Mercury (planet)
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.