Astronomy Picture of the Day . Mariner s Mercury . ... Credit Copyright: Mariner 10 , Astrogeology Team , U.S. Geological Survey 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 three executed flybys of Mercury in 1974 and 1975, surveying approximately 45 percent of its surface. ... Publications with keywords: Mariner 10 - Mercury - Mercury surface . ...