Mon. Sep 26th, 2022
A 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 behind a high-altitude lab 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 metal trellis runs down the middle, and a thick black curtain partially obscures the device. 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 on the trellis and drew a dark curtain around the setup before stepping out of the shipping container. Then he pressed the light switch.

When the solar simulator was sweltering hot, Benkoski began pumping liquid helium through a small embedded tube that snaked across the plate. The helium absorbed heat from the LEDs as it twisted and expanded through the channel until it was finally released through a small nozzle. It may not sound like much, but Benkoski and his team just demonstrated solar thermal propulsion, a previously theoretical type of rocket engine powered by the sun’s heat. They think this could hold the key to interstellar exploration.

“It’s very 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’ll never get those theoretical numbers,'” says Benkoski, a scientist at the Applied Physics Laboratory and the leader of the team working on a solar thermal propulsion system. “What this shows 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 proper instruments to study the boundary between our star’s planetary fiefdom and the rest of the universe. In addition, the Voyager twins slow† They plod along at 30,000 miles per hour and it took them nearly half a century to escape the influence of the sun.

But the data they sent back from the edge is tempting. 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 begging for a special interstellar probe to explore this new frontier.

In 2019, NASA engaged the Applied Physics Laboratory to study concepts for a special interstellar mission. By the end of next year, the team will submit its research to the Heliophysics decadal study of the National Academies of Sciences, Engineering and Medicine, which sets solar-related scientific priorities for the next 10 years. APL 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 biggest and most important piece of the puzzle.

Do not pause during heliopause

The edge of the solar system – called the heliopause – is extremely far away. By the time a spacecraft reaches Pluto, it is only a third of the way into interstellar space. And the APL team is studying a probe that would travel three times beyond the edge of the solar system, traveling 80 billion miles, in about half the time it took the Voyager spacecraft to reach the edge. To carry out those kinds of missions, they need a probe unlike anything ever built. “We want to create a spacecraft that goes faster and farther and closer to the sun than ever before,” Benkoski said. “It’s the hardest thing you can do.”

In mid-November, the Interstellar Probe researchers gathered online for a week-long 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 about three times more efficient than the best conventional chemical engines currently available. “From a physics standpoint, it’s hard for me to imagine anything that can beat solar thermal propulsion in terms of efficiency,” Benkoski says. “But can you keep it from exploding?”

Unlike a conventional motor mounted on the rear end of a rocket, the solar thermal motor the researchers are studying would integrate with the spacecraft’s shield. The rigid flat shell is made of a black carbon foam with one side coated in a white reflective material. Outwardly, it would be very similar to the heat shield on the Parker Solar Probe. The crucial difference is the tortuous pipeline hidden just below the surface. As the interstellar probe gets close to the sun and pushes hydrogen into the vascular system of its shield, the hydrogen will expand and explode from a nozzle at the end of the pipe. The heat shield generates 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-mechanical hack that turns the sun into a giant catapult. The sun’s gravity acts as a force multiplier that dramatically increases the spacecraft’s speed if a spacecraft fires its engines as it orbits 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 swirling surface.

To put this into 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, booking it at nearly 430,000 miles per hour. That’s about twice the speed the interstellar probe aims to reach, and the Parker Solar Probe built up speed with gravitational assistance from the sun and Venus over the course of seven years. The Interstellar Probe will need to accelerate around the sun from about 30,000 miles per hour to about 200,000 miles per hour all at once, which means getting close to the star. Really close.

Empathizing with a sun-sized thermonuclear explosion poses all sorts of material challenges, says Dean Cheikh, a materials technologist at NASA’s Jet Propulsion Laboratory, who presented a case study on the solar thermal rocket at the recent conference. For the APL mission, the probe would spend about 2.5 hours in temperatures around 4500 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 as it is flashed by the sun. “You want to maximize the amount of energy you kick back,” Cheikh says. “Even small differences in material reflectivity start to heat up your spacecraft significantly.”

“We don’t have many options”

An even bigger problem is dealing with the hot hydrogen flowing through the channels. At extremely high temperatures, the hydrogen would pass right through the carbon-based core of the heat shield, meaning the inside of the channels would need to be coated 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,” Cheikh says. “In a way, that’s a good thing, because we just need to look at these materials. But it’s also bad because we don’t have many options.”

The big conclusion of his research, Cheikh says, is that a lot of testing needs to be done on heat shield materials before sending a solar thermal rocket around the sun. But it’s not a deal breaker. Thanks to incredible advances in materials science, the idea finally seems feasible, more than 60 years after it was first conceived by US Air Force engineers. “I thought I had come up with this great idea on my own, but in 1956 people were talking about it,” Benkoski says. “Additive manufacturing is an important part of this, and we couldn’t do that twenty years ago. Now I can 3D print metal in the lab.”

Even if Benkoski wasn’t the first to bring up the idea of ​​a solar thermal drive, 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 channels in a heat shield. These experiments had several limitations. They didn’t use the same materials or propellant that would be used on a real mission, and the tests took place at temperatures well below what an interstellar probe would experience. Most importantly, Benkoski says, the data from the low-temperature experiments matched the models that predict how an interstellar probe would perform on its actual mission once the different materials were modified. “We did it on a system that would never actually fly. And now the second step is we’re going to replace each of these components with the things you would do 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 go on a mission – and with only a year to go in the Interstellar Probe study, there isn’t enough time to launch a small satellite to conducting 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 forms the basis for tests in space. There is no guarantee that the National Academies will pick the concept of the interstellar probe as their top priority for the next decade. But when we’re ready to put the sun behind us, chances are we’ll need to use it for a boost on our way out.

This story originally appeared on wired.com.

By akfire1

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