Astronomy Picture of the Day . ... Washington Explanation: For the first time, the entire surface of planet Mercury has been mapped. Detailed observations of the innermost planet 's surprising crust have been ongoing since the robotic MESSENGER spacecraft first passed Mercury in 2008 and began orbiting in 2011. Previously, much of the Mercury's surface was unknown as it is too far for Earth-bound telescopes to see clearly, while the Mariner 10 flybys in the 1970s observed only about half. ...