NASA launches Mariner 10. The last of the Mariner Program, Mariner 10 was the first spacecraft to flyby two planets - Venus and Mercury. The probe flew by Venus on February 5, 1974, and did 3 flybys of Mercury on March 29, 1974, September 21, 1974, and on March 16, 1975, after which communications with the probe were terminated.
Mariner 10