... Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. December 17, 1996 . Mariner's Mercury . ... Explanation: Mercury, the closest planet to the Sun, remains the most mysterious of the Solar System's inner planets. ... The only spacecraft to explore Mercury close-up was Mariner 10 which executed 3 flybys of Mercury in 1974 and 1975, surveying approximately 45 percent of its surface. ...