Mariner 9 -  image courtesy NASAOn 30 May 1971 NASA launched Mariner 9 on top of an Atlas-Centaur launch vehicle. After a journey of more than 600 million kilometers Mariner 9 arrived and entered Mars orbit on 14 November 1971, the first spacecraft to orbit another planet.

The main mission goal was to map over 70% of the Martian surface with resolutions ranging from 1 km per pixel to as good as 100 m per pixel during successive Mars orbits from an altitude of about 1,500 km. On arrival the surface was obscured by a giant dust storm so Mariner 9 was kept in orbit until the dust began to settle out of the atmosphere and the systematic imaging of the planet's surface began in January of 1972. After 349 days in Mars orbit, 7329 images (including images of Mars' two moons, Phobos and Deimos) had been sent back to Earth and a total of 54 billion bits of data had been transmitted. Our view of Mars had been completely changed by the discovery of the giant volcanoes such as Olympus Mons and the Valles Marinaris, a canyon stretching 4,800 kilometers (3,000 miles) across its surface.

Other features discovered included ancient river beds, craters, layered polar deposits, evidence of wind-driven deposition and erosion of sediments, weather fronts, ice clouds, localized dust storms, morning fogs and more. With evidence of flow features, and therefore the possibility of liquid water having been present in the past interest in the possible existence of life on Mars was intensified.

Mariner 9 completed its final transmission on 27 October 1972. It had more than met its objectives, paving the way for future lander missions. The enormous Valles Marineris canyon system is named after Mariner 9 in honour of its achievements.

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