A solar-powered rocket might be our ticket to interstellar space

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If Jason Benkoski is right, the path to interstellar space begins in a shipping container tucked behind a laboratory high bay in Maryland. The setup looks like something out of a low-budget sci-fi film: one wall of the container is lined with thousands of LEDs, an inscrutable metal trellis runs down the center, and a thick black curtain partially obscures the apparatus. This is the Johns Hopkins University Applied Physics Laboratory solar simulator, a tool that can shine with the intensity of 20 Suns. On Thursday afternoon, Benkoski mounted a small black-and-white tile onto the trellis and pulled a dark curtain around the setup before stepping out of the shipping container. Then he hit the light switch.

Once the solar simulator was blistering hot, Benkoski started pumping liquid helium through a small embedded tube that snaked across the slab. The helium absorbed heat from the LEDs as it wound through the channel and expanded until it was finally released through a small nozzle. It might not sound like much, but Benkoski and his team just demonstrated solar thermal propulsion, a previously theoretical type of rocket engine that is powered by the Sun’s heat. They think it could be the key to interstellar exploration.

“It’s really easy for someone to dismiss the idea and say, ‘On the back of an envelope, it looks great, but if you actually build it, you're never going to get those theoretical numbers,’” says Benkoski, a materials scientist at the Applied Physics Laboratory and the leader of the team working on a solar thermal propulsion system. “What this is showing is that solar thermal propulsion is not just a fantasy. It could actually work.”
Only two spacecraft, Voyager 1 and Voyager 2, have left our Solar System. But that was a scientific bonus after they completed their main mission to explore Jupiter and Saturn. Neither spacecraft was equipped with the right instruments to study the boundary between our star’s planetary fiefdom and the rest of the universe. Plus, the Voyager twins are slow. Plodding along at 30,000 miles per hour, it took them nearly a half-century to escape the Sun’s influence.

But the data they have sent back from the edge is tantalizing. It showed that much of what physicists had predicted about the environment at the edge of the Solar System was wrong. Unsurprisingly, a large group of astrophysicists, cosmologists, and planetary scientists are clamoring for a dedicated interstellar probe to explore this new frontier.

In 2019, NASA tapped the Applied Physics Laboratory to study concepts for a dedicated interstellar mission. At the end of next year, the team will submit its research to the National Academies of Sciences, Engineering, and Medicine’s Heliophysics decadal survey, which determines Sun-related science priorities for the next 10 years. APL researchers working on the Interstellar Probe program are studying all aspects of the mission, from cost estimates to instrumentation. But simply figuring out how to get to interstellar space in any reasonable amount of time is by far the biggest and most important piece of the puzzle.

Don’t pause at the heliopause

The edge of the Solar System—called the heliopause—is extremely far away. By the time a spacecraft reaches Pluto, it’s only a third of the way to interstellar space. And the APL team is studying a probe that would go three times farther than the edge of the Solar System, a journey of 50 billion miles, in about half the time it took the Voyager spacecraft just to reach the edge. To pull off that type of mission, they’ll need a probe unlike anything that’s ever been built. “We want to make a spacecraft that will go faster, further, and get closer to the Sun than anything has ever done before,” says Benkoski. “It’s like the hardest thing you could possibly do.”

In mid-November, the Interstellar Probe researchers met online for a weeklong conference to share updates as the study enters its final year. At the conference, teams from APL and NASA shared the results of their work on solar thermal propulsion, which they believe is the fastest way to get a probe into interstellar space. The idea is to power a rocket engine with heat from the Sun, rather than combustion. According to Benkoski’s calculations, this engine would be around three times more efficient than the best conventional chemical engines available today. “From a physics standpoint, it’s hard for me to imagine anything that’s going to beat solar thermal propulsion in terms of efficiency,” says Benkoski. “But can you keep it from exploding?”

Unlike a conventional engine mounted on the aft end of a rocket, the solar thermal engine that the researchers are studying would be integrated with the spacecraft’s shield. The rigid flat shell is made from a black carbon foam with one side coated in a white reflective material. Externally, it would look very similar to the heat shield on the Parker Solar Probe. The critical difference is the tortuous pipeline hidden just beneath the surface. If the interstellar probe makes a close pass by the Sun and pushes hydrogen into its shield’s vasculature, the hydrogen will expand and explode from a nozzle at the end of the pipe. The heat shield will generate thrust.

430,000mph

It’s simple in theory but incredibly hard in practice. A solar thermal rocket is only effective if it can pull off an Oberth maneuver, an orbital-mechanics hack that turns the Sun into a giant slingshot. The Sun’s gravity acts like a force multiplier that dramatically increases the craft’s speed if a spacecraft fires its engines as it loops around the star. The closer a spacecraft gets to the Sun during an Oberth maneuver, the faster it will go. In APL’s mission design, the interstellar probe would pass just a million miles from the Sun’s roiling surface.

To put this in perspective, by the time NASA’s Parker Solar Probe makes its closest approach in 2025, it will be within 4 million miles of the Sun’s surface and booking it at nearly 430,000 miles per hour. That’s about twice the speed the interstellar probe aims to hit, and the Parker Solar Probe built up speed with gravity assists from the Sun and Venus over the course of seven years. The Interstellar Probe will have to accelerate from around 30,000 miles per hour to around 200,000 miles per hour in a single shot around the Sun, which means getting close to the star. Really close.

Cozying up to a Sun-sized thermonuclear explosion creates all sorts of materials challenges, says Dean Cheikh, a materials technologist at NASA’s Jet Propulsion Laboratory who presented a case study on the solar thermal rocket during the recent conference. For the APL mission, the probe would spend around 2.5 hours in temperatures around 4,500 degrees Fahrenheit as it completed its Oberth maneuver. That’s more than hot enough to melt through the Parker Solar Probe’s heat shield, so Cheikh’s team at NASA found new materials that could be coated on the outside to reflect away thermal energy. Combined with the cooling effect of hydrogen flowing through channels in the heat shield, these coatings would keep the interstellar probe cool while it blitzed by the Sun. “You want to maximize the amount of energy that you’re kicking back,” says Cheikh. “Even small differences in material reflectivity start to heat up your spacecraft significantly.”

“We don’t have a lot of options”

A still greater problem is how to handle the hot hydrogen flowing through the channels. At extremely high temperatures, the hydrogen would eat right through the carbon-based core of the heat shield, which means the inside of the channels will have to be coated in a stronger material. The team identified a few materials that could do the job, but there’s just not a lot of data on their performance, especially extreme temperatures. “There’s not a lot of materials that can fill these demands,” says Cheikh. “In some ways that’s good, because we only have to look at these materials. But it’s also bad because we don’t have a lot of options.”

The big takeaway from his research, says Cheikh, is there’s a lot of testing that needs to be done on heat shield materials before a solar thermal rocket is sent around the Sun. But it’s not a deal-breaker. In fact, incredible advances in materials science make the idea finally seem feasible more than 60 years after it was first conceived by engineers in the US Air Force. “I thought I came up with this great idea independently, but people were talking about it in 1956,” says Benkoski. “Additive manufacturing is a key component of this, and we couldn’t do that 20 years ago. Now I can 3D-print metal in the lab.”

Even if Benkoski wasn’t the first to float the idea of a solar thermal propulsion, he believes he’s the first to demonstrate a prototype engine. During his experiments with the channeled tile in the shipping container, Benkoski and his team showed that it was possible to generate thrust using sunlight to heat a gas as it passed through embedded ducts in a heat shield. These experiments had several limitations. They didn’t use the same materials or propellant that would be used on an actual mission, and the tests occurred at temperatures well below what an interstellar probe would experience. But the important thing, says Benkoski, is that the data from the low-temperature experiments matched the models that predict how an interstellar probe would perform on its actual mission once adjustments are made for the different materials. “We did it on a system that would never actually fly. And now the second step is we start to substitute each of these components with the stuff that you would put on a real spacecraft for an Oberth maneuver,” Benkoski says.

A long way to go

The concept has a long way to go before it’s ready to be used on a mission—and with only a year left in the Interstellar Probe study, there’s not enough time to launch a small satellite to do experiments in low Earth orbit. But by the time Benkoski and his colleagues at APL submit their report next year, they will have generated a wealth of data that lays the foundation for in-space tests. There’s no guarantee that the National Academies will select the interstellar-probe concept as a top priority for the coming decade. But whenever we are ready to leave the Sun behind, there’s a good chance we’ll have to use it for a boost on our way out the door.