the first spacecraft to visit mercury was nasa's mariner 10. it stayed in mercury's orbit from 1974 to 1975. mariner 10 took the first close-up pictures of mercury's surface. it showed many features, such as the craters. unfortunately, the same side of mercury was day each time mariner 10 flew close to mercury. this made observing of both sides of mercury impossible. in the end, less than 45% of the mercury's surface was mapped.