Launched on 3rd November 1973, Mariner 10 flew past Venus in February 1974 on its way to Mercury. It returned some 3500 pictures of Venus taken by its twin TV cameras, clearly visible in the picture. It flew past Mercury at a height of only 271 kilometres on 29th March 1974 sending over 2000 pictures back to Earth. These revealed a heavily cratered lunar-like surface. Mariner 10 flew past Mercury again in September 1974 and March 1975.
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