"Mercury is hard to study because it is so close to the sun. People have never gone to Mercury. Spacecraft without people have gone. Mariner 10 was the first to visit Mercury. It flew by in 1974 and 1975. Not even half of Mercury was seen then. After that, nothing was sent to Mercury for more than 30 years. NASA's MESSENGER spacecraft flew by Mercury in 2008 and 2009. In March 2011, it began to orbit Mercury. MESSENGER will study parts of Mercury that have not been seen before. It will let scientists learn many new things about the plane."