less is known about mercury than about other planets of our solar system. even with telescopes only a small, bright crescent can be seen. it is also hard to put a satellite in orbit around it. two spacecraft have visited mercury. the first one was mariner 10. it only made a map of about 45% of the mercury's surface from 1974 to 1975. the second is the messenger spacecraft, which finished mapping mercury in march 2013.