Launched January 2, 1959,
Decommissioned August 1976

During the late 1950s and 1960s the United States and the Soviet Union were engaged in a “space race,” which began when the Soviets launched Sputnik I (see entry), the first artificial satellite, on October 4, 1957. Space exploration had become possible after World War II when powerful rockets were developed, and the Soviet Union had taken advantage of the new technology with its Sputnik program. In response to Sputnik I, which had successfully orbited the Moon, the United States launched its first Earth-orbiting satellite, Explorer I (see entry), on January 31, 1958. The race was now in high gear: in fact, during the decade following Sputnik I the United States and the Soviet Union would collectively launch 50 unmanned space probes to explore the Moon.

In the first stages of experimentation the probes were designed either to fly close to the Moon—to execute a...