Mariner 10

The first mission to employ gravity assist, or using the gravity of a planet to alter a spacecraft's speed and trajectory to fly by its target planet, the Mariner 10 mission flew by both Venus and Mercury, snapping photos and collecting data.

Also the first spacecraft to visit Mercury, a feat that wouldn't happen again for more than 30 years, Mariner 10 data revealed a surprising magnetic field and a metallic core comprising about 80 percent of Mercury's mass.