Humans had landed on the moon the previous summer, the 16-year-old noted in the treatise his mother transcribed for him on her Royal No. 10 typewriter. Soon, he was sure, we would venture to all the other planets of the solar system. Then it would be time for the next step: “Going to the stars.”
On a sweaty summer afternoon, McNutt sits in his office at the Johns Hopkins University Applied Physics Laboratory, a 65-year-old with a Mickey Mouse wristwatch and thinning hair. On his computer screen is the latest draft of his boyhood dream: a plan for a probe that would travel 1,000 times farther than Earth is from the sun, leaving behind the safety of our solar system to explore the wilds of interstellar space.
From that far-flung vantage point, Interstellar Probe
will help humans finally see ourselves for what we truly are, McNutt says: citizens of a galaxy. Our home planet will be just one world among many, and the sun that gives us life just another pinprick of light in the endless dark.
It’s an audacious proposal, even by space travel standards. The probe would take 50 years to reach its destination, by which time nearly everyone currently involved in the project will be dead.
Nevertheless, McNutt and a cadre of fellow dreamers hope to get an important endorsement in a few years, when the nation’s space scientists release a list of their top research priorities. To get Interstellar Probe on the agenda, its supporters must convince their colleagues that its goal is scientifically valuable, not to mention politically viable, when there are so many questions inside the solar system still unanswered and so many Earthly squabbles still unsolved.
What makes McNutt believe it’s possible?
The scientist leans back in his chair and crosses his arms. When he answers, it’s in the form of poetry.
“I think man’s reach should exceed his grasp,” he says, paraphrasing Robert Browning. “Otherwise, what is a heaven for?”
93 billion miles from the sun
Our sun sits on a minor arm of the spinning, star-strewn pinwheel of the Milky Way, about 25,000 light-years from the galactic core. Zooming through the cosmos at roughly half a million miles per hour, the solar system is buffeted by gusts of gas and dust and bombarded by energetic particles whose origins are a mystery.
But we on Earth are partly shielded from this chaos by the heliosphere, a balloon-like structure inflated by the solar wind. Charged particles flowing from the sun stream out to the edge of the solar system — past the planets, beyond Pluto, through the frozen halo of the Kuiper belt, to a place called the heliopause.
This is the liminal zone between the river of solar particles and the ocean of interstellar space; the boundary between our celestial neighborhood and the wider universe.
Only two spacecraft have reached that zone and lived to tell the tale: the twin Voyager probes, which launched in 1977 and took more than 35 years to reach the heliopause. (The Pioneer probes left the solar system but were defunct by that time.) Now their radio communications are increasingly feeble, and several instruments have failed.
Voyager 1, the most distant human-built object in the universe, is now 145 astronomical units from Earth (an astronomical unit is equal to the distance between Earth and the sun). At that pace, it would take 283 years to reach 1,000 AU — 93 billion miles from the sun — the place McNutt hopes to reach.
“To really explore what’s out there . . . you want to get out of the solar system as quickly as possible,” he said.
And for that, you need a really big rocket.
NASA might soon have one. The ultrapowerful (but long-delayed) Space Launch System, which is capable of nearly twice as much thrust as the biggest rocket in operation, is expected to make its first flight sometime in 2020 or 2021.
With the SLS, Interstellar Probe could leave Earth at a speed of about 9 miles per second. After looping around Jupiter, the probe would fall back toward the sun, picking up speed from our star’s gravitational pull. It would pass the orbits of the inner planets and soar through the solar corona until finally, just above the sun’s blazing surface, it would fire a second rocket and zoom off into the dark as fast as 60 miles per second. At that blistering — and admittedly aspirational — pace, it would need little more than a decade to reach the heliopause.
The travel time would not be wasted. Kathy Mandt, a planetary scientist, has been exploring the potential for Interstellar Probe to fly past Uranus, Neptune or an icy body in the Kuiper belt called Quaoar.
Abigail Rymer, a physicist, is dreaming up ways for the mission to assist research on exoplanets. One experiment might involve looking back at the planets with the same techniques scientists on Earth use to study alien worlds.
“Against the backdrop of the stars,” she says, “we will see our habitable home . . . and we’ll have a better understanding of what habitability means.”
Crossing the boundary into interstellar space, the probe could scan for dust and slurp up particles to help researchers understand the structure of the heliosphere and the material from which our solar system formed.
And once it departed the sun’s protective bubble, it could finally study phenomena the heliosphere obscures: galactic cosmic rays from exploding stars; light from the afterglow of the big bang; disks of debris where planets are forming around other suns.
For now, Interstellar Probe exists only in the form of PowerPoint presentations and a twinkle in McNutt’s eye. His team has received about $700,000 for concept studies, and they are waiting to hear whether NASA will give them an additional $6.5 million over the next three years to pull together a more detailed science plan and mission design.
Their do-or-die moment will come in 2023, when the National Academies of Sciences, Engineering and Medicine are slated to publish their next decadal survey for solar and space physics. These assessments, conducted every 10 years at the request of Congress and NASA, represent the official consensus on the nation’s space science goals and guide NASA’s budget in subsequent years.
If Interstellar Probe is going to launch in McNutt’s lifetime, it needs to be ranked as a top priority.
“It was always something we couldn’t do immediately, but set aside maybe for the future,” says Richard Mewaldt, a Caltech physicist who served as chair for the solar and heliospheric physics panel during the most recent decadal survey, published in 2013. That report ranked “advance planning” for an interstellar probe eighth among nine imperatives for NASA.
Mewaldt notes that NASA’s heliophysics division — which would oversee an interstellar mission — gets the least funding of any of the agency’s science divisions. Interstellar Probe might fare better if the planners get an endorsement from the planetary science community, who could benefit from flights past the ice giants or through the Kuiper belt. Yet the scientific world tends to be siloed, he says, making it difficult to get missions funded across multiple NASA divisions.
Even if the project goes forward, it’s not clear how a spacecraft could survive the solar flyby. The best heat shield humans have ever made, currently flying on NASA’s Parker Solar Probe, is designed to keep a spacecraft safe within 3.8 million miles of the sun’s surface. To achieve its desired speed, Interstellar Probe would need to get more than twice as close.
“There is a moment for every big mission, almost an ‘aha’ moment, when the technology is ready and you’ve got a plan and it makes sense and is going to answer the science questions,” says Nicky Fox, director of NASA’s heliophysics division. The heat shield problem, she says, still stands between Interstellar Probe and that moment.
But then again, she says, there also comes a moment for every big mission when scientists simply decide that now is the right time to try.
Another question looms over the mission, one that goes beyond issues of budgets and bureaucracy to the boundaries of what humans can accomplish.
By 2050 — the year in which the probe would reach the Interstellar Medium — the United Nations’ Intergovernmental Panel on Climate Change has projected that the global average temperature will already be more than 2 degrees Celsius higher than in pre-industrial times. Unless the world dramatically cuts back its carbon consumption, we face a future in which cities are submerged beneath several feet of sea-level rise or are heated to unlivable extremes. But most large emitters are nowhere near meeting their climate goals.
Rarely has the gulf between what the world can do and what it will do seemed so vast.
‘They just can’t wait for the future to come’
But maybe, Mandt says, the apparent audacity of an interstellar mission is exactly what makes it worth trying.
“This would be an example of a large group of people working together on something multigenerational,” she says. “Which is the same thing we need with climate change.”
Members of the Interstellar Probe team, she noted, range from fellows just out of graduate school to people staving off retirement. They come from at least eight countries. They include planetary scientists, astronomers, engineers and a particle physicist.
Last fall, Mandt invited Janet Vertesi of Princeton, who has conducted ethnographic studies of spacecraft teams, to advise the team on organizational issues. It is the first time they know of that a sociologist has been involved in the conception of a NASA mission.
Her job is to “remind them of the human side,” Vertesi says: How to resolve conflict. Where to store data. How to conduct outreach so that the demographics of the project team today reflect the more diverse nation that will launch the probe in decades to come.
“We’re testing out this notion that you can actually plan a mission up front to achieve certain social objectives, too,” Vertesi says.
In these “uncertain times,” she adds, it’s a heady feeling to take part in something so inherently optimistic. To watch as a computer calculates the precise location of the planets on the date 50 years from now. To see scientists commit the remainders of their careers to an idea whose fruition they may never live to see.
“These people,” she says, “they just can’t wait for the future to come.”
At his office in Maryland, McNutt turns away from the unfinished plan on his computer screen and tries to visualize the moment when Interstellar Probe reaches the void between the stars.
There’s no way of knowing what it will find out there, beyond the veil of the solar wind. But of one thing, he is certain.
When the probe turns toward Earth to beam back the data it has gathered, it will have in its sights “one of the most special places in the universe,” McNutt says: the small, watery world where it was first dreamed into being.