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).