Prior to missions to the Moon and the planets in the solar system our knowledge of what lay beyond Earth was minimal. Five millennia of astronomical observation had produced an incomplete picture of the solar system. Although the Moon and planets were neighbors, there was only so much that could be learned from even the best telescopes. Only by sending spacecraft and astronauts on programs of exploration could we examine our neighbors in space more closely.

The first objective for both the United States and the Soviet Union was reaching the Moon. In September 1959 the Soviet probe Luna 2 struck the Moon. Three weeks later, Luna 3 sent back the first grainy images of the Moon's farside. For the United States, the Ranger project of the 1960s marked the first effort to launch probes toward the Moon. A variety of difficulties plagued the first several Ranger missions, and...