Mariner 10

Mariner 10 was a NASA robotic space probe launched in 1973 that became the first spacecraft to use a planetary gravity assist, flying past Venus to bend its trajectory toward Mercury. It was the first spacecraft to visit Mercury, making three flybys of the planet between 1974 and 1975 and returning the first close up images of its cratered surface, and it remained the only spacecraft to visit Mercury until NASA MESSENGER mission arrived more than three decades later. Mariner 10 also returned the first close range images of Venus cloud cover during its flyby of that planet.