In January 1959, only a little more than a year after the launching of Sputnik 1, the Soviet Union's Luna 1 flew 5,955 kilometers (3,692 miles) above the surface of the Moon, thus quickly heralding the age of planetary exploration. Since then, nearly 100 successful robotic missions to obtain closer looks at the planets, their moons, and asteroids have been launched, mainly by the United States and the Soviet Union. Among the planets, only Pluto, so far away that a signal from it would take five hours to reach Earth, has not had a spacecraft fly at least close by it. During the more than four decades of planetary exploration, continued improvements and miniaturization in electronics and computers, and in rocketry and instrument techniques in general, have been used to gain scientific knowledge.

The term "robotic" is used for any spacecraft without a human pilot. In addition to...