The fundamental concepts of the gravity-assist manoeuvre

Abstract

A typical undergraduate course in mechanics does not cover the fascinating and important gravity-assist manoeuvre that allows satellites or other spacecrafts to navigate through our solar system on efficient and desired paths. Instead, it usually remains a mystery to students how energy is conserved when a spacecraft gains speed as it flies past a planet. Indeed, one might be led to believe that the curved path of the planet is the root cause for the gain in speed, requiring consideration of gravity-assist within the framework of the restricted three-body problem. This contribution will emphasize that this extension is not required to explain the gain in kinetic energy. Instead, a simple, scaffolded analysis of the planet-satellite system alone, using elementary physics, two reference frames and analytical methods, provides a sufficient explanation. Our simplified analysis is successfully validated against mission data from Voyager 2's gravity-assist manoeuvre around Jupiter.