Historians writing a hundred or two hundred years from now may well look on the latter part of the twentieth century as another turning point in civilization. For the first time, we explored beyond Earth—first the Moon, then the neighboring planets, and finally the outermost planets, the very fringe of our solar system. How will the historian evaluate this period of exploration? First, perhaps, he will describe the Apollo program as a visionary example of great cooperative ventures that can be accomplished by many individuals, private companies, and government institutions. He will describe the subsequent space ventures that weave a fabric of cooperation and goodwill between nations. He will point out the technological advances incorporated in unmanned spacecraft, sophisticated robots able to control their own activities and solve their own problems. He will mention the revolution in microelectronics—the art of fabricating complex electrical control circuits so small the eye cannot perceive them, a revolution accelerated by the requirement to conserve weight and generate performance in interplanetary spacecraft. He will point to the introduction of new products, particularly in areas of communication, medical treatment, and energy conversion. [Illustration: _Galileo orbiter and probe mission to Jupiter in 1985 will expand upon the Voyager investigations of the Jovian system._] [Illustration: _A solar electric propulsion spacecraft would eject an instrumented probe toward Halley’s comet in 1986 and continue on to rendezvous with another comet, Tempel 2._]