25. How do spaceships stay up in space? (A K2S Question)

From The Space Library

Simply put, a spacecraft moving along in space will continue as long as all external forces are balanced. Try this: hold a ball in your hand with your arm outstretched. Why doesn't the ball fall? It doesn't fall because your hand exerts an upward force equal to the Earth's gravity. In other words, the forces on the ball are balanced. Now throw the ball as hard as you can sideways. It will go out some distance and fall to the Earth. Why? Because the forces on the ball are unbalanced. But if you could throw the ball at a speed of about 25,000 feet per second (and ignore the drag of the Earth's atmosphere), the ball would orbit the Earth continually on a circular path. And if the speed is increased, the path will become more and more oblong, forming an oval called an ellipse. Why? Because the centrifugal force on the ball due to its speed around the center of the Earth will be counteracted by the force of Earth's gravity. This will cause the ball to speed up and slow down as it proceeds around the orbit, but it will continue along the orbital path indefinitely. Now if the spacecraft is flying out into the solar system, the path and speed will be influenced by gravitational forces of the Sun and the planets. On board rockets will be necessary to make thrust corrections to balance out these forces to maintain a controlled trajectory.

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Answer provided by Jon H. Brown

Question and Answer extracted from the book Kids to Space - by Lonnie Schorer

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