Gravity Assist or "Planet Hopping"

Spacecraft journeying to other planets often get a boost in energy from the motion of a nearby planet—a space-travel trick called gravity assist.  As a spacecraft overtakes a planet, the planet’s gravity flings the spacecraft in a new direction and gives it more energy. By careful planning, a spacecraft can “hop” from planet to planet, picking up energy from each planet’s orbital motion and greatly shortening its travel time.