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, before rendezvousing with Mercury in March 1974. Although similar in size and internal structure to Earth, Venus is a very different world. It has a thick atmosphere of mainly carbon dioxide with clouds of sulphuric acid, creating a runaway greenhouse effect with surface temperatures of 485 degrees Celsius and atmospheric presure 90 times that at sea level on Earth. The impenetrable clouds meant that nothing was known about the planet's surface until the Soviet Venera probe landed and was briefly able to transmit images in 1975.
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