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.
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.”
Mission of a lifetime
Researchers are pushing NASA to build the $3.1 billion Interstellar Probe (IP), which would explore the plasma environment of the Sun and interstellar space. It would follow in the footsteps of Voyager 1 and 2, which NASA launched in 1977. By most accounts, the twin probes crossed the heliopause into interstellar space a decade ago. If launched by a heavy lift rocket in 2036, IP could leave the heliosphere by 2052, in less than half the time it took Voyager.
*One astronomical unit (AU) is 150 million kilometers, the average distance between the Sun and Earth.
A solar cocoon
The heliosphere is the Sun’s zone of influence, a bubble inflated by charged particles in the solar wind.
1 Termination shock
Buffeted by neutral gas and dust that penetrate the heliosphere, the solar wind drops from supersonic to subsonic speeds here.
2 Heliopause
At this boundary, the solar wind is stopped by plasma in the interstellar medium. Cosmic rays and energetic neutral atoms should become more prevalent. Magnetic field directions should also shift, although this has not been detected.
3 Bow shock
When the interstellar wind slows suddenly to subsonic speeds, a shock wave analogous to the termination shock can form. But some studies suggest the Solar System is not moving fast enough to create one.
Shape shifting
In the classic picture, the heliosphere is shaped like a comet—squashed in the direction of the Sun’s orbit around the Galaxy. But data from NASA’s Cassini mission point to a spherical shape, whereas other models suggest it may be lobed, like a croissant.
Voyager on steroids
Like Voyager, IP would measure magnetic fields, plasma flux, and cosmic rays. It would also bring new tools such as a dust analyzer. Transmissions would be much faster. “It’s like comparing a rotary telephone from 1935 to an iPhone 13,” says project leader Ralph McNutt.
A cloudy future
Sometime in the next 2000 years, the Sun is expected to leave a local cloud of gas and dust. After that, it may enter the G cloud. If it is denser, it would squash the protective magnetic bubble of the heliosphere, potentially exposing the Solar System to a barrage of cosmic rays.
Space physicists later worked out that models had largely ignored interstellar magnetic fields, which compress the heliosphere below the ecliptic, Provornikova says. The models also assumed the solar wind is a steady gale. But it fluctuates with the Sun’s 11-year cycle of magnetic activity, another reason why the two probes reached the shock at different distances.
To explain the weakness of the termination shock, space physicists turned to findings from other planetary probes such as Cassini, the spacecraft that peeled away the mysteries of Saturn and its rings. One was a better understanding of “pickup” ions: neutral atoms, primarily hydrogen, from interstellar space that become ionized when they encounter the solar wind or the Sun’s ultraviolet radiation. “Voyager was not equipped to measure pickup ions,” Brandt says. “And these are really central here.” Scientists inferred that pickup ions riding along with the solar wind would gain enough energy crossing the termination shock to explain why the wind did not slacken as much as predicted.
After navigating that first boundary, the Voyager probes entered the heliosheath, the region where the diminished solar wind continues to wither under a fusillade of gas and dust as the Solar System plows through space. Before Voyager’s encounter, the heliosheath was viewed as the heliosphere’s thin “skin.” But with a stronger solar wind emerging from a weak termination shock, the sheath ought to be thicker. The solar wind would run farther before sputtering to a stop at the heliopause, where the hot, wispy plasma of our heliosphere gives way to the cold, dense plasma of interstellar space.
Without a working plasma detector, Voyager 1 was hard-pressed to confirm that picture. But in early 2013, mission scientists, sifting data from other detectors, declared that the probe had in fact left the heliosphere months earlier, on 25 August 2012—some 122 AU from Earth. A precipitous drop in higher energy solar wind ions and a concomitant rise in cosmic rays clinched the case, they said. Six years later, Voyager 2 hit the heliopause at nearly the same distance from the Sun, in a different phase of the solar cycle—suggesting that unlike the termination shock, the heliopause is insensitive to solar variation. “That was just incredible,” Provornikova says.
Other data didn’t add up. The Sun’s magnetic field, embedded in the solar wind, is twisted into a spiral by the Sun’s rotation. Crossing the heliopause, Voyager 1 should have observed a change in the direction of the magnetic field, as the solar wind’s twisting field gives way to differently oriented interstellar fields. “But it was basically the same damn direction from the Sun,” Brandt says. “All the people that know the theory behind it, they’re perplexed.”
Fisk thinks it’s a sign the probes haven’t yet reached interstellar space. In the 1 March issue of The Astrophysical Journal, he and UM colleague George Gloeckler propose Voyager 1 and 2 are still in the heliosheath, where they have encountered a unique plasma containing two magnetic fields: not just the field embedded in the wind, but an additional one created by mobile ions that aren’t swept up into the wind. “The physics changes dramatically when you account for that,” he says. Provornikova and others hold firm that the probes are in interstellar space, arguing that the solar wind’s magnetic field evidently dissipates over much greater distances than earlier models augured. “I don’t see a scenario where Voyager’s still inside the heliosphere,” Opher says.
No matter who’s right, scientists find Voyager’s interstellar space oddities irresistible. “Voyager didn’t give us the answers we’re looking for, and we should capitalize on that,” Fisk says. McNutt agrees, and in 2017 he assembled a 45-person team of collaborators—including Fisk and Opher—to flesh out a mission concept. Scientists have contemplated an interstellar mission for 50 years, since before Voyager, McNutt says, but “nobody had sat down and run the numbers and done the engineering.” The group released its 498-page report at the American Geophysical Union meeting in December 2021.
The mission concept study decisively settles one engineering question: whether to launch the probe toward the Sun and use its enormous gravity as a slingshot, an idea called the Oberth maneuver. After huddling with experts at a thermal materials firm, the IP team worked out that the heat shield needed for the probe to pass so close to the Sun would add too much mass—and risk. “You wouldn’t get there any faster” compared with a conventional launch of a lighter probe, McNutt says—provided the launch is on a heavy lift rocket with a rare third and fourth stage. McNutt is eyeing NASA’s Space Launch System, a mammoth rocket, bigger than the Saturn V, that could see its first launch this summer as NASA contemplates sending astronauts back to the Moon. And he has put out feelers to SpaceX and Blue Origin about a ride on one of the big launchers those private companies are developing. After the gravity boost from Jupiter, IP should peak at speeds of more than 7 AU per year, about twice as fast as the Voyager probes.
China’s Interstellar Express mission would send two probes in opposite directions: one toward the heliosphere’s nose, where modelers think it is squashed by the oncoming wind of particles in interstellar space, and the other toward its tail. Observations from both missions “will give us a more comprehensive picture of the heliosphere,” says Wang Chi, director-general of the National Space Science Center of the Chinese Academy of Sciences. When his team proposed the mission in 2014, it envisioned a third probe that would launch on a path perpendicular to the ecliptic plane, using nuclear propulsion to escape the heliosphere. But the technical challenges are daunting, and that probe for now is on ice. “As the old Chinese saying goes, a journey of a thousand miles begins with a single step,” Wang says. “We should make the two probes successful first.”
Although McNutt says the Chinese team “holds their cards close to their vest,” he, too, views the missions as complementary. “The more the merrier!” he says. “To the extent you’ll get different cuts through the heliospheric structure and the nearby interstellar medium, you’ll learn a lot about what’s going on out there.”
APL’s concept report lays out a smorgasbord of science IP could tackle, depending on the instruments it carries. High on the list is a suite of four detectors that would measure particles across a broad energy spectrum—from the chilliest plasma and feeblest pickup ions on up to cosmic rays hot enough to fry a DNA strand. “With Voyager we have giant energy gaps,” says APL physicist Alice Cocoros. Better detection of pickup ions may be the most important capability, Brandt says, with space physicists just beginning to appreciate the unheralded role they play at the heliosphere’s edges.
A dust detector would cure another Voyager blind spot. “We know pretty much nothing about how much interstellar dust actually gets into the Solar System,” Provornikova says, or how it interacts with the solar wind. On its outward journey IP could also map the cloud of dust in the outer reaches of the Solar System, leftover from its formation. The contours of this “zodiacal” dust could refine formation models, but they are largely unknown because measurements have only been taken from inside the cloud, McNutt says.
Legging it out beyond the zodiacal cloud would offer another perk: an unobscured view of the extragalactic background light (EBL)—the sum of all radiation produced since the big bang. The New Horizons spacecraft, cruising out beyond Pluto, recently uncovered a mystery when it observed a patch of dark sky and recorded about twice as much visible light as the current census of galaxies can explain, the mission team reported in the 1 March issue of The Astrophysical Journal. Equipped with the right instruments, McNutt says, IP “could for the very first time determine the EBL’s absolute brightness” across all wavelengths.
Once in interstellar space, IP could also follow in the tradition of other far-ranging probes and look back toward home. But instead of the pale blue dot of Earth, it would capture an image of the entire heliosphere. “You can solve this in one fell swoop,” McNutt says, using a unique camera that images a swirling Van Gogh–esque nightscape of energetic neutral atoms (ENAs) generated in the heliosheath when solar wind ions collide with interstellar hydrogen atoms, neutralizing the ions. High-energy ENAs—those above 50 kiloelectron volts (keV)—are especially revealing. “Simulations show that once you get above 50 keV, something remarkable happens—you start seeing images of the shape of the heliosphere,” Brandt says. But he says with a grin, “It will probably be the most expensive picture in history.”
NASA got a preview in 2008, when it put a spacecraft the size of a bus tire called Interstellar Boundary Explorer (IBEX) in orbit around Earth. Its two ENA cameras captured the first all-sky map of ENAs in the outer heliosphere—and they revealed a stunner: a winding ribbon that is richer in ENAs than surrounding areas. “The great circle in the sky,” as Brandt calls it, may be a region just beyond the heliopause where ions snared in a magnetic field spawn ENAs. IP’s ideal launch in 2036 would shoot the probe right through the ribbon.
Early IBEX data supported the traditional notion of a comet-shaped heliosphere, with a tail extending two to three times farther into space than the nose. But subsequent measurements from IBEX, Cassini, and Voyager point to a more rounded heliosphere, and recent modeling suggest it is concave on one side, like a croissant. As an encore to IBEX, NASA in 2025 plans to launch the Interstellar Mapping and Acceleration Probe (IMAP), which would peer out at the heliopause from an orbital station between the Sun and Earth with much finer imaging resolution. “IMAP will bring a lot to the party,” McNutt says. But IP, he says, will be able to provide the most revealing ENA map of all once it exits the heliosphere and snaps that picture of high-energy ENAs lighting up the heliosheath.
The science wouldn’t stop after the probe reaches interstellar space. Opher says IP would be a “game changer” in our understanding of the interstellar clouds of gas and dust the Solar System hurtles through during the Sun’s 230-million-year orbit around the center of the Milky Way. Like oases in a desert, these clouds are likely remnants of stellar nurseries, rich wellsprings of hydrogen that collapsed under gravity to form stars. Space physicists have put together a rudimentary cloud atlas of the local interstellar neighborhood. “It looks like a child’s sketch—but that’s all we’ve got,” Brandt says.
IP would directly sample gas, dust, and other properties of the Local Interstellar Cloud, the Solar System’s home for the past 60,000 years. And by measuring the absorption of starlight by dust and hydrogen atoms, it could probe the nearby G cloud, into which we will plunge in the next 2000 years—if the transition hasn’t already begun. “We have no clue what’s going to happen next,” Brandt says. The denser and colder a cloud is, the more momentum it will sap from the solar wind. That could squash the Sun’s magnetic cocoon, to our biosphere’s detriment.
Voyager found that 75% of the cosmic rays heading our way from interstellar space get filtered out in the heliosphere’s outer reaches. If the encounter with the next cloud squeezes the heliosphere all the way down to Earth’s orbit, life forms would be exposed to an intense radiation environment that would riddle DNA with mutations, Brandt says.
There’s evidence of such an event around the time early hominids were just beginning to pick up stone tools, and Brandt muses on a possible connection. “Let that creep up your spine for a moment,” he says. In recent years, scientists have discovered iron-60 isotopes in ocean crust samples dating from 2 million to 3 million years ago. Iron-60 is not found naturally on Earth: It’s forged in the cores of large stars. So, either a nearby supernova blasted the heliosphere with the iron dust, or the heliosphere drifted through a dense cloud laden with iron-60 from a previous supernova. Either way, Brandt says, “The heliosphere was way in, and we had a full blast of galactic cosmic rays and interstellar matter for a long, long time.” To look for relics of other such events, IP could use plasma wave antennas to essentially take the temperature of nearby electrons. Hot regions might mark the blast paths of material from past supernovae.
The IP team is thinking big in other ways—even the possibility that the probe will ultimately stray near another star and fall into alien hands. Each Voyager probe carries a golden record filled with music and voices sampling Earth’s cultures. IP would likely bear an updated digital version: a thumb drive, perhaps, offering a flavor of life on Earth for aliens cosseted in their own heliospheres—as long as an extraterrestrial IT department can figure out how to read it.
If the decadal survey endorses IP and NASA embraces it, the space agency would need to persuade Congress that the Star Trekkian emissary is worth the price tag—and then decide which lab would honcho it. Fresh off APL’s latest success—the $1.5 billion Parker Solar Probe, which is flying closer to the Sun than any mission ever—McNutt’s team is eager to put their sales hats back on. “We have a lot of missions under our belt. Parker came in around $100 million underbudget,” Brandt says.
In the meantime, APL is assiduously breeding the next generation of IP scientists. Literally. “We counted 13 babies born during the concept design study,” says Cocoros, who has high hopes her son Luke, who’s about to turn 2, will become enamored with space. She’s expecting a daughter in November. “I guess she makes 14!” During the design study, Cocoros served as a bridge between scientists and engineers as they strove for a sweet spot: an instrument payload that would meet key science objectives without growing too fat for a conventional launcher. “I loved being the glue in the middle of the project,” she says.
Cocoros sees McNutt as a mentor. “I adore him. If you ask him a question, he tells you a story. A novel,” she says. She says his wealth of knowledge is mirrored in his APL office, which is cluttered with the memorabilia of a life devoted to space. “It’s like his brain. Piles and piles of VHS tapes of past missions, giant binders of I don’t know what.”
With future IP leaders waiting in the wings—and some still in diapers—McNutt hopes his push for the stars will at last get the go-ahead. “We don’t want to kick this can down the road farther than it already has been,” he says. For this Voyager veteran, after all, it’s a trip that’s been a half-century in the making.