Space mission design depends on orbital mechanics to conserve propellant. This classroom-tested text derives the subject of orbital mechanics, as well as important subtopics such as Lambert's time-of-flight equation, Kepler's laws of planetary motion, and the Gauss and Laplace methods of orbit determination, from the first principles of Newton's laws of motion and the law of gravitation. Special attention is paid to gravity-assist maneuvers and orbit transfers (such as Hohmann transfer and minimum-fuel transfers using more than two impulses), topics particularly relevant to modern mission planning.