For over three decades, globes of Mercury were blank on one side. Though Mariner 10 explored the small planet in three flybys in 1974 and 1975, no more than half was ever seen. Of the four terrestrial planets – Mercury, Venus, Earth and Mars - we knew the least about Mercury.
On Oct. 6, 2008, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, better known as MESSENGER, made its second close-approach flyby of Mercury. 30 years after man's first look, MESSENGER has revealed Mercury in its entirety - well, mostly.