Some of the most exciting moments in the exploration of space in the last thirty years have been provided by a succession of unmanned spacecraft that have explored more and more remote reaches of the Solar System. The early Moon-orbiters, scouts for later Moon landers, were succeeded by spacecraft that visited Mercury Venus, Mars, Jupiter, Saturn, Uranus, Neptune, various comets, and the Sun itself.

But to explore the Solar System in this way requires stronger and stronger rockets, much stronger than are required simply to get a spacecraft away from the Earth's gravitational pull. In order to do the most with the rockets available to them, planetary scientists have used a remarkable trick, called the gravitational slingshot: they have used the gravitational pull of another planet, such as Jupiter, to give their spacecraft an extra kick in the direction they want it to go. In this chapter we will try to understand how this works, not only for getting spacecraft into the outer parts of the Solar System, but also for getting them very close to the Sun.