A canopy of vapor around a planet is not outside the realm of natural possibility, for such a canopy presently surrounds the planet Venus. Venus is often called Earth’s twin, but extensive research over the past 15 years has shown it is nothing like Earth. Shrouded in thick sulfuric acid clouds and high-speed upper atmospheric winds, the surface is a hot, dry, poisonous place. In the spring of 1979, NASA reported that Pioneer 2 discovered the intense Venus heat can be explained by a massive greenhouse