Right now our solar system is barreling into a region of space that we know next to nothing about. For about 60,000 years, our sun has been traversing the local interstellar cloud (LIC), a region of gas and dust within a mostly empty bubble that was carved out of the Milky Way by supernovae millions of years ago. In as little as 2,000 years, however, our star’s sphere of influence will move on to uncharted space. “We have no clue what’s going to take place” when that happens, says Pontus Brandt of the Johns Hopkins University Applied Physics Laboratory (APL). Beyond the boundaries of the LIC, a profoundly different “new normal” could arise: Earth could be subjected to more cosmic rays that alter planetary climates and fry DNA. And the sun’s region of influence—its heliosphere—could radically change in size. “We just don’t know,” Brandt says.
Finding out what will happen as we leave the LIC’s homely confines is one goal of an ambitious and thrilling mission proposal by Brandt and his APL colleagues. Called the Interstellar Probe, this notional spacecraft would study the sun’s cosmic environs in unprecedented detail. First announced in 2019 but dreamed of for decades, today the Interstellar Probe concept reflects the labors of more than 1,000 scientists working across the past few years to formalize the mission’s design, instruments and scientific goals. Last month, at the Fall Meeting of the American Geophysical Union (AGU) in New Orleans, the APL team unveiled the mission concept report that arose from these discussions—a nearly 500-page study seeking to bring the Interstellar Probe one major step closer to reality. And it is not alone: efforts in China are underway to launch an independent but equally enthralling interstellar mission, currently referred to as “Interstellar Express.”
“Interstellar Probe will allow us to understand where we come from and where we are going,” says Brandt, who is the proposal’s project scientist. “It is a mission to go beyond the boundaries of the heliosphere, the vast magnetic bubble that encases the entire solar system.”
The upcoming U.S.-based Solar and Space Physics Decadal Survey is expected to help dictate which missions NASA and other federal agencies should be focused on for a 10-year period between the early 2020s and the 2030s. APL hopes that the report will direct NASA to develop the Interstellar Probe. Geopolitical conflicts between the U.S. and China currently preclude the prospect of collaboration between the two nations, but other partners, such as the European Space Agency, could come onboard in the future to make either project a truly international endeavor. Such discussions are premature for now but may arise at a later date—along with the potential for Interstellar Probe and Interstellar Express to complement one another, in lieu of formal collaboration, if they are both selected by their respective agencies.
A Multigenerational Mission
The APL concept is designed, in part, to be a successor to NASA’s Voyager 1 and 2 missions—to date the only active spacecraft to leave our solar system. Both Voyagers made enormous strides in understanding interstellar space: in particular, they found that the edge of the heliosphere—the so-called heliopause, where our star’s magnetic influence ebbs to insignificance and interstellar space can be said to truly begin—lies more than 120 times the Earth-sun distance (more than 120 astronomical units, or AU) from the sun. But neither Voyager mission was specifically designed to explore this region so far from Earth, limiting the scientific returns on offer. “The Voyagers accidentally got there,” says Ralph McNutt of APL, principal investigator of the Interstellar Probe mission proposal. “They just barely scratched the surface.”
Interstellar Probe would be a dedicated mission to explore this far-flung locale—designed for liftoff in the 2030s, with a launch year of 2036 used as a baseline in the study. Launching on a powerful rocket such as NASA’s upcoming Space Launch System (SLS) and weighing 860 kilograms (about 1,900 pounds), similar to the Voyagers, Interstellar Probe would leave Earth at a speed of about 60,000 kilometers per hour, faster than any spacecraft in history, eclipsing the record of about 43,000 kilometers per hour set by the New Horizons spacecraft on its way to Pluto. At such speeds, Interstellar Probe would reach Jupiter in seven months, arrive at the heliopause in 15 years—compared with 35 years for Voyager 1—and overtake the Voyager spacecraft by the end of the century.
“I’m a strong supporter,” says Don Gurnett, a professor emeritus at the University of Iowa and former principal investigator of the plasma wave instruments on both Voyager spacecraft. “Voyager 1 will probably operate another 10 years and be at 165 AU. It’s not far enough out. We would like to go [farther] into the pristine interstellar medium.”
The overall goal is to design a mission that lasts at least 50 years, reaching a distance of more than 300 AU by that time and then progressing well into interstellar space, as much as 1,000 AU from Earth, in nearly 150 years. Such a mission would require unprecedented cross-generational planning, with today’s Baby Boomers taking the lead on a project that would be passed down to Gen Xers, Millennials, members of Generation Z and beyond. “This is the way that science progresses,” McNutt says. “You pay it forward; you keep handing those things down to the next generations and set them up for success.” He and his colleagues also hope their notional spacecraft will carry a compendium of humanity and life on Earth for any alien civilizations to discover that might happen to stumble across the spacecraft, akin to the Golden Record included on both Voyagers. Unlike those physical records, however, this would likely be some sort of “solid-state memory” that can store the equivalent of “hundreds of Libraries of Alexandria,” Brandt says.
The Shape of Things to Come
The spacecraft’s instruments would be specifically crafted to revolutionize our understanding of interstellar space. Topping the list would be an energetic neutral atom (ENA) imager, designed to take a picture of the atoms streaming from our sun as they strike the interstellar medium, creating an image of the heliosphere we inhabit. Currently it is unclear what shape this heliosphere takes against prevailing galactic winds: some models extrapolating from the sparse available data favor a heliosphere contoured like a croissant, whereas others lean toward a tadpolelike shape with a distinct nose and tail. Interstellar Probe would ideally be sent on a lateral trajectory to this great structure, revealing its true shape for all to see.
“The biggest mystery we have right now is ‘What is that shape of the heliosphere?’” says Elena Provornikova of APL, heliophysics lead of Interstellar Probe. An ongoing mission called the Interstellar Boundary Explorer (IBEX), launched in 2008, suggested this shape is elongated. But data from the Cassini spacecraft that orbited Saturn between 2004 and 2017 pointed to a more rounded structure without a long tail. “The only way to find out is to fly out of the heliosphere and take a look back,” Provornikova says. “The Voyagers cannot do that because they do not have ENA cameras.”
Another alluring goal is to determine our location in the LIC. Using four 50-meter-long radio antennas, Interstellar Probe would measure the density of electrons in the surrounding plasma. This would reveal our proximity to edge of the LIC and give us a definitive forecast of when we would leave it. “We may be able to say where we are,” Provornikova says. That could tell us if we are set to enter a lower-density region between the great interstellar clouds that could allow the heliosphere to dramatically expand—or if we are instead bound for a neighboring cloud called the G-cloud complex, where the density could be much higher, and our heliosphere would thus be squashed. “If you have a very compressed heliosphere, the galactic cosmic ray flux may [increase] 10-fold, who knows,” Brandt says. “Does that affect atmospheric chemistry [and] biological evolution?”
Other instruments, including a magnetometer and interstellar dust analyzer, would further investigate the interaction of our sun and the interstellar medium. Such information would be invaluable, says Shami Chatterjee, an astronomer at Cornell University, who is not involved in the mission. “Can we actually send something out to give us a direct sample?” he says. “If we could, that would be truly remarkable. That would be absolutely unique information [on] the neighborhood of our sun.”
Interstellar Probe would have some intriguing additional goals it could tackle, too. That would include studies of Jupiter as it flew past but also potential observations of dwarf planets in the outer solar system and Kuiper belt objects (KBOs) consisting of asteroids and comets that are found beyond Neptune. There are “a number of different candidates,” says Alice Cocoros of APL, a payload systems engineer for the mission. One is Eris, the most massive known dwarf planet, whose discovery in 2005 led to Pluto’s demotion to dwarf planet in 2006. Another is the large dwarf planet Gonggong. Perhaps even Planet Nine, the hypothesized ninth planet of our solar system, could be visited—if it is found to exist. “This report is really capturing the menu” of science on offer, McNutt says.
The spacecraft could perform other transformational science, such as studying the extragalactic background light, the collective cosmic glow from all luminous objects in the observable universe. From its distant locale, Interstellar Probe could see “how many photons are really coming in,” McNutt says. “That would help us constrain some of the big bang models.” Another possibility still would be studying the ancient remnants of a nearby supernova. Studies of ice cores from glaciers and polar caps on Earth have found a telltale spike in gamma radiation that seemed to arrive about three million years ago, seemingly from a star that exploded some 300 light-years away. By traveling beyond the heliopause, Interstellar Probe could more clearly see and study the remote remnants of this ancient explosion. “If we could see the dust composition, we could tell whether that [spike] was from a supernova,” says Jesse Miller of the University of Illinois at Urbana-Champaign, who presented research on that idea at the AGU’s Fall Meeting. “It could tell us what happened to material that’s been processed by the blast wave [and] how ancient supernova remnants evolve and fade away.”
Meanwhile the Interstellar Express mission China is considering would see two—or even three—spacecraft launched into interstellar space. One would go toward the supposed “nose” of the heliosphere, and another would travel toward the “tail.” The potential third spacecraft would possibly travel in a lateral direction, much like the general trajectory planned for Interstellar Probe. Initially planned for launch as soon as 2024, China’s spacecraft could then reach 100 AU by 2049, the 100th anniversary of the founding of the People’s Republic of China, but the coronavirus pandemic has so slowed development that this target liftoff date may no longer be feasible.
Along the way, each of China’s probes would independently study targets such as Jupiter, Neptune and some select KBOs such as the dwarf planet Quaoar. Their collective goal, however, would be to explore the outer heliosphere and interstellar space. Like Interstellar Probe, the two or three spacecraft would be equipped with ENA imagers to provide snapshots of our heliosphere, along with instruments to measure the local magnetic field and plasma density, among others. “We have similar instruments,” says Linghua Wang of Peking University, who is part of Interstellar Express’s ENA imager team. “But the instrument specifics could be different.”
Having these spacecraft complementing Interstellar Probe would be a boon for scientists hoping to study interstellar space. “It’s better to have more,” Wang says. “I hope we can have four spacecraft or more.” The prospects for China supporting the mission seem promising, with the country eager to cement its spacefaring capabilities after already returning samples from the moon, landing on Mars and building a crewed space station in low-Earth orbit. The groups involved in the mission, based at Peking University and the Chinese Academy of Science, “got funding from the government for concept studies,” Wang says. “So the chance probably is not that low.”
Whether one, two or all of these spacecraft will be selected and funded remains to be seen. But there is little doubt that Interstellar Probe and Interstellar Express could be transformational in our understanding of our place in space. From learning our position and direction in the galaxy to understanding the boundary between our sun’s outermost reaches and the surrounding cosmos, these missions could inspire multiple generations of scientists and the public alike. “Inevitably that is where space exploration is going to take us,” Brandt says. “We do this for future generations.”