Why can’t an aeroplane just fly into space? Why do we need rockets? Most of the time air may not seem like a very real substance. But when the wind blows so hard it nearly knocks you down, air seems very real indeed.Aeroplanes can fly because air moving under their wings is strong enough to hold them up. If you could slice across an aeroplane wing, you would see it is curved over the top and flat on the bottom. As the plane’s engines push the wing forward, air moves over and under the wing. Because the top of the wing is curved and the bottom is flat, the air going above has a little farther to travel than the air going below.The air molecules on top are thus a little farther apart, making the air there a little thinner, and the pressure on the top of the wing a little less than the pressure on the bottom. So what happens? The wing is pushed up by the air underneath it.Large passenger planes can’t fly much higher than about 7.5 miles. The air is too thin above that altitude to hold the plane up. Some kinds of planes can fly much higher. But no plane can fly as high as even the lowest Earth-orbiting satellite at around 125 miles above Earth’s surface.So, to boost a spacecraft to even the lowest Earth orbit requires a rocket. But how is a rocket different from an aeroplane?Rockets do not depend on air, even for burning their fuel. Rockets work because of an important law of nature called Newton’s Third Law. This law says that for every action there is an equal and opposite reaction.For example, when you blow up a balloon and let it go, the air rushes out of the hole and the balloon takes off in the opposite direction. Most rockets use high-speed exhaust gases from burning rocket fuel to propel themselves up and away from Earth’s surface to the vacuum of space. Unlike planes, they don’t need air to lift them up.But like everything else that burns, rocket fuel needs oxygen to burn. A rocket carries its own oxygen in tanks and mixes it with the fuel just before it is burned.Read about it at The Space Place, http://spaceplace.nasa.gov/galex-pegasus/.This article was provided through the courtesy of the US Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, under a contract with NASA and NOAA.