In 1687 the physicist Isaac Newton presented the first explanation of orbital motion in his book Principia Mathematica. Newton imagined a cannon firing balls at different speeds. The greater the speed, the farther the ball travels. With enough speed, a ball travels so far that its landing point depends on the Earth’s curvature. With even more speed, the curve of its path will match that of the Earth’s surface. With no drag to slow it, the ball will continue to fall but never reach the ground.