Spacecraft must steer through the curvature of space-time taking into account the accelerating effects of gravitation created by the Sun, planets, and other planetary objects. Gravitational forces create warps in space-time. We often speak of such warps as gravity wells, in the sense that it takes a great deal of energy to climb out of a well once you have fallen in. Upon close approach we can use a gravity assist to slingshot around the planet, accelerating to reach our true destination even faster. While we characterize a gravity assist by the planet the spacecraft goes near, it's not just the planet that does the trick.
Gravity assist refers to the technique of borrowing energy by means of a close planetary flyby.
Using vector analysis, we can plan a gravity assist. We track from the ground using the Deep Space Network's antennas to measure the Doppler shift of the spacecraft's radio signal. Once we are near the planet itself, the Doppler shift changes more rapidly, as the radio signals from the spacecraft shift slightly as it accelerates. The velocity of the spacecraft is affected on its approach as it nears the planetary gravitational force. Because we know the mass of the planet we can calculate the change in the velocity-and we can improve our knowledge of its whereabouts and, consequently, better direct its path.