Image of Mariner 10 Courtesy of NASA/JPL

Mariner 10 was launched on Nov. 3, 1973, intended as a flyby of planets Venus and Mercury.

It passed Venus in February of 1974, returning over 4000 photographs of it's thick cloud cover and information on the atmosphere composition. Mariner 10 then used Venus's gravity to gain speed and travel to Mercury, passing by the planet three times over the next year.

From the Mariner 10 mission, 57% of Mercury's surface was mapped, revealing many craters similar to those on our Moon. The last Mariner spacecraft is now out of energy and in orbit around the sun.