Mercury’s magnetic field is distorted by the solar wind, which compresses the magnetic field on the dayside and stretches it out to form a long tail on the nightside. Interaction between the solar wind and the planetary field creates the bow shock and magntopause boundaries, as well as features noted in the figure. Two representative MESSENGER orbits are shown in green. When the spacecraft is closest to the planet at night the orbit takes the form of the solid green ellipse. One-half Mercury year later, the spacecraft is closest to the planet during the daytime and the orbit takes the form of the dashed green ellipse. The MESSENGER orbit carries the spacecraft across the magnetospheric boundaries and through each region of Mercury’s magnetic field. The challenge for MESSENGER scientists is to extract telltale signatures that indicate how Mercury’s magnetic field would look without the solar wind.