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And that was just the beginning. Over the more than four decades since, NASA and other space agencies have taken us to vicarious close encounters with all eight planets (and if you refuse to accept Pluto's recent demotion to "dwarf planet," the New Horizons mission will reach a ninth in 2015). Robotic explorers have boldly gone where fragile human spacefarers can't, at least not anytime soon, sending back extraordinarily detailed pictures and measurements of dozens of moons and even a few asteroids. They've touched down on Venus, on Saturn's giant moon Titan, and most significantly, on Mars—the first time for the U.S. when Viking 1 landed in 1976, and most recently this past May, when the Phoenix probe arrived to dig into the Martian surface.

The result of this sustained campaign of exploration is a wealth of information about the nature and origin of our solar system. During the 1960s and 1970s, for example, Ranger 7's crash landing was followed by a series of soft lunar landings and orbital mapping missions, beginning with the Soviet Union's Luna 9 in 1966. Most of the early flights were designed to gather information for the Cold War goal of being first to put humans on the moon. But after the U.S. won that race, the Soviets continued to send unmanned missions that collected surface samples and returned them to Earth. In the 1990s the U.S.'s Clementine and Lunar Prospector probes found hints that the moon might hold deposits of ice in shadowed crater floors at the lunar poles—a potentially crucial resource for any future moon base.

Impressive as the moon probes have been, they merely filled in the details about a world we knew relatively well. As soon as Galileo pointed his telescope skyward in 1609, the general topography of our nearest heavenly neighbor, with its craters, mountains, and plains, was evident. But earthbound telescopes couldn't tell us very much about the more distant planets. It was only with the first flyby of Venus by Mariner 2, in 1962, that scientists began to understand that the planet's atmosphere is principally made of carbon dioxide, that Venus rotates in the opposite direction from Earth, and that its surface temperature, thanks to the heat-trapping ability of CO₂, is a withering 900°F. Later probes, including the Magellan orbiter, which reached Venus in 1990, have shown that beneath its permanent shroud of clouds, the planet that is Earth's near twin in size is covered mostly by hardened lava and is probably still geologically active. Mariner 10 was the first space probe to visit Mercury, in 1974 and 1975, revealing that the tiny planet's surface looks very much like the moon's, while its dense, iron-rich core is more like Earth's.

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