Goals: Mariner 10 was the first spacecraft sent to study Mercury. Mariner 10 also studied Venus while using the planet's gravity to modify its speed and trajectory, enabling it to reach Mercury.
Accomplishments: On its way to Mercury, Mariner 10 became the first spacecraft to use the gravity of one planet (in this case, Venus) to reach another. This has become an extremely important technique. While flying by Venus, the spacecraft took ultraviolet pictures that revealed great bands of clouds, providing data on atmospheric circulation which included upper-atmosphere wind speeds of 300 km/hr.