The other signature mission from Stone’s days as JPL director is Mars Pathfinder and its little rover, Sojourner, which landed in 1997. Pathfinder served two important functions, he notes: “It was part of learning how to do things on a smaller scale, and how to rove on the surface of Mars — because you’re unlikely to land on the most interesting location.” Sojourner’s mobility became the template for the successor Mars rovers: Spirit, Opportunity, Curiosity and the upcoming Mars 2020. Stone points out that Pathfinder was also the first NASA mission that became an Internet sensation. “For the first time, it was possible for the public to engage whenever they wanted to,” he says.
Stone stepped down from JPL in 2001, but his touch remains in the current cadence of Mars exploration, alternating between big-budget missions like Curiosity and smaller ones like the MAVEN orbiter, which studies the Martian atmosphere.
To the Edge and Beyond
Once Stone was free of his director’s duties at JPL, he could again focus on the Voyager probes, both now headed inexorably to interstellar space. Their targets were no longer planets but the particles and fields that define the sun’s outer environment. Ever since the 1990s, team scientists had been waiting to reach the termination shock — the boundary that Stone modeled for me in his sink, where the 250-mile-per-second solar wind plows into interstellar material moving less than a tenth as fast. The sudden slowdown should be accompanied by a distinctive pattern of radio signals and particle flows, but detecting it was no simple matter.
As the faster of the two probes, Voyager 1 was poised to reach interstellar space first, but its instrument for measuring the solar wind had stopped functioning in 1980. That forced Stone and his colleagues to rely on indirect evidence, leading to a difficult series of efforts to determine when the craft had truly crossed over.
According to Stone, Voyager 1 reached the termination shock in December 2004, at a distance of 9 billion miles from Earth. Voyager 2, pointing almost 90 degrees away from its twin, hit the shock in 2007 at a distance of just under 8 billion miles. Evidently the sun’s outer boundary layer is lopsided. Piling on the confusion, the termination shock still doesn’t qualify as true interstellar space because it is thoroughly blended with the solar wind. Stone watched over the readings from the Voyagers’ remaining five active instruments, waiting for the momentous entry into the undiluted interstellar flow. And waiting some more.
At long last, in 2013 all the data were in, all the disputes were settled. Voyager 1 officially entered interstellar space on Aug. 25, 2012. Let that stand as the day humans became an interstellar species.
It’s hard to imagine a more extreme territory to conquer, but Stone sounds as restless as ever. “We’re outside, but we’re still not out in pristine interstellar wind. We’re just barely into interstellar space,” he says. “And we don’t actually leave the solar system for another roughly 40,000 years.” He means that the sun’s gravitational influence extends far beyond the solar bubble, all the way to the edge of the Oort Cloud some 9 trillion miles away — and Voyager 1 won’t get there for 400 more centuries.
For a moment Stone seems frustrated, but a moment later he’s eagerly discussing his next projects: He is helping to build the world’s largest telescope, which will scrutinize distant galaxies, and serving as adviser on a space probe that will dive closer to the sun than ever before. After 56 years, Stone is still working full time, seeking out frontiers as methodically as ever.
No plans to retire, then? He offers a half-quizzical, half-smiling look. “My job is my relaxation, really. I’m very lucky.”