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 Venus, Mariner 10 first crossed the orbit of Mercury on March 29, 1974.