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 the thick cloud cover which surrounds the planet, and information on the atmosphere composition.

Mariner 10 then used the gravitational pull of Venus to accelerate itself toward Mercury, becoming the first spacecraft to visit that planet, on March 29, 1974. During the next year, two more flybys produced a total of 10,000 images, revealing a heavily-cratered surface similar to that on our Moon.

After mapping 57% of Mercury, Mariner 10 ran out of energy and is now in an orbit around the Sun.