Exploration is one thing, science another—but they’ve come together rather nicely in the Voyager mission to the outer planets, outbound for the past 35 years yet still making discoveries.
The twin Voyager probes are currently poised on the brink of interstellar space. Both are immersed in the foamy walls of the transparent “heliospheric bubble,” where the solar wind, consisting of particles blown off the Sun, stalls against the stellar winds that permeate the rest of the galaxy. Astronomers don’t know how thick the bubble walls are—that’s for the Voyagers to ascertain—but they expect the probes to burst free and begin reporting from the great beyond within the next three years. This final phase of the probes’ scientific mission should last until around 2020 to 2025, when their plutonium power sources will falter and their radios fall silent.
Thereafter the Voyagers will wander forever among the stars, mute as ghost ships but with stories to tell. Each carries a time capsule, the “Golden Record,” containing information about where, when and by what sort of species they were dispatched. Whether they will ever be found, or by whom, is utterly unknown. In that sense, the probes’ exploratory mission is just beginning.
Having played an incidental role in the mission, as producer of the Golden Record, I attended the first launch, on August 20, 1977—Carl Sagan embracing me and shouting, “We did it!” over the rolling thunder of the Titan-Centaur rocket as it climbed into a blue Florida sky atop a roiling pillar of smoke—and was among the hundreds of journalists who showed up at the Jet Propulsion Laboratory (JPL) outside Los Angeles each time the probes swept by another planet. These “encounters,” as they were called, resembled school reunions, where those of us drawn together by passion or profession witnessed one another’s journeys from young upstarts to senior citizens.
Recently I caught up with a stalwart regular, Edward Stone, Voyager’s first and only mission scientist. Bright-eyed, mantis-thin and famously unflappable, Ed is now in his late 70s. He continues to work enthusiastically on Voyager plus three other NASA missions—including the upcoming Solar Probe Plus, designed to boldly fly a mere four million miles above the Sun’s blazing surface.
“One has to remember that when the Voyagers were launched,” Ed recalled, “the space age was only 20 years old. There was no way to know how long these things would work.” The space agency launched two probes, instead of just one, as an insurance policy against catastrophic failures at Jupiter and beyond.
Yet the Voyagers worked, not just for the 5 years demanded of its builders but for 35 years and counting.
They reached Jupiter in 1979, taking thousands of photos that revealed the complexity of the giant planet’s atmosphere and the surprising diversity of its satellites, from icy Europa to the lava lakes and spewing volcanoes of hellish Io. Slingshotting past Jupiter, they picked up enough speed (in exchange for an imperceptible reduction in Jupiter’s orbital inertia) to exceed the Sun’s escape velocity, inadvertently attaining starship status. The probes have been cruising ever since, as enthralled by gravitational fields as square-riggers are by winds.
Their newfound alacrity flung the Voyagers from Jupiter to Saturn in less than three years. They found that Saturn has not just the few rings observed from Earth but thousands of them, rippled and twisted into kinks by the gravitational interactions of Saturn’s many moons.
There the two spacecraft parted company. Voyager One took a close look at Saturn’s mysterious, cloud-enshrouded satellite Titan—of intense scientific interest because it has a dense atmosphere thought to resemble that of the infant Earth. The maneuver enabled scientists to nail down Titan’s diameter (3,200 miles) and to improve their understanding of its surface, where ethane lakes are thought to glisten under an atmosphere 60 percent denser than Earth’s. But it also flung Voyager One out of the plane of the solar system, ending its planetary mission.