On August 20, 1977, at Cape Canaveral, Dr. Edward C. Stone, in his role as chief scientist on the NASA Voyager mission, carried out final checks on Voyager 2 before the Titan-Centaur rocket blast it into space. Days later, on Sept. 5, Voyager 1 joined its twin spacecraft and headed out into the dark beyond.
Almost 40 years later, after its flyby of Jupiter and Saturn, Voyager 1 is now in interstellar space, 20.6 billion kilometers, or 137 Astronomical Units (AU), from Earth. Some 17 billion kilometers from Earth, Voyager 2 took a slightly different route, going past Uranus and Neptune, and is currently in the heliosheath, the outermost layer of the heliosphere where solar wind is slowed by the pressure of interstellar gas. Simply put, both spacecraft have traveled further than any spacecraft has boldly gone before.
Remarkably, Dr. Stone is still in his role as chief scientist, despite having just celebrated his 81st birthday. He has been principal investigator on nine NASA spacecraft missions, co-investigator on five other NASA missions, director of NASA's Jet Propulsion Laboratory (JPL) itself from 1991 to 2001, and received many honors, including the NASA Distinguished Service Medal and National Medal of Science. He's also a full-time professor at the California Institute of Technology (Caltech), which manages JPL for NASA.
After a brief stop at NASA JPL Mission Control, where all signals from missions are monitored 24/7 by the Deep Space Network—a geek's thrill indeed—PCMag met up with Dr. Stone, who talked us through a full-size replica of Voyager 1.
Before sitting down to talk, he pointed out the instruments onboard, which include a Magnetic field instrument, Low energy charged particle instrument, Cosmic ray instrument, Plasma instrument, and Plasma wave instrument (Voyager 1 also has an Ultraviolet spectrometer subsystem). They directly support the five scientific investigation teams participating in the Interstellar Mission: Magnetic field investigation, Low energy charged particle investigation, Plasma Investigation (Voyager 2 only), Plasma wave investigation, and Cosmic ray investigation.
Can you take us back and describe the atmosphere at Cape Canaveral on August 20, 1977?
It was a very intense period. Thousands of things have to happen at the right time. You've invested five years in the project, and now, on that day, it's all sitting on top of a large Titan-Centaur rocket. Both of the twin spacecraft were built here, at NASA JPL, then trucked to Florida, roughly three months before launch. That's when I went down there too, and where we put it all back together.
That's also where the team installed the radioisotope thermoelectric generators (RTGs) that convert the heat produced from the natural radioactive decay of plutonium into electricity to power the spacecraft, instruments, radio and on-board computers. The spacecraft fly too far from the sun to use solar panels. So, during that period, the entire team moved to Florida, some stayed here at JPL in Operations, but most of us were there in the summer of 1977. It was an amazing time.
What inspired you to study astrophysics and space science in the first place?
I went to the University of Chicago in 1956, in the graduate program for physics. I wanted to study nuclear physics as that was the frontier back then. One year later, Sputnik was launched and heralded a new era of exploration—and the first major discovery of the Space Age—the Van Allen radiation belts around the Earth. It became apparent there was a lot to learn, if you could build the instruments and get them into space. As part of my work, I had the opportunity to launch scientific instruments looking at cosmic rays on a polar orbiting spacecraft.
So, after your Ph.D., you came to join Caltech in early 1960s, became chief scientist on the Voyager and then director. Did you always know you wanted to return to Voyager after finishing your time as director?
I never left the Voyager mission, continued on right through, as chief scientist, while I was director. It was several years later that Voyager 1 reached the first milestone in the Voyager Interstellar Mission: the termination shock of the supersonic wind. Voyager 1 crossed the termination shock in December 2004 at about 94 AU from the Sun while Voyager 2 crossed it in August 2007 at about 84 AU. Then in August 2012, Voyager 1 finally entered Interstellar space.
And they're still out there.
It's a long journey. Yes, they're still sending back signals. When Voyager launched, the Space Age was only 20 years old and there was no empirical evidence that spacecraft could last more than a few years. Voyager 1 and 2 have been up there for 40 years now, and we expect they'll deliver us valuable data until ~2030 when their nuclear power sources will no longer supply enough electrical energy to power critical subsystems.
What are the two main questions about the universe that the mission has answered thus far?
Before Voyager, we thought the only active volcanoes were on Earth. Suddenly, on Jupiter's moon Io, we found 10 times more volcanic activity, and that's just on a moon. We're no longer as "terracentric" in our view of the bodies in the solar system. Time after time we were surprised by what we discovered. On Triton, a moon of Neptune, where the nitrogen is frozen, we found geysers erupting—at 40 degrees above absolute zero! We know that on Earth, water is present in three different states—frozen, liquid, and gas—and we've now found moons where other substances, like nitrogen and methane, possess similar states. Suddenly, because of Voyager, we realize how complex and interesting the planetary system is.
Is Voyager 2 still on course to go interstellar soon?
We don't know exactly when, but the number I keep using is "a few years." But this is space exploration—it could be another surprise.
Talking of surprises, both Voyager spacecraft carry the 12-inch gold-plated copper disk which Carl Sagan and his committee put together as a greeting for other life forms.
It was actually astrophysicist Frank Drake, on Carl Sagan's committee, that suggested the phonograph record for the Voyager mission, instead of a plaque which was onboard earlier missions: Pioneer 10 and 11.
Have you been disappointed that Voyager hasn't received a response from other interplanetary spacecraft?
(Pauses) You mean Search for Extraterrestrial Intelligence?
Yes
It will be 40,000 years before the Voyagers pass by other stars, so it was never expected there would be a response to the golden record during their operational lifetimes.
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Perhaps we've been a bit dull as a prospect to anyone/thing out there?
(Laughs) Maybe! [But] intelligent life is really very rare. There are now searches for microbial life, and that's to find the beginning of life. If a planet—or exoplanet—has the right thermal and geophysical conditions, with the development of scientific instruments it will be possible to study planets and exoplanets for evidence of microbial life, which is the initial step leading to intelligent life.
Back on firmer scientific footing for the final question: The Voyager spacecraft won't ever return to Earth, will they?
No, in fact both spacecraft are escaping the solar system at a speed of about 3.6 AU per year. They'll continue communicating to Earth until the power runs out. The radioisotope thermoelectric generators (RTGs) have a radiation half-life of 88 years, and the spacecraft could keep going until ~2030.
What will happen then?
Then Voyager 1 and 2 will both speed in their orbit around the center of the Milky Way Galaxy every 225 million years—until the Milky Way collides with another galaxy.