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3.17 The Idea of Space Exploration

The dream of space travel is an ancient one. The first known description of a flight to the Moon is from an ancient Greek writer, Lucian of Samosata (190 A.D.). He had one of the characters in his "A True Story" don eagle wings and fly to the Moon in order to learn how the stars came to be "scattered carelessly up and down the universe." Since then, many writers, scientists, and others have dreamed of traveling to the Moon, other planets, and even stars.

Most people don't realize that our ability to fly spaceships to the Moon or Mars is a direct result of Newton's work in the 1600s! The idea of space exploration is an extension of the Copernican revolution, and the actual scientific basis of space travel came from Newton. The understanding of force and his kinematical equations answer the fundamental questions of space flight. Specifically, they allow one to calculate escape velocities, and orbital trips not just around the Earth, but all the way to the Moon or Mars.

Newton realized that a satellite could be launched into space. In his 1687 masterwork, "Principia," he described a thought experiment on how it could be done. A thought experiment is a hypothetical experiment run through in ones mind that is difficult to carry out in practice. For example, one of Newton's laws of motion states that an object in uniform motion (constant velocity) will continue in uniform motion (the same velocity) until acted upon by an outside force. In reality, projectiles are subject to air resistance, and rolling objects are subject to friction and outside forces are always present. Thus, thought experiment allow one to imagine that if friction or air resistance was eliminated, uniform motion would be the result, while reality precludes doing the experiment.

This was Newton's thought experiment for launching something into orbit: imagine a mountain so high that it projects above the Earth's atmosphere (you see why this would be a difficult experiment to actually perform!). Now imagine a cannon pointing out from the mountain top, parallel to the Earth's surface. If you fired a cannon ball at modest speed, the ball would fall near the foot of the mountain. At a higher speed, the ball would fall farther away. At a high enough speed, the ball would be falling towards the Earth (due to the force of gravity) at exactly the same rate that the curved surface of the Earth is “falling away” from it. In this case, it would continue to travel all the way around the Earth. This is exactly how we define an orbit. Note that each path obeys Kepler's laws: even the paths that hit the Earth's surface are segments of ellipses around the center of the Earth. The final orbit is the special case of an ellipse that is a circle. Thus, launching a satellite into orbit around Earth is in fact a 17th-century idea of Isaac Newton!