it would not be orbiting would be if it had been hit by an asteroid or gravitationally perturbed by a close encounter with a large body. Discoveries During its flyby of Venus, Mariner 10 discovered evidence of rotating clouds and a very weak magnetic field. Using a near-ultraviolet filter, it photographed Venus's chevron clouds and performed other atmospheric studies. The spacecraft flew past Mercury three times. Owing to the geometry of its orbit – its orbital period was almost exactly twice Mercury's – the same side of Mercury was sunlit each time, so it was only able to map 40–45% of Mercury's surface, taking over 2,800 photos. It revealed a more or less Moon-like surface. It thus contributed enormously to our understanding of Mercury, whose surface had not been successfully resolved through telescopic observation. The regions mapped included most or all of the Shakespeare, Beethoven, Kuiper, Michelangelo, Tolstoj, and Discovery quadrangles, half of Bach and Victoria quadrangles, and small portions of Solitudo Persephones (later Neruda), Liguria (later Raditladi), and Borealis quadrangles. Mariner 10 also discovered that Mercury has a tenuous atmosphere consisting primarily of helium, as well as a magnetic field and a large iron-rich core. Its radiometer readings suggested that Mercury has a nighttime temperature of and maximum daytime temperatures of . Planning for MESSENGER, a spacecraft that surveyed Mercury until 2015, relied extensively on data and information collected by Mariner 10. Mariner 10 Commemoration In 1975, the US Post Office issued a commemorative stamp featuring the Mariner 10 space probe. The 10-cent Mariner 10 commemorative stamp was issued on 4 April 1975, at Pasadena, California. Since the backup spacecraft was never launched, it was put on exhibition at the National Air and Space Museum of the Smithsonian Institution. See also 1973 in spaceflight Exploration of Mercury List of missions to Venus Timeline of artificial satellites and space probes References Notes Bibliography and Further reading External links The Voyage of Mariner 10: Mission to Venus and Mercury (NASA SP-424) 1978 Entire book about Mariner 10, with all pictures and diagrams, on-line. Scroll down to click on the "Table of Contents" link. PDF version Mariner 10, NASA's 1973–75 Venus/Mercury Mission Mariner 10 image archive Mariner 10 mission bulletins Mariner 10 Mission Profile by NASA's Solar System Exploration Calibrated images from the Mariner 10 mission to Mercury and Venus Master Catalog entry for Mariner 10 at the National Space Science Data