For 25 years, a pair of sturdy robotic explorers have revolutionized humanity's picture of our solar system.
Today, as Voyagers 1 and 2 hurtle toward the little-known boundary of the solar system, they have come to symbolize mankind's deepening understanding of Earth's place in the cosmos, and what some see as a historic transition taking place in space exploration.
Even if they fall silent tomorrow, these craft, launched Aug. 20 and Sept. 5, 1977, will have set the stage for an era of increasingly specialized unmanned space missions.
The Voyagers revealed that active volcanoes exist beyond Earth, that rings of dust are not limited to Saturn, and that auroras shimmer around Jupiter's poles while lightning bolts lance through its clouds.
But perhaps the Voyagers' most fundamental legacy is perspective, says Carolyn Porco, a planetary scientist who was deeply involved with the mission.
In 1990, when Voyager 1 was 4 billion miles from the Sun, it turned its cameras back for a "family portrait" of six of the nine planets. Earth appeared as a single pixel, a white speck in the vast blackness of space.
"That to me spoke of evolution that humankind had reached the point where we could send a robotic explorer that far away to see ourselves," says Dr. Porco, who works at the Southwest Research Institute in Boulder, Colo. "It's like our early ancestors finally stepping down from the trees onto the savanna to take on a new life, and casting one last look over their shoulders to see where they've come from."
Today, humanity's quest to extend that epic journey is shifting from the age of reconnaissance, marked by flyby investigations, to the age of understanding, typified by focused missions to specific planets, moons, comets, and asteroids. "Twenty-five years ago, we were explorers first putting our oars into the water," Porco says. "Now we are poised at a threshold of understanding how our planet fits into the scheme of things ... all with an eye toward learning why Earth became a successful abode of life."
The shift began with the highly successful Galileo mission to Jupiter and the current Cassini-Huygens mission to Saturn both triggered by questions that the Voyager flybys raised. And its momentum is growing.
For the coming decade, solar-system scientists in the United States have given top priority to 11 new or existing projects from a flyby of Pluto to a trip back to Jupiter's moon Europa and missions that would return samples from frigid comets and the furnace-like surface of Venus. They detailed their choices and the rationale for them in a report issued last month by the National Research Council in Washington.
The technological risks these missions face are mirrored in last week's apparent breakup of the Contour spacecraft after a rocket motor ignited to send the craft from Earth orbit toward a planned encounter with two comets Encke in 2003 and Schwassmann-Wachmann 3 in 2006. At press time, mission controllers at the Johns Hopkins University's Applied Physics Laboratory were trying to contact the craft, which fell silent last Thursday.
Coming projects, proposed in the July 11 report, focus on better understanding the solar system's history, currently estimated at 5 billion years and counting. The scientists also hope to trace the history of chemical compounds that were necessary for organic life to emerge, the processes that led at least one planet to harbor life, and the processes that shape planetary systems.
As they draw new plans, researchers are quick to acknowledge their debt to Voyager.
"Much of the impetus for the exploration of the outer solar system has come from the Voyager spacecraft," says Ellis Miner, science manager for the Cassini mission at Cal Tech's Jet Propulsion Laboratory in Pasadena, Calif., and former deputy project scientist for Voyager. "Each Voyager flyby carried definite surprise. Our theories bore little resemblance to what we actually saw."
For example, he says, many researchers expected the satellites of planets such as Jupiter or Saturn to be rather inert objects, like Earth's moon. Instead, they found a volcano erupting on Jupiter's moon Io, the first object beyond Earth to display active vulcanism. Icy Europa tantalized researchers with evidence of cracks on its surface that hinted at an evolving surface.
Meanwhile, Voyager returned images from Jupiter of auroras, like those around Earth's poles. And for the first time, researchers had a supply of photos of Jupiter's Great Red Spot and other cloud features to begin to piece together a detailed picture of the planet's circulation patterns.
Rings the spectacular halos of dust and rock once thought to be the province of Saturn were found to be ubiquitous among the giant planets, Dr. Ellis says.
If Voyager represented a scientific gold mine, it also represented a technological tour de force, adds John Casani, who was Voyager's project manager. Technologies developed for the mission from small plutonium-heated electrical generators to computer-aided systems designed to run without human commands have found their way onto a growing number of craft.
Perhaps it's this theory-busting, precedent setting nature of Voyager that sets it apart from its more focused descendants.
Veteran space-imaging scientist Porco says Voyager was unlike any mission before or since. "It was far more romantic than anything we'll ever do again. It was a mission of adventure as well as scientific inquiry."