In 1957, Ed Stone, SM'59, PhD'64, was returning to the University of Chicago for his second year as a graduate student. He had planned to pursue nuclear physics—the frontier of his field at the time—but then Sputnik launched October 4. "The space age began, and suddenly there was another whole frontier." Less than a year later, Stone jointed John Simpson's lab to study astrophysics.
Now a Caltech professor and retired NASA Jet Propulsion Laboratory director, Stone has been an investigator on more than a dozen NASA missions. The most extraordinary, he says, is the Voyagermission. Stone has been its sole project scientist since the mission's 1972 inception, and last year NASA commended this work with the Distinguished Public Service Medal, its highest civilian honor.
In 1977, five years after the mission began, two identical, nuclear-battery-poweredVoyager spacecraft were launched with the goal of conducting a "grand tour" of our planets—each visiting Jupiter and Saturn, and Voyager 2 continuing on to Uranus and Neptune. Each craft carries ten instruments, including cameras; particle, wave, and magnetic field detectors; and tools that measure cosmic rays and determine what elements are present.
Both Voyagers have sent back revelatory information: Jupiter's moon Io has ten more volcanic activity than Earth; Neptune's moon Triton, only 40 degrees above absolute zero, has geysers erupting from its polar cap; Saturn's moon Titan has a nitrogen atmosphere. Previously, the only known active volcanoes, geysers, and nitrogen atmospheres were on Earth. "We realized we were in for not just more understanding of what we already knew but in fact things we could not have imagined."
Voyager 2 took its last picture in 1989 during the Neptune flyby. Voyager 1 cameras, off since 1980 after visiting Saturn, were briefly turned back on in 1990 for a solar system "family portrait," a mosaic of 60 frames. It includes the famous "pale blue dot" photograph. NASA then permanently turned off its cameras to preserve power. Tour completed, there was nothing left to see within Voyager's life span. But both spacecraft continue to collect data on whatever invisible mysteries lay beyond.
The space age began, and suddenly there was another whole frontier.
Thirty-five years after launch, Voyager 1 had crossed the heliosphere's boundary: more galactic cosmic rays, denser plasma, and a shift in magnetic field. Galactic cosmic rays—high-energy particles created by distant supernovae—can penetrate our heliosphere bubble, but the slowest are deflected by solar wind. Models predicted that galactic cosmic rays would increase dramatically outside the heliosphere, while low-energy particles from inside would disappear. On August 25, 2012, Voyager 1 data revealed exactly that. But this data was not enough to prove its crossing.
The strongest evidence would be the density, temperature, and speed of the plasma itself, but Voyager 1's plasma instrument hasn't been functional since 1980. Stone relied on magnetic field measurements as a proxy for plasma density because plasma carries magnetic field lines. In August 2012, when the galactic cosmic rays were spiking, the magnetic fields strengthened by 60 percent, but the direction changed by no more than two degrees, far less than expected.
Without direct observation of plasma density or corroborating magnetic field shift, Stone could say only that Voyager was in a new but undetermined region. But in April 2013, there was a "large outburst from the sun like a tsunami, which reachedVoyager" and distributed its surrounding plasma. Voyager 1 still has a functional plasma instrument, "which can't measure the plasma directly but measures electrical waves," says Stone. "The plasma is an ionized medium, so when it oscillates, it generates an electric field, which all oscillates." The waves produce "a particular tone that tells you how dense the plasma is. It turns out it was 80 times denser than it had been. So then we knew we were indeed outside."
Extrapolation between that solar tsunami and a previous weaker disturbance indicated Voyager 1 entered interstellar space around August 25, 2012, confirming the cosmic ray measurement. Because the magnetic field did not shift as expected, the models needed to be refined.
But had Voyager 1 really left the solar system? Not exactly. While "solar system" usually means planetary system in vernacular, it technically encompasses the entirety of the sun's influence. That includes gravity, which dominates halfway to the closest star, where that star's gravity becomes stronger. Outside the heliosphere, there is a cloud of comets anchored by our suns's gravity, called the Oort cloud. "It will be 300 years before we reach the inner edge and 38,000 years before we finally get through the cloud."
If Voyager were still collecting data after traversing the Oort cloud, astrophysicists might find that some of the outer comets actually orbit another star. But, says Stone, "by 2025, we will have turned off the last instrument and after that the spacecraft will orbit the center of the Milky Way" as a silent ambassador—a lifeless vessel carrying a message from Earth in the form of a golden record.
As long as Voyager transmits, Voyager 1 from north of the planetary plane andVoyager 2 from south, two or three years from interstellar space, Stone will be listening. In the meanwhile, he turns his attention the other way toward the innermost reaches of the solar system. In 2018, NASA will launch Solar Probe Plus, a spacecraft, carrying an instrument that Stone helped develop, on a mission to study the sun's corona where the solar wind begins. —M.S.