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These New Technologies Could Make Interstellar Travel Real

Long considered science fiction, leaving the solar system and speeding amid the stars may soon be within reach

By Corey S. Powell
Mar 13, 2019 12:00 AMJan 3, 2020 5:49 PM
Beyond Solar System - NASA
(Credit: Charles Carter/Keck Institute for Space Studies via NASA)


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On October 31, 1936, six young tinkerers nicknamed the “Rocket Boys” nearly incinerated themselves in an effort to break free of Earth’s gravity. The group had huddled in a gully in the foothills of California’s San Gabriel Mountains to test a small alcohol-fueled jet engine. They wanted to prove that rocket engines could venture into space, at a time when such ideas were widely met with ridicule. That goal was disrupted when an oxygen line caught fire and thrashed around wildly, shooting flames.

The Rocket Boys’ audacity caught the attention of aerodynamicist Theodore von Karman, who already worked with two of them at Caltech. Not far from the location of their fiery mishap, he established a small test area where the Rocket Boys resumed their experiments. In 1943, the site became the Jet Propulsion Laboratory (JPL), and von Karman its first director. JPL has since grown into a sprawling NASA field center with thousands of employees, yet it has managed to retain its founding motivation: test the limits of exploration, convention be damned.

They’ve had many successes over the years. In the early 1970s, JPL engineers built Pioneer 10, the first spacecraft to reach escape velocity from the solar system. A few years later, they followed up with Voyagers 1 and 2, the fastest of the many objects aimed at interstellar space. From the beginning of the Space Age to the launch of the Voyager spacecrafts — a span of just two decades — rocket scientists more than doubled flight speeds. But in the decades since, only one more spacecraft has followed the Voyagers out of the solar system, and nothing has done so at such a high speed. Now JPL’s rocketeers are getting restless again, and quietly plotting the next great leap.

The consistent theme of the new efforts is that the solar system is not enough. It is time to venture beyond the known planets, on toward the stars. John Brophy, a flight engineer at JPL, is developing a novel engine that could accelerate space travel by another factor of 10. Leon Alkalai, a JPL mission architect, is plotting a distant journey that would begin with an improbable, Icarus-esque plunge toward the sun. And JPL research scientist Slava Turyshev has perhaps the wildest idea of all, a space telescope that could provide an intimate look at a far-off Earth-like planet — without actually going there.

These are all long shots (not entirely crazy, according to Brophy), but if even one succeeds, the implications will be huge. The Rocket Boys and their ilk helped launch humans as a space-faring species. The current generation at JPL could be the ones to take us interstellar.

NASA's Dawn spacecraft used ion propulsion to explore Ceres. Future missions could take the tech even further. (Credit: NASA-JPL/Caltech)

Rocket Reactions

For Brophy, inspiration came from Breakthrough Starshot, an extravagantly bold project announced in 2016 by the late Stephen Hawking and Russian billionaire Yuri Milner. The ultimate aim of the project is to build a mile-wide laser array that could blast a miniature spacecraft to 20 percent the speed of light, allowing it to reach the Alpha Centauri star system (our closest stellar neighbor) in just two decades.

Brophy was skeptical but intrigued. Ambitious aspirations are nothing new for him. “JPL encourages people to think outside the box, and my wacky ideas are getting wackier in time,” he says. Even by that standard, the Starshot concept struck him as a little too far from technological reality. But he did begin to wonder if he could take the same concept but scale it down so that it might actually be feasible within our lifetimes.

What especially captivated Brophy was the idea of using a Starshot-style laser beam to help deal with the “rocket equation,” which links the motion of a spacecraft to the amount of propellant it carries. The rocket equation confronts every would-be space explorer with its cruel logic. If you want to go faster, you need more fuel, but more fuel adds mass. More mass means you need even more fuel to haul around that extra weight. That fuel makes the whole thing heavier still, and so on. That’s why it took a 1.4 million-pound rocket to launch the 1,800-pound Voyager probes: The starting weight was almost entirely fuel.

Since his graduate student days in the late 1970s, Brophy has been developing a vastly more efficient type of rocketry known as ion propulsion. An ion engine uses electric power to shoot positively charged atoms (called ions) out of a thruster at high velocity. Each atom provides just a tiny kick, but collectively they can push the rocket to a much greater velocity than a conventional chemical rocket. Better yet, the power needed to run the ion engine can come from solar panels — no heavy onboard fuel tanks or generators required. By squeezing more speed out of less propellant, ion propulsion goes a long way toward taming the rocket equation.

But ion engines come with drawbacks of their own. The farther they get from the sun, the more limited they are by how much electricity their solar panels can generate. You can make the panels huge, but then you add a lot of weight, and the rocket equation slams you again. And ion engines have such gentle thrust that they can’t leave the ground on their own; it then takes them a long time in space to accelerate to their record-breaking speeds. Brophy knows these issues well: He helped design the ion engine aboard NASA’s Dawn spacecraft, which just completed an 11-year mission to asteroid Vesta and dwarf planet Ceres. Even with its formidable 65-foot span of solar cells, Dawn went from zero to 60 in an unhurried four days.

An orbiting laser system could power an ion propulsion vehicle through the solar system, and prove reusable. (Credit: Jay Smith/Discover)

Ion the Prize 

While Brophy was pondering this impasse between efficient engines and insufficient solar power, the Breakthrough Starshot concept came out, and it got the gears turning in his head. He wondered: What if you replaced sunshine with a high-intensity laser beam pointed at your spacecraft? Powered by the more efficient laser, your ion engine could run much harder while still saving weight by not having to carry your power source on board.

Two years after his epiphany, Brophy is giving me a tour of an SUV-size test chamber at JPL, where he puts a high-performance ion engine through its paces. His prototype uses lithium ions, which are much lighter than the xenon ions Dawn used, and therefore need less energy to attain higher velocities. It also runs at 6,000 volts compared with Dawn’s 1,000 volts. “The performance of this thing would be very startling if you had the laser to power it up,” he says.

There’s just one minor issue: That laser does not exist. Although he drastically downsized the Starshot concept, Brophy still envisions a 100-megawatt space-based laser system, generating 1,000 times more power than the International Space Station, aimed precisely at a fast-receding spacecraft. “We’re not sure how to do that,” he concedes. It would be by far the biggest off-world engineering project ever undertaken. Once built, though, the array could be used over and over, with different missions, as an all-purpose rocket booster.

As an example, Brophy describes a lithium-ion-powered spacecraft with 300-foot wings of photovoltaic panels powering a full-size version of the engine he is developing at JPL. The laser would bathe the panels in light a hundred times as bright as sunshine, keeping the ion engine running from here to Pluto, about 4 billion miles away. The spacecraft could then coast along on its considerable velocity, racking up another 4 billion miles every year or two.

At that pace, a spacecraft could rapidly explore the dim areas where comets come from, or set off for the as-yet-undiscovered Planet 9, or go ... almost anywhere in the general vicinity of the solar system.

“It’s like we have this shiny new hammer, so I go around looking for new nails to pound in,” Brophy says dreamily. “We have a whole long list of missions that you could do if you could go fast.”

Only the Voyager probes have passed the heliopause, leaving the sun’s influence. New probes may one day study the interstellar medium lying beyond. (Credit: NASA-JPL/Caltech)

Interstellar Medium Well 

After Brophy’s genial giddiness, it is a shock to talk to Alkalai, in charge of formulating new missions at JPL’s Engineering and Science Directorate. Sitting in his large, glassy office, he seems every bit the no-nonsense administrator, but he, too, is a man with an exploratory vision.

Like Brophy, Alkalai thinks the Breakthrough Starshot people have the right vision, but not enough patience. “We’re nowhere near where we need to be technologically to design a mission to another star,” he says. “So we need to start by taking baby steps.”

Alkalai has a specific step in mind. Although we can’t yet visit another star, we can send a probe to sample the interstellar medium, the sparse gas and dust that flows between the stars.

“I’m very interested in understanding the material outside the solar system. Ultimately, we got created from that. Life originated from those primordial dust clouds,” Alkalai says. “We know that there’s organic materials in it, but what kind? What abundances? Are there water molecules in it? That would be huge to understand.”

The interstellar medium remains poorly understood because we can’t get our hands on it: A constant blast of particles from the sun — the solar wind — pushes it far from Earth. But if we could reach beyond the sun’s influence, to a distance of 20 billion miles (about 200 times Earth’s distance from the sun), we could finally examine, for the first time, pristine samples of our home galaxy.

Alkalai wants answers, and he wants to see the results firsthand. He’s 60, so that sets an aggressive schedule — no time to wait for giant space lasers. Instead, he proposes a simpler, albeit still unproven, technology known as a solar thermal rocket. It would carry a large cache of cold liquid hydrogen, protected somehow from the heat of the sun, and execute a shocking dive to within about 1 million miles of the solar surface. At closest approach, the rocket would let the intense solar heat come pouring in, perhaps by jettisoning a shield. The sun’s energy would rapidly vaporize the hydrogen, sending it racing out of a rocket nozzle. The combined push from the escaping hydrogen, and the assist from the sun’s own gravity, would let the ship start its interstellar journey at speeds up to 60 miles per second, faster than any human object yet —and it only gets faster from there.

“It’s very challenging, but we’re modeling the physics now,” Alkalai says. He hopes to begin testing elements of a thermal-rocket system this year, and then develop his concept into a realistic mission that could launch in the next decade or so. It would reach the interstellar medium another decade after that. In addition to sampling our galactic environment, such a probe could examine how the sun interacts with the interstellar medium, study the structure of dust in the solar system and perhaps visit a distant dwarf planet along the way.

It would be a journey, Alkalai says, “like nothing we’ve done in the past.”

How a solar gravitational lens works. (Credits: Courtesy of Slava Turyshev; The Aerospace Corp.; Jim Deluca/Jimiticus via YouYube (2); Jay Smith)

Catch A Glimpse 

Solar thermal rockets and laser-ion engines, impressive as they may be, are still absurdly inadequate for crossing the tremendous gulf between our solar system and exoplanets — planets orbiting other stars. In the spirit of the Rocket Boys, Turyshev is not letting absurdity stop him. He is developing a cunning workaround: a virtual mission to another star.

Turyshev tells me he wants to send a space telescope to a region known as the solar gravitational lens (SGL). The area begins a daunting 50 billion miles away, though that’s still hundreds of times closer than our closest stellar neighbors. Once you get far enough into the SGL, something marvelous happens. When you look back toward the sun, any object directly behind it appears stretched out, forming a ring, and hugely magnified. That ring is the result of our star’s intense gravity, which warps space like a lens, altering the appearance of the distant object’s light.

If you position yourself correctly within the SGL, the object being magnified from behind the sun could be an intriguing exoplanet. A space telescope floating at the SGL, Turyshev explains, could then maneuver around, sampling different parts of the light ring and reconstructing the snippets of bent light into megapixel snapshots of the planet in question.

I have to interrupt him here. Did he say megapixel, like the resolution you get on your camera phone? Yes, he really is talking about an image measuring 1,000 by 1,000 pixels, good enough to see details smaller than 10 miles wide on a planet up to 100 light-years (600 trillion miles!) away.

“We could peek under the clouds and see continents. We could see weather patterns and topography, which is very exciting,” Turyshev says. He doesn’t mention it, but he doesn’t need to: That kind of resolution could also reveal megacities or other giant artificial structures, should they exist.

Assuming the JPL boffins can solve the transportation issues of getting to the SGL, the mission itself is fairly straightforward, if enormously challenging. Turyshev and his collaborators (Alkalai among them) will need to develop a Hubble-size space telescope,

or a mini-fleet of smaller telescopes, that can survive the 30-year journey. They will need to perfect an onboard artificial intelligence capable of running operations without guidance from home. Above all, they will need a target — a planet so intriguing that people are willing to spend decades and billions of dollars studying it. NASA’s TESS space telescope is doing some of that reconnaissance work right now, scanning for Earth-size worlds around local stars.

“Ultimately, to see the life on an exoplanet, we will have to visit. But a gravity lens mission allows you to study potential targets many decades, if not centuries, earlier,” Turyshev says merrily.

A journey to the SGL would take us beyond Alkalai’s baby steps, well onto the path toward interstellar exploration. It’s another audacious goal, but at least the odds of catching fire are much lower this time around.

Corey S. Powell, a contributing editor of Discover, also writes for the magazine's Out There blog. Follow him on Twitter: @coreyspowell. This story originally appeared in print as "Boldly Go."

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