Airplanes can fly because of air. The air moving under their wings is strong enough to hold them up. An airplane wing is round on top. The bottom is flat. The plane's engines push the wing forward. At the same time, air moves over and under the wing. The air going above the wing has to go a little farther than the air going below.
This is because of the wing's shape. The air molecules on top are a little farther apart. This makes the air there a little thinner. It also makes less pressure on the top of the wing. There is more pressure on the bottom. So what happens? The wing is pushed up by the air under it.
Large planes can only fly as high as about 7.5 miles. The air is too thin above that height. It would not hold the plane up. Some kinds of planes can fly much higher. One special NASA plane, Helios, flew to about 19 miles.
This is far higher than any other plane has gone. At that height, the air is about 100 times thinner than at sea level. The air gets thinner the higher you go. It gets so thin there's hardly any air at all. This is called a vacuum.
Even a spacecraft in a low orbit is about 125 miles high. This is far above the thick air that we are used to. It is much higher than any plane can reach. You need a rocket to get to even the lowest Earth orbit. So, how is a rocket different from an airplane?
Rockets do not need outside air to lift them. Rockets use some of the basic laws of nature. Scientist Isaac Newton discovered these laws over 300 years ago. One of these is called Newton's third law.
It says for every action there is an equal and opposite reaction. You can see this law when you blow up a balloon and let it go without tying it. The air rushes out of the mouth of the balloon.
The balloon flies the other way. This law also tells us how to build a powerful rocket. The rocket needs to shoot out a lot of material. It must come out at a high speed. And, it must come out in the direction opposite of the way we want the rocket to go.
That is what a rocket engine is made to do. Most rockets use high-speed gases. These gases come out of the burning rocket fuel. These gases push the rockets up and away from Earth. They push the rocket up into space where there is no air.
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