted in that direction by the rotation of the planet. There is no way for an orbiting vessel to be 'relatively stationary' with respect to the ground without expending almost as much energy as was required to get it up there in the first place.

Answer 2

It would take too much energy (i.e. fuel) for a controlled slow descent. Therefore the vehicle has to just drop through the atmosphere and at a very high speed. This causes friction between the vehicle and the atmosphere which in turn creates intense heat. If the vehicle hits the atmosphere at the wrong angle it will bounce off into space, a bit like a moving car hitting a sudden gust of wind.

Answer 3

you have to consider that when a spaceship is in orbit around the Earth, it is traveling at 16,000 miles per hour. And if it were to go that fast in Earth's atmosphere, it would burn up. And for a ship to enter the Earth's atmosphere slowly, well, it wouldn't be able to carry that much fuel to do so. The space shuttle can't fly in the Earth's atmosphere, it glides. But first it has to slow down, and coming from the vacuum of space to the Earth's atmosphere is quite a big deal, and N.A.S.A. takes it very seriously. And if they ever built a lift into space, they wouldn't need a heat shield in the floor if the lift didn't go very fast, but that is sci-fi compared to plain science.

Answer 4

they don't use a ship "propeller" to land, they don't bother. Enough fuel is used as it is (to take off). So the earths gravity does the job. Now before they enter the atmosphere, there're going really fast right? Cause there's no air to slow them down. So at such a high speed, hitting the atmospere, so to speak, would be like crashing on the ground. They need an angle.
The lift would have a propelling system, so it wouldn't reach the 100's of miles/hr like a spaceship.

Answer 5

The friction is caused by the spacecraft hitting the atmosphere at 17,500 mph. In order to eliminate the friction, you'd need to slow the spacecraft down to subsonic speed. Unfortunately, this would require a phenominal amount of fuel - and the spacecraft would have fallen out of orbit long before it managed to slow down to subsonic speed. The spacecraft would have to make a sudden stop - something which is impossible at present, and would also be fatal to the crew.

The bouncing effect by coming in at the wrong angle is like throwing stones across the pond. If you were able to come in slowly, then you'd be able to come in at any angle.

A lift wouldn't need a heatshield for re-entry if it were going at subsonic speed. If you stepped out of the lift, then you'd fall straight down as you're not in orbit.

Answer 6

if a space ship re-enters the atmosphere at too sharp of an angle, the atmosphere will not slow it down too much. and its drag chutes won't be strong enough to take the pressure.

if the ship comes down at too shallow of an angel it can bounce right off the atmosphere and back into space.

so a returning ship can only enter the atmosphere at a certain angle.

Answer 7

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Answer 8

In a nut shell, because re-entry is so extremely hard and dangerous. It is not possible to build any structure extending to space. I read somewhere any structure taller than 3 miles would collapse under its own weight. But if you succeeded in building one, whether a heat shield in needed or not depends on how fast its car can descend. If it is spaceship-like speeds, heat shield is needed. If it is much slower, not needed.

Answer 9

Basically, you wave your hand through the air, you feel a slight drag. You send a space ship hurtling through the atmosphere you get a lot of drag, the friction heats the surface up and that can be dreadfully dangerous for the people inside.

Answer 10

becuase the earth's atmospheric friction at those speeds will burn you up like a a tiny piece of charcoal!