The Messenger spacecraft captured a sliver of the southern hemisphere of Mercury in this recent image. Although Mercury is relatively close to Earth, it stays so near the Sun in our sky that it is difficult to study with ground-based telescopes, which reveal almost no surface features. In fact, astronomers couldn't even measure the length of Mercury's day until they began using radio telescopes in the 1960s. Even today, the only way to get a clear view of Mercury is from a spacecraft like Messenger, which is the first probe ever to orbit the Sun's closest planet. [NASA/JHUAPL/CIW]