Mariner 10 spacecraft, 1973.

UNITED STATES - MAY 28: Model (scale 1:48). Mariner 10 was the last of a series of probes designed by NASA to survey the inner planets of the Solar System. Launched on 3rd November 1973, it flew past Venus in February 1974 on its way to Mercury. It returned some 3500 pictures of Venus taken by its twin TV cameras. Mariner 10 flew past Mercury on 29th March 1974, sending over 2000 pictures back to Earth. These revealed a heavily cratered surface similar to that of the Moon. It passed Mercury again in September 1974 and in March 1975 made its closest pass to the planet at a distance of 327 kilometres. Mariner 10 was the first spacecraft to use a gravity assist trajectory, using the gravity of Venus to throw it onto an altered course to Mercury. (Photo by SSPL/Getty Images)