If Jason Benkoski is right, the path to interstellar space begins in a shipping container that is located behind a laboratory bay in Maryland. The setup looks like something out of a low-budget science fiction film: one wall of the container is lined with thousands of LEDs, an unfathomable metal grille runs in the middle and a thick black curtain partially covers the device. This is the Johns Hopkins University Applied Physics Laboratory's 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 grille and pulled a dark curtain around the setup before stepping out of the shipping container. Then he pressed the light switch.
As soon as the solar simulator was red 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 snaked through the channel and expanded until it was finally released through a small nozzle. It may not sound like much, but Benkoski and his team have just demonstrated solar thermal propulsion, a previously theoretical type of rocket engine powered by the heat of the sun. They think it could be the key to interstellar exploration.
"It's really easy for someone to dismiss the idea and say, 'It looks great on the back of an envelope, but when you actually build it you never get those theoretical numbers," says Benkoski, a materials scientist at the lab for applied physics and leader of the team working on a solar thermal drive system. "This shows that solar thermal drive is not just a fantasy. It could actually work."
Only two spaceships, Voyager 1 and Voyager 2, have left our solar system. But that was a scientific bonus after completing their main mission to explore Jupiter and Saturn. No spaceship was equipped with the proper instruments to examine the boundary between our star's planetary fiefdom and the rest of the universe. Plus, the Voyager twins are slow. At 30,000 miles per hour, 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 turned out 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 demanding a special interstellar probe to explore this new frontier.
In 2019, NASA used the Applied Physics Laboratory to study concepts for a special interstellar mission. Late next year, the team will present its research to the National Academies of Science, Engineering, and Medicine Decadal Survey on Heliophysics, which will set solar-related scientific 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 instruments. But figuring out by far how to get into interstellar space in a reasonable time is by far the biggest and most important piece of the puzzle.
Do not take a break from the heliopause
The edge of the solar system – called the heliopause – is extremely far away. Before a spaceship reaches Pluto, it is only a third of the way to interstellar space. And the APL team is studying a probe three times farther than the edge of the solar system, a journey of 50 billion miles, about half the time it takes the Voyager spacecraft to reach the edge. To accomplish this type of mission, you need a probe unlike any other. “We want to build a spaceship that goes faster and further and comes closer to the sun than ever before,” says Benkoski. "It's the hardest thing you can do."
In mid-November, Interstellar Probe researchers met 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 solar heat rather than combustion. According to calculations by Benkoski, this motor would be about three times more efficient than the best conventional chemical motors currently available. "From a physical point of view, it is difficult for me to imagine something that will surpass solar thermal propulsion in terms of efficiency," says Benkoski. "But can you keep it from exploding?"
Unlike a conventional motor mounted on the rear end of a rocket, the solar thermal motor studied by the researchers would be integrated into the shield of the spacecraft. The rigid, flat shell consists of a black carbon foam, one side of which is coated with a white reflective material. Outwardly, it would look very similar to the Parker Solar Probe's heat shield. The key difference is the tortuous pipeline that is hidden just below the surface. When the interstellar probe makes a narrow passage through the sun and pushes hydrogen into the vasculature of its shield, the hydrogen expands and explodes from a nozzle at the end of the tube. The heat shield generates thrust.
430,000 miles per hour
It's easy in theory, but incredibly difficult in practice. A solar thermal rocket is only effective if it can perform an Oberth maneuver, a hack in orbital mechanics that turns the sun into a giant sling. The sun's gravity acts like a force multiplier that increases the vehicle's speed dramatically when a spaceship fires its engines while it rotates around the star. The closer a spaceship comes to the sun during an Oberth maneuver, the faster it gets. In APL's mission design, the interstellar probe would be only a million miles from the sun's surface.
To put this in perspective, by the time NASA's Parker Solar Probe gets closest in 2025, it will be within 4 million miles of the sun's surface and be booked at speeds close to 430,000 miles per hour. This is roughly twice the speed the interstellar probe is supposed to reach, and the Parker solar probe has built speed over the course of seven years using the gravity of the Sun and Venus. The interstellar probe must accelerate around the sun from about 30,000 miles per hour to about 200,000 miles per hour, meaning it is approaching the star. Very close.
Dean Cheikh, a materials technologist at NASA's Jet Propulsion Laboratory, presented a case study of the solar thermal rocket preparing for a solar-sized thermonuclear explosion at the recent conference. For the APL mission, the probe would spend around 2.5 hours at temperatures around 4,500 degrees Fahrenheit after completing its Oberth maneuver. That's more than hot enough to melt through the Parker Solar Probe's heat shield. That's why Cheikh's team at NASA found new materials that can be coated on the outside to dissipate thermal energy. Combined with the cooling effects of hydrogen flowing through channels in the heat shield, these coatings would keep the interstellar probe cool while it is being flashed by the sun. "You want to maximize the amount of energy that you fight back," says Cheikh. "Even small differences in material reflectivity can heat up your spaceship considerably."
"We don't have many options."
An even bigger 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 that the inside of the channels must be coated with a stronger material. The team identified some materials that could do the job, but little data is available on their performance, especially at extreme temperatures. "There aren't many materials that can meet these requirements," says Cheikh. "In a way, it's good because we just have to look at these materials. But it's also bad because we don't have many options."
The big takeaway from his research, says Cheikh, is that many tests must 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 have made the idea finally feasible more than 60 years after it was first conceived by US Air Force engineers. "I thought I came up with this great idea independently, but it was discussed in 1956," says Benkoski. “Additive manufacturing is a key component in this, and we couldn't do that 20 years ago. Now I can 3D print metal in the lab. "
Even if Benkoski was not the first to put the idea of a solar thermal drive into practice, he believes he will be the first to demonstrate a prototype of an engine. During his experiments with the channeled tile in the shipping container, Benkoski and his team showed that it was possible to use sunlight to generate a thrust to heat a gas that was passed through channels embedded in a heat shield. These experiments had several limitations. They didn't use the same materials or propellants used for an actual mission, and the tests were conducted at temperatures well below those of an interstellar probe. However, according to Benkoski, it is important that the data from the low-temperature experiments match the models that predict how an interstellar probe would behave on its actual mission, if adjustments are made for the different materials. “We did it on a system that would never fly. And now, in the second step, we're starting to replace each of these components with the material you would put on a real spaceship for an Oberth maneuver, ”says Benkoski.
A long way to go
The concept still has a long way to go before it can be used on a mission. With only one year left for the Interstellar Probe study, there isn't enough time to launch a small satellite to conduct experiments in near-earth orbit. By the time Benkoski and his colleagues from APL submit their report next year, however, they will have generated a wealth of data that will form the basis for tests in space. There is no guarantee that the National Academies will select the concept of the interstellar probe as a top priority for the next decade. But if we're ready to leave the sun behind, there's a good chance we'll need to use it for a boost on the way to the door.
This story originally appeared on wired.com.