Venus is unmistakable in our skies. Never straying terribly far from the Sun, it blazes brilliantly in the evening and morning skies, shining as brightly as magnitude –4.6, the most luminous permanent object after the Sun and Moon. Long called Earth's “sister planet,” the similarity is slight. It lies relatively close to us in the solar system, the next planet inward toward the Sun. Like Earth, it is a terrestrial planet, a predominantly rocky body, is about the same size as Earth, just about 95 percent the diameter of our planet, and contains about 82 percent of Earth's mass. But that's where the similarity ends. In most respects, Venus could hardly be any more different than Earth.

On this world that lies about 30 percent closer to the Sun than Earth, ironies abound. They begin even with the planet's name, which comes from the Roman goddess of love and beauty. When this bright and mesmerizing light wandering among the stars was named, the idea might have made sense. But early spacecraft studies of Venus betrayed the hellish nature of the planet.

Venus played a key role in the turning point of understanding the solar system in 1609, when Galileo Galilei observed it with his newly constructed telescope, from Padua, Italy. Galileo watched the planet and sketched it over the course of months, seeing that it underwent phases analogously to the Moon.

But the nature of Venus itself remained largely mysterious until the first spacecraft missions visited the planet in the 1960s. The first such probe, the Soviet Venera 1, was launched in early 1961. Mission successes were slow, from both the Soviet Union and the United States, marked by spacecraft failures. In 1962, the US craft Mariner 2 became the first successful interplanetary mission, measuring the surface temperature of Venus to be a searing 425 °C (800 °F), and ending speculation that the planet might harbor life.

In 1966, the Soviet Venera 3 probe became the first spacecraft to enter the atmosphere and crash land on the surface of another planet.