The first spacecraft to fly past Venus was Mariner 2 in 1962. Since it was well known at the time that Venus was covered in clouds, scientists figured that putting a camera on Mariner 2 would just be a waste of time, so we had to wait until February 5, 1974, for our first close-up picture of Venus from Mariner 10, shown above.
At optical wavelengths, Venusian clouds are mostly featureless, but in the ultraviolet (which is what we're looking at here, with enhanced color), Mariner 10 revealed an unexpected level of detail that was quite a surprise to astronomers, who had (up until that point) been restricted to earthbound telescopes.
After Venus, Mariner 10 made three flybys of Mercury, but by early 1975, the spacecraft was out of maneuvering propellant. Unable to keep its antenna pointed at Earth, communications with the spacecraft ceased on March 24th, and as far as we know, Mariner 10 is still in orbit around the Sun, getting slowly fried by radiation.
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