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 humankind's deepening understanding of Earth's place in the cosmos, and what some people see as a historic transition occurring in space exploration.
Even if they fall silent tomorrow, these craft, launched in 1977 on August 20 and September 5, 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, said Carolyn Porco, a planetary scientist who was deeply involved with the mission.
In 1990, when Voyager 1 was 4 billion miles (6.4 billion kilometers) 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 evolutionthat humankind had reached the point where we could send a robotic explorer that far away to see ourselves," said Porco, who works at the Southwest Research Institute in Boulder, Colorado. "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."
New Phase of Discovery
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 said. "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.