A shaded relief map of Mercury as imaged by Mariner 10 in 1973 and 1974. Since the three flybys occurred at intervals of two Mercurian years (= one Mercurian day), the same side of Mercury was facing the Sun during each flyby. As a result only half of the planet could be imaged, and small portions of even that half were not observed and are left blank (on the right side of this map). (Source: NASA, USGS Flagstaff).