Mariner 10 used a Venus gravity assist in 1974 to become the first spacecraft to visit Mercury.
NASA launched Mariner 10 on 3 November 1973. Its trajectory used Venus’s gravity to reshape the spacecraft’s orbit around the Sun, allowing it to reach Mercury without requiring an impractically large amount of propellant. This was the first mission to use a gravity-assist maneuver to reach another planet.
Mariner 10 made its first Mercury flyby on 29 March 1974 and completed two further flybys later that year. It photographed roughly 45 percent of Mercury’s surface and discovered that the planet has a magnetic field and a large iron-rich core.
Mariner 10 did not enter orbit around Mercury; it performed flybys. MESSENGER later became the first spacecraft to orbit Mercury, entering orbit in 2011. BepiColombo, a joint European-Japanese mission, arrived for Mercury flybys before its planned orbital mission.