To design and build a spacecraft, you need to be able to find out how big you can make it, how heavy it can be, how fast it will have to go, how much fuel( 燃料 ) it needs and so on. For that, you need a theory ( 原理 ) of how objects move in space and how to make the calculations( 计算 ). Almost all theory of space fight was worked out by three men over nearly three centuries-from 1600 to 1900. u Johannes Kepler——was a German mathematician who, in 1609, worked out the equations( 方程 ) for orbiting planets and satellites. He decided that the planets move in orbits rather than true circles. u Isaac Newton——in 1687 discovered the basic laws of force, motion( 运动 ) and gravitation( 重力 ), and invented a new branch of mathematics in his study——calculus( 微积分 ). He did all this to show how the force of gravity is the reason that planets, orbits follow Kepler's equations. u Konstantin Tsiolkovsky——was a Russian schoolteacher who was the first to work out all the basic equations for rocketry——in 1903! From his very wide reading, including Jules Verne's From the Earth to the Moon, he concluded that space travel was possible, that it was in fact man's destiny, and that rockets would be the way to pull it off. He solved many of the problems that were going to come up for rocket flight and drew up several rocket designs. He decided that liquid( 液体 )fuel rockets would be needed to get into space, and that the rockets would need to be built in stages. He concluded that oxygen( 氧 )and hydrogen( 氢 ) would be the most powerful fuels to use. He had predicted how,65 years later, the Saturn V rocket would operate for the first landing of men on the Moon.