Mariner 10

Mariner 10 was the seventh successful launch in the Mariner series and the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury). It was also the first probe to visit two planets. Launched on November 3, 1973, it reached Venus on February 5, 1974. Using a gravity assist from this planet, Mariner 10 first crossed the orbit of Mercury on March 29, 1974 and did so a second time on September 21, 1974. A third and last Mercury encounter took place on March 16, 1975. It measured the environments of both Venus and Mercury.