Mariner 10 was the first spacecraft sent to the planet Mercury; the first mission to explore two planets during a single mission; the first to use a gravity assist to change its flight path; the first to return to its target after an initial encounter; and the first to use the solar wind as a major means of spacecraft orientation during flight. But what did it capture with its two onboard cameras?
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 night time temperature of −183 °C (−297 °F) and maximum daytime temperatures of 187 °C (369 °F).
Planning for MESSENGER, a spacecraft that surveyed Mercury until 2015, relied extensively on data and information collected by Mariner 10.