It is expensive to place an object in orbit around Earth, let alone reach another planet such as Mercury. To reduce the cost of putting the probe in orbit around Mercury, scientists and engineers will exploit the laws of physics to minimize the amount of fuel needed. Mission planners will use the gravity of Venus and Mercury to help adjust the velocity of MESSENGER so that it can enter an orbit around the inner planet. This will be done in two flybys of Venus, and three of Mercury.
During its three flybys of Mercury, MESSENGER will map nearly the entire planet in color, collecting images of most parts of the planet not seen by Mariner 10, and will take measurements of surface, atmosphere and magnetosphere composition. The flyby results will then be used to plan the yearlong portion of the mission in which MESSENGER orbits the planet.
Learn about the Spacecraft Design of MESSENGER