In 1973 , NASA launched Mariner 10, the first space probe to reach Mercury.
Mariner 10 was also the first spacecraft to use the gravitational pull of one planet (Venus) to reach another (Mercury) as well as the first spacecraft mission to visit two planets.
Learn more at http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-085A.