NASA’s Mariner 10 made the first successful flyby of Mercury on 29 March 1974. It was also the first spacecraft to use a gravity assist, using Venus to alter its trajectory toward the innermost planet.
Mariner 10 flew past Mercury three times and returned the first close-up images of its heavily cratered surface. The spacecraft found that Mercury has a magnetic field and measured extreme temperature differences between its day and night sides.
Because Mariner 10 repeatedly encountered the same side of Mercury, it mapped only about 45 percent of the planet’s surface. NASA’s MESSENGER later became the first spacecraft to orbit Mercury, entering orbit in 2011. Europe and Japan’s BepiColombo mission launched in 2018 to continue detailed study of the planet.