The Voyager probes raised perplexing questions as they exited the Solar System. Now, scientists have conceived new missions to interstellar space

Before embarking on his Ph.D., Ralph McNutt had never been east of the Mississippi River. But soon after the young Texan arrived at the Massachusetts Institute of Technology (MIT) in the fall of 1975, he found himself on a voyage to the edge of the Solar System—and beyond. Casting around for a research assistantship, he ended up in the office of plasma physicist Herbert Bridge, a towering figure in space science who had overseen the cloak-and-dagger effort to dismantle and ship Harvard University’s cyclotron to New Mexico for the Manhattan Project during World War II. Bridge evidently saw a familiar spark in McNutt and invited him to work on a plasma detector for Voyager, the epic mission to the outer planets that began in 1977. “I said, ‘Where do I sign up before you change your mind?’”

Now, this veteran of Voyager, one of NASA’s greatest scientific triumphs, wants to wheel his own passion project onto the launchpad. McNutt and colleagues at the Johns Hopkins University Applied Physics Laboratory (APL) have laid out a concept for Interstellar Probe (IP), a $3.1 billion mission to pick up a scientific gauntlet that the two Voyager probes threw down a decade ago after leaving the heliosphere, the Sun’s zone of influence. Few expected the spacecraft to survive that long, yet their beguiling observations, still trickling in, have upended many beliefs about the Solar System’s outer limits. “A lot of our preconceived notions didn’t work out too well,” McNutt says.

The Voyager data are so mystifying that some prominent researchers assert the probes haven’t made it to interstellar space yet, perhaps because the bounds of the heliosphere stretch farther than generally thought. Gazing out from Earth’s perch won’t settle the matter. “The only way to see what our fishbowl looks like is to be outside looking in,” McNutt says. “We need to get modern instruments out there,” adds Lennard Fisk, a space physicist at the University of Michigan (UM), Ann Arbor. “In that sense, Interstellar Probe would be revolutionary.”

Now, McNutt needs to convince a jury of his peers. His team has delivered a concept study of IP to the decadal survey of solar and space physics, a community exercise led by the National Academies of Sciences, Engineering, and Medicine that will set the field’s priorities for the next 10 years. The panel is set to begin deliberating next month and deliver its verdict in 2024. A thumbs-up for IP would go a long way toward securing NASA support for a probe that would, ideally, lift off in 2036. The timing would allow it to rendezvous with Jupiter and its potent gravity, which would sling the probe toward interstellar space. It would arrive about 16 years later, in half the time it took Voyager.

Chinese scientists are designing a similar mission, called Interstellar Express, that could launch around the same time. Buckle up, enthuses Jim Bell, a planetary scientist at Arizona State University, Tempe, and past president of the Planetary Society. “It’s a space race to the edge of the Solar System!”

One challenge for McNutt and his colleagues is selling a mission expected to last at least 50 years, requiring three or more generations of scientists. More daunting may be winning hearts and minds in space physics, which is dominated by experts on space weather—the solar flares and coronal mass ejections that can wreak havoc on satellites and power grids. “People are overly scared that one big project will suck out all the funding for the rest of the science we want to do,” says APL space physicist Pontus Brandt, chief scientist on the IP mission concept study. But Merav Opher, an astrophysicist at Boston University, says broadening the field’s boundaries is important. “It’s myopic if we continue to fund just space weather.”

“Voyager on steroids,” as McNutt calls IP, may stumble at this first hurdle. “It’s a long shot,” says Bell, who doesn’t have a stake in the project. But IP has a powerful champion in McNutt, says Opher, who calls him “a fantastic mover and shaker.” According to Bell, McNutt’s superb mentoring abilities will also be critical. “You really do have to think beyond your own lifetime,” he says.

Interstellar space has been a lifelong pursuit for McNutt, who says he was “an introverted and nerdy kid” with a passion for science fiction. One work that left a deep impression was Robert Heinlein’s Time for the Stars, whose premise was the twin paradox, an early 20th century thought experiment that sought to explain a mind-bending aspect of Albert Einstein’s special theory of relativity. In the novel, a telepathic teenager joins an expedition to search for habitable planets around other stars; he spends 4 years on a spaceship that can travel at close to light-speed. He returns home to find his earthbound identical twin had aged 71 years. That premise inspired McNutt, age 16, to concoct an interstellar mission as his project for the 1970 Fort Worth, Texas, science fair. He laid out the physics hurdles of such an epic voyage and even crafted a prototype spacecraft from poster board, balsa wood, and Elmer’s glue.

Can Ralph McNutt rally support for a mission he’s mused over for half a century?Marvin Joseph/The Washington Post via Getty Images

In high school, McNutt struggled to satisfy his thirst for science. School administrators “were more interested in preventing kids from dropping out,” but he and several fellow students successfully petitioned for a physics course. A few years later, McNutt got a chance to meet the “father of space travel”: Wernher von Braun, a former Nazi rocket scientist who moved to the United States after World War II and became the chief architect of NASA’s Moon program. Von Braun was giving a talk at Texas Christian University and McNutt was tapped for a student panel that would pose questions. He asked von Braun whether NASA had plans to put humans on Mars by, say, 1990. That wasn’t in the cards, von Braun responded dryly. Instead, he said, the space agency would focus on robotic probes. “I was really irritated,” McNutt says. “I was thinking something like, ‘What the hell’s wrong with you?’”

McNutt came away from the encounter with von Braun’s autograph—now in his basement, along with the science fair model—and a fierce determination to become a space scientist. He had a knack for math—“I used to do slide rule speed competitions,” he confesses—and majored in physics at Texas A&M University, College Station. At MIT, as a junior member of the Voyager team, he got to go to Cape Canaveral for Voyager 1’s launch in 1977, and he vividly remembers a visit 2 years later to mission control at the Jet Propulsion Laboratory (JPL). TV monitors in JPL’s cafeteria were showing the first images of Io, Jupiter’s flamboyantly colorful volcanic moon. “It looked like a rotting orange or a pizza pie. I thought, ‘Oh my God, it’s so beautiful.’”

Voyager’s revelations about the enigmatic outer planets kept coming. And the dauntless probes kept going. By the early 2000s, it seemed plausible that one or both would reach the heliopause—the boundary between the heliosphere and interstellar space, where the Sun’s blast of charged particles, the solar wind, peters out. Tempering that thrilling prospect was the fact that the probes were engineered primarily to interrogate Jupiter’s powerful magnetosphere, not the far weaker fields and particles of the interstellar medium. “By today’s standards, the information you can get from the Voyager spacecraft is primitive,” Bell says. Still, McNutt adds, “The fact that we could get something was a whole lot better than nothing.”

A big surprise came in 2007, when Voyager 2, diving below the ecliptic plane in which the planets orbit, crossed the termination shock: where the solar wind first starts to falter as it is buffeted by the interstellar gas and dust the Solar System is barreling through. Voyager 1 had crossed the shock 3 years earlier, some 94 astronomical units (AU) from Earth. (One AU, the average distance between Earth and the Sun, is approximately 150 million kilometers.) But its plasma detector had failed at Saturn in 1980, so it could not measure the slowing of the solar wind. Models had predicted the wind would decelerate from 1.2 million kilometers per hour to about 300,000 kilometers per hour. But Voyager 2 clocked a windspeed of 540,000 kilometers per hour. “Going through the termination shock, people said, ‘WTF?’” Brandt says.

Also mystifying was that Voyager 2 crossed the shock a full 10 AU closer to Earth than Voyager 1. After a Voyager team member broke the news at a conference in Switzerland, “Everybody was like, ‘What’s going on?’ says APL’s Elena Provornikova, IP’s heliophysics lead, who was then at the Russian Academy of Sciences’s Space Research Institute in Moscow. “We immediately started talking about what could cause this asymmetry—what could be the physics behind it.”

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