Venus was one of the last planets to be explored, despite its position as the closest planet to Earth. This is largely because it is perpetually shrouded in a uniformly bland covering of clouds. The cloud cover made looking at Venus through a telescope about as exciting as staring at a billiard ball. While Mars and the Moon were objects of much attention by early telescopic observation, the surface of Venus remained a mystery. It was even easier to say something about the outer planets, such as Jupiter and Saturn, than it was to make meaningful observations of Venus.

The absence of information about Venus was particularly ironic because Venus is the most like Earth in size and position within the solar system, thus suggesting that it could be more like Earth than any of the other planets. Venus's diameter is only 651 kilometers (404 miles) smaller than Earth's diameter...