When American astronaut Neil Armstrong piloted the Apollo 11 lunar module to a soft landing on the surface of the moon in July 1969, he gazed upon a landscape that was, in some respects, already familiar to him. As the spacecraft descended toward the surface, Armstrong scanned the ground below, looking for landmarks he had learned to recognize from photographs. By the time Armstrong walked on the moon, it had been 10 years since Luna 2, an unmanned probe from the Soviet Union, became the first spacecraft to reach the moon. In the decade that followed, more than a dozen other Soviet and United States space vehicles had studied the moon in fine detail. Photographs and data radioed to Earth by these probes were instrumental in the success of the six U.S. manned missions to the lunar surface.

Since then, probes have visited every planet in the solar system except the dwarf planet Pluto, and some have been visited several times. In November and December 1996, NASA sent two probes named Mars Global Surveyor and Mars Pathfinder on the first survey missions to the red planet since 1975, when the Viking 1 and Viking 2 spacecraft were launched. The new spacecraft were scheduled to reach Mars in the summer of 1997, the Surveyor to observe the planet from orbit and the Pathfinder to explore its surface. Coincidentally, these missions were launched on the heels of a 1996 announcement that a meteorite of Martian origin contains the apparent remains of bacterialike microorganisms, raising the possibility that primitive life forms lived on Mars billions of years ago. Although their development was already too far along to alter their missions, the Surveyor and Pathfinder may uncover evidence that points toward the existence of life on Mars.

With Mars back on the public's radar screen, some space-exploration enthusiasts advocated sending a manned mission to the red planet. The cost of such a venture, however, would be enormous—up to $400 billion by some estimates. In contrast, probe missions to Mars or other planets can be more than 1,000 times cheaper. Increasingly limited budgets have forced space programs of all nations to abandon the large, complex probe designs of the past in favor of smaller, simpler spacecraft that employ more advanced technologies. Engineers are now building sophisticated probes far more economically than ever before. The Mars Pathfinder Mission, for example, was projected to cost only about one-tenth as much as a Viking mission. Although human beings may set foot on faraway planets and moons at some time in the future, for now probes are by far our best option for exploring the solar system.