Mercury’s "dark side" glimpsed for first time.

The MESSENGER spacecraft has returned this image of the unexplored hemisphere of Mercury. When Mariner 10 flew past Mercury three times in 1974 and 1975, the same hemisphere was in sunlight during each encounter. As a consequence, Mariner 10 was able to image less than half the planet. Planetary scientists have wondered for more than 30 years about what spacecraft images might reveal about the hemisphere of Mercury that Mariner 10 never viewed.

Credit: NASA / JHUAPL / CIW