Why can't an airplane just fly into space? Why do we need rockets?

Delta rocket.

Some of our friends who are members of the Back Bay Astronomy Club in Virginia Beach, Virginia, often hear the questions "Why can't an airplane just fly into space? Why do we need rockets?"Well, most of the time air may not seem like a very real substance, like water or wood or metal. But when the wind blows so hard it nearly knocks you down, air seems very real indeed. If we think about it, we can begin to understand how essential it is to how airplanes work.

Airplanes are able to fly because air moving along the wings holds them up. We often think of air as being too thin and lightweight to exert much of a force, but for a fast aircraft with sufficiently large wings of a cleverly chosen shape, our atmosphere can do the job. As the plane's engines move the wings forward, the air has to flow both over and under them. The wings are designed so that net effect is that the air pushes them upward, countering the downward pull of gravity.

Helios flew higher than any other plane.

Large passenger planes can't fly much higher than about 12 kilometers (7.5 miles). The air is too thin above that altitude to hold the plane up. Some kinds of planes can fly much higher, and one special NASA plane, named Helios, flew to about 30 kilometers (19 miles), which is far higher than any other plane has traveled. At that altitude, the air is 100 times thinner than at sea level. The air gets thinner and thinner the higher you go, until there's hardly any air at all. In other words, it's nearly a vacuum up there.Even the lowest Earth-orbiting spacecraft orbit at around 200 kilometers (125 miles) above Earth's surface, far above the thick air we are accustomed to and much higher than any plane can reach. Most Earth orbiters, though, are placed at much higher altitudes to do their jobs. Some of these jobs include scientific observations of the universe that are better than scientists can make when looking through the air, as well as observing Earth's weather and long-term climate changes, taking pictures of Earth's changing surface, and studying pollution in the atmosphere.

To get a spacecraft to even the lowest Earth orbit requires a rocket. So how is a rocket different from an airplane? Rockets do not depend on air, even for burning their fuel. Rockets take advantage of some basic laws of nature that were discovered by the brilliant scientist Isaac Newton late in the 17th century. One of these, called Newton's third law, says that for every action there is an equal and opposite reaction. This explains what is going on when you blow up a balloon, then let it go without tying a knot. The air rushes out of the mouth of the balloon and that pushes the balloon in the opposite direction. This law also tells us that, to build a powerful rocket, we need to shoot out a lot of high-speed material opposite the direction we want the rocket to go. That is exactly what a rocket engine is designed to do. 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.

Rocket engine diagram.

Like everything else that burns, rocket fuel cannot burn without oxygen. Because it operates where the air is too thin to provides enough oxygen, a rocket carries its own oxygen in tanks and mixes it with the fuel just before it is burned.

Rockets not only make space exploration possible, but they also enable us to explore our own planet in ways we could never do even from an airplane. Do a crossword about a "nine-eyed Earth watcher" that studies pollution in the atmosphere from space. Also, make and launch your own bubble-powered rocket. And learn all about the amazing Helios airplane.