Launched in 1978, Pioneer Venus Orbiter returned over 1000 pictures of the clouds of Venus in its first two years operation and was still operating after 10 years in orbit. 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. The surface has few impact craters and is extensively covered by lava flows. Volcanic activity persists in places.
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