Mercury is the smallest and closest planet to the sun in our Solar System. People have recorded seeing Mercury since the time of the Sumerians, about five thousand years ago. The first space probe to visit Mercury was called Mariner 10. It made the first close observation of Mercury. In January 2008, another probe, called MESSENGER, flew past Mercury. It will fly past two more times in 2008 and 2009, before going into an orbit around Mercury.