... Mercury in Stereo: Craters Within Craters . ... Credit: Mariner 10 , NASA Explanation: This Stereo image pair of craters on on Mercury was produced using data from NASA's robot explorer Mariner 10 which performed three close flybys of the Sun's closest companion , two in 1974 and one in 1975. ... The image on the left was actually taken during the first flyby, the one on the right during the second. A crater within a crater is visible at the upper left, the outer one is about 70 miles in diameter. ...