The scientific goals of the planetary part of the multi-encounter fly-by mission to Mercury combined with a solar observatory (INTERHELIOS) are presented. Mercury represents the last frontier of the knowledge of the Solar System and a challenge for our understanding of the physics of the terrestrial planets. The origin of Mercury is puzzling because of impact/accretion problems. The existence of a magnetic field remains unexplained because Mercury's core lacks the high temperatures necessary to keep the massive core liquid. Similarly, the role of tidal energy dissipation due to the high eccentricity of Hermean orbit remains unknown. One of the most important experiments on the internal structure will be determination of the polar dimensionless moment of inertia. The mantle and crust of the planet are unusually thin. Despite a similarity in reflectivity of their surfaces, the compositions of Mercury and the Lunar regolith must differ, reflecting completely different total composition. The Morphology of the surface, only 30% of which is known, will provide clues to the history of the planet. The rarefied Mercury atmosphere is enriched with helium, potassium and sodium. The latter are of unknown origin. The absence of a dense atmosphere combined with an intrinsic magnetic field provides a unique possibility to study the direct interaction of the solar wind with a magnetized planet unobservable at any other terrestrial planet.