The Earth has a force that pulls thing toward itself. We call this force gravity (地心引力). This is something we live with all the time, and we take it for granted and hardly ever think about it. But it is a most important factor (因素) in rocket (火箭) operation and must overcome (克服) if we are to get anywhere in space, or off the ground at all.
Take the throwing of a ball as an example. The harder the ball is thrown, the faster and higher it will go. What is the secret Its speed. If we could throw the ball hard enough it would go up and up forever and never come down. The speed at which it would have to be thrown do this is known as escape speed. Of course, we cannot throw a ball hard enough because the speed required to escape completely from the Earth’s gravity is seven miles per second, or over twenty-five thousand miles per hour.
Once a spacecraft has reached escape speed, no further power is needed. A rocket aimed at the Moon, for instance, will "coast" (滑行) the rest of the way because the Earth’s gravity cannot then pull it back, and there is no air resistance (阻力) in space to slow it down. This "coasting" is known as "free fall" Free fall is an important feature (特点) of space travel: it would be impossible to carry enough fuel to provide powered flight all the time. Why is it not necessary to provide powered flight in space all the time