On February 5, 1974, Mariner 10 took this first close-up photo of Venus.
Made using an ultraviolet filter in its imaging system, the photo has been color-enhanced to bring out Venus's cloudy atmosphere as the human eye would see it. Venus is perpetually blanketed by a thick veil of clouds high in carbon dioxide and its surface temperature approaches 900 degrees Fahrenheit.
Launched on November 3, 1973 atop an Atlas-Centaur rocket, Mariner 10 flew by Venus in 1974.
Image Credit: NASA
Explanation from: http://www.nasa.gov/multimedia/imagegallery/image_feature_2443.html