This video is based on images taken by the MESSENGER probe, the first spacecraft to actually orbit Mercury. It has allowed astronomers to get the first up-close view at the planet's geology; until now, the sum total of our knowledge was limited to a single Mariner flyby in 1975. Over the past year, MESSENGER has taken over 80,000 images with plans to take 80,000 more. That's allowed us to assemble this complete visual representation of Mercury's surface, with each pixel representing about a square kilometer; the video expands on this previously published image.