Solar-powered rocket could be our ticket to interstellar space

If Jason Benkoski is right, the path to interstellar space begins in a shipping container tucked away behind a high lab bay in Maryland. The setup looks like something out of a low-budget sci-fi movie: One wall of the container is lined with thousands of LEDs, an impenetrable wire mesh runs down the center, and a thick black curtain partially obscures the device. This is the Solar Simulator from the Applied Physics Laboratory at Johns Hopkins University, a tool that can shine with the intensity of 20 suns. On Thursday afternoon, Benkoski mounted a small black and white tile over the trellis and pulled a dark curtain around the facility before exiting the shipping container. Then he hit the light switch.

Once the solar simulator was hot, Benkoski started pumping liquid helium through a small built-in tube that snaked through the slab. The helium absorbed heat from the LEDs as it coiled in the channel and expanded until it was finally released through a small nozzle. It may not seem like much, but Benkoski and his team have just demonstrated solar thermal propulsion, a previously theoretical type of rocket motor powered by heat from the Sun. They believe this 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 scientist in the applied physics lab and team leader working on a solar thermal propulsion system. “What this shows is that solar thermal propulsion is not just a fantasy. It might actually work.

Only two spaceships, Voyager 1 and Voyager 2, have left our solar system. But that was a science bonus after completing their main mission of exploring Jupiter and Saturn. None of the spaceships were equipped with the right instruments to study the boundary between our star’s planetary stronghold and the rest of the universe. In addition, the Voyager twins are slow. Traveling at 30,000 miles an hour, it took them almost half a century to escape the influence of the Sun.

But the data they sent back from the edge is enticing. It showed that much of what physicists had predicted about the environment on the outskirts of the solar system was wrong. Unsurprisingly, a large group of astrophysicists, cosmologists and planetologists are calling for a dedicated interstellar probe to explore this new frontier.

In 2019, NASA called on the Applied Physics Laboratory to study the concepts of a dedicated interstellar mission. At the end of next year, the team will submit their research to the decennial heliophysical survey of the National Academies of Science, Engineering and Medicine, which determines the scientific priorities related to the Sun for the next 10 years. PLA researchers working on the Interstellar Probe program study all aspects of the mission, from cost estimates to instrumentation. But simply figuring out how to get to interstellar space in a reasonable amount of time is by far the most important and important piece of the puzzle.

Don’t stop at 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, traveling 50 billion miles, in about half the time it took the Voyager spacecraft just to reach the edge. To successfully complete this type of mission, they will need a probe unlike anything that has ever been built. “We want to create a spaceship that will go faster, further and closer to the Sun than anything that has been done before,” says Benkoski. “It’s like the hardest thing you can do.”

In mid-November, researchers from the interstellar probe gathered online for a week-long conference to share updates as the study enters its final year. At the conference, teams from the PLA 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 about three times more efficient than the best conventional chemical engines available today. “From a physical point of view, it’s hard for me to imagine anything that can beat solar thermal propulsion in terms of efficiency,” says Benkoski. “But can you prevent it from exploding?”

Unlike a conventional engine mounted on the back of a rocket, the solar thermal engine the researchers are studying would be integrated into the shield of the spacecraft. The hard flat shell is made of black carbon foam with one side coated with white reflective material. Externally, it would look a lot like the heat shield of the Parker solar probe. The critical difference is the tortuous pipeline hidden just below the surface. If the interstellar probe passes close to the Sun and pushes hydrogen through its shield’s vasculature, the hydrogen will expand and explode from a nozzle at the end of the pipe. The heat shield will generate a thrust.

430,000 mph

It’s simple in theory but incredibly difficult in practice. A solar thermal rocket is only effective if it can perform an Oberth maneuver, an orbital mechanic hack that turns the Sun into a giant slingshot. The Sun’s gravity acts as a force multiplier that dramatically increases the speed of the craft 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 the design of the APL mission, the interstellar probe would pass just a million kilometers from the rough surface of the Sun.

To put that into perspective, when NASA’s Parker solar probe makes its closest approach in 2025, it will be within 4 million miles of the Sun’s surface and will reserve it at nearly 430,000 miles per hour. This is about twice the speed that the interstellar probe aims to achieve, and the Parker solar probe has increased its speed with gravity assists from the Sun and Venus over the course of seven years. The interstellar probe will need to accelerate from about 30,000 miles per hour to about 200,000 miles per hour in a single shot around the Sun, which means getting closer to the star. Very close.

Getting close to a thermonuclear explosion the size of a sun creates all kinds of materials challenges, says Dean Cheikh, materials technologist at NASA’s Jet Propulsion Laboratory who presented a solar thermal rocket case study. at the recent conference. For the APL mission, the probe would spend about 2.5 hours in temperatures of around 4,500 degrees Fahrenheit to complete its Oberth maneuver. It’s more than hot enough to melt through the heat shield of the Parker solar probe, so Sheikh’s team at NASA found new materials that could be coated on the outside to reflect thermal energy. Combined with the cooling effect of hydrogen flowing through the heat shield channels, these coatings would keep the interstellar probe cool as it blew under the influence of the sun. “You want to maximize the amount of energy you collect,” says Sheikh. “Even small differences in the reflectivity of materials start to heat up your spaceship considerably.”

“We don’t have a lot of options”

An even bigger problem is how to handle the hot hydrogen flowing through the channels. At extremely high temperatures, the hydrogen would eat through the heat shield’s carbon-based core, meaning the inside of the channels would have to be lined with a stronger material. The team identified a few materials that could do the job, but there just isn’t much data on their performance, especially extreme temperatures. “There aren’t many materials that can meet these demands,” says Sheikh. “In some ways it’s good, because we just have to look at these materials. But it’s also bad because we don’t have a lot of options.

The big advantage of his research, Sheikh says, is that there is a lot of testing that needs to be done on heat shield materials before a solar thermal rocket is sent around the Sun. But this is not a deciding factor. In fact, incredible advancements 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 had this great idea on my own, but people were talking about it in 1956,” says Benkoski. “Additive manufacturing is a key part of it, and we couldn’t do it 20 years ago. Now I can 3D print metal in the lab. “

Although Benkoski was not the first to launch the idea of ​​thermal solar propulsion, he believes he was the first to demonstrate a prototype engine. During his experiments with the ducted tile in the shipping container, Benkoski and his team showed that it was possible to generate a thrust by using sunlight to heat a gas as it passed through ducts embedded in it. a heat shield. These experiences had several limitations. They did not use the same materials or propellants that would be used in an actual mission, and the tests were conducted at temperatures well below those of an interstellar probe. But the important thing, says Benkoski, is that the data from the low-temperature experiments matches the models that predict how an interstellar probe would behave 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 that we start replacing each of these components with the material that you would put on a real spaceship for an Oberth maneuver, ”Benkoski explains.

a long way to go

The concept has a long way to go before it is ready to be used for a mission – and with only a year in the study of the interstellar probe, there isn’t enough time to launch a small satellite to do so. experiments in low earth orbit. But by the time Benkoski and his colleagues at the PLA submit their report next year, they will have generated a wealth of data that will lay the foundation for testing in space. There can be no assurance that national academies will choose the concept of the interstellar probe as a top priority for the coming decade. But whenever we’re ready to leave the sun behind, there’s a good chance we need to use it for a boost on our way out.

This story originally appeared on wired.com.