If Jason Benkoski is right, the road to interstellar space begins in a shipping container hidden behind a laboratory high bay in Maryland. The setting 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 grille runs down the center, and a thick black curtain partially obscures the device. This is the solar simulator of the Johns Hopkins University Laboratory for Applied Physics, an instrument that can shine with the intensity of 20 suns. On Thursday afternoon, Benkoski placed a small black-and-white tile on the grille and pulled a dark curtain around the installation before leaving the shipping container. Then hit the light switch.
After the solar simulator inflates hot, Benkoski begins to pump liquid helium through a small built-in tube that twists over the plate. Helium absorbs heat from the LEDs as it winds through the channel and expands until it is finally released through a small nozzle. It may not sound like much, but Benkoski and his team have just demonstrated solar heat propulsion, formerly a theoretical type of rocket engine powered by the sun̵
“It’s really easy for someone to dismiss the idea and say, ‘It looks great on the back of an envelope, but if you actually build it, you’ll never get those theoretical numbers,'” says Benkoski, a materials scientist at the Applied Physics Lab and team leader. working on a solar thermal drive system. “What it shows is that solar heat is not just a fantasy. It can actually work. “
Only two spacecraft, Voyager 1 and Voyager 2, have left our solar system. But it was a scientific bonus after fulfilling their main mission to explore Jupiter and Saturn. None of the spacecraft has been equipped with the appropriate tools to explore the boundary between the planetary power of our star and the rest of the universe. Plus, the Voyager twins are slow. Continuing at 30,000 miles per hour, it took them nearly half a century to avoid the influence of the Sun.
But the data they sent back from the edge is excruciating. He showed that much of what physicists had predicted about the environment at the edge of the solar system was wrong. Not surprisingly, a large group of astrophysicists, cosmologists and planetary scientists are pushing for a special interstellar probe to explore this new boundary.
In 2019, NASA used the Applied Physics Laboratory to study concepts for a special interstellar mission. At the end of next year, the team will present its research on the decimal study of the National Academy of Sciences, Engineering and Medicine, which determines the decimal studies of the Sun for the next 10 years. APL researchers working on the Interstellar Probe program study all aspects of the mission, from cost estimates to tools. But just figuring out how to get to interstellar space in any reasonable amount of time is the biggest and most important part of the puzzle.
Do not pause during heliopause
The end of the solar system – called heliopause – is extremely far away. By the time the spacecraft reaches Pluto, it’s only a third of the way to interstellar space. And the APL team is exploring a probe that will go three times farther from the edge of the solar system, traveling 50 billion miles, for about half the time it takes the Voyager spacecraft just to reach the edge. To perform this type of mission, they will need a probe, unlike anything that has ever been built. “We want to make a spaceship that goes faster, farther and closer to the Sun than it has ever done before,” Benkoski said. “It’s like the hardest thing you could do.”
In mid-November, Interstellar Probe researchers met online for a one-week 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 insert a probe into interstellar space. The idea is to drive a rocket engine with heat from the sun, not 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 physics point of view, it’s hard to imagine anything that would beat solar heat in terms of efficiency,” says Benkoski. “But can you protect it from an explosion?”
Unlike a conventional engine mounted on the rear end of a rocket, the solar thermal engine that researchers are studying will be integrated with the spacecraft’s shield. The hard flat shell is made of black carbon foam with one side covered with white reflective material. Externally, it would look very similar to the heat shield of the Parker solar probe. The critical difference is the winding pipeline hidden just below the surface. If the interstellar probe passes close to the Sun and pushes hydrogen into the vasculature of its shield, the hydrogen will expand and explode from the nozzle at the end of the tube. The heat shield will generate traction.
430,000 miles per hour
In theory it is simple, but in practice it is extremely difficult. The solar thermal rocket is only effective if it can maneuver Oberth, an orbital mechanic that turns the Sun into a giant slingshot. The Sun’s gravity acts as a force multiplier that dramatically increases the speed of the vessel if a spacecraft starts its engines as it orbits the star. The closer the spacecraft approaches the Sun during the Oberth maneuver, the faster it will go. In the design of the APL mission, the interstellar probe will travel only a million miles from the rotating surface of the Sun.
To put this in perspective, by the time NASA’s Parker solar probe takes 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 strike, and the Parker solar probe has gained speed with gravitational assists from the Sun and Venus for seven years. The interstellar probe will have to accelerate from about 30,000 miles per hour to about 200,000 miles per hour with a single shot around the Sun, which means getting closer to the star. Really close.
The coziness of a sun-sized thermonuclear explosion poses all sorts of material challenges, said Dean Cheikh, a materials technologist at NASA’s Jet Propulsion Laboratory, who presented a case of a solar thermal rocket at a recent conference. For the APL mission, the probe will spend about 2.5 hours at temperatures around 4,500 degrees Fahrenheit while completing its maneuver in Obert. It’s more than hot enough to melt through the Parker Solar Probe’s heat shield, so NASA’s Cheikh team has found new materials that can be coated on the outside to reflect distant heat. Combined with the cooling effect of hydrogen flowing through channels in the heat shield, these coatings will keep the interstellar probe cool while it shines from the Sun. “You want to maximize the amount of energy you kick back,” says Sheikh. “Even small differences in the reflectivity of the material begin to significantly heat up your spacecraft.”
“We don’t have many opportunities”
An even bigger problem is how to deal with the hot hydrogen flowing through the ducts. At extremely high temperatures, hydrogen will feed right through the carbon base of the heat shield, which means that the inside of the ducts will have to be covered with a stronger material. The team has identified several materials that could do the job, but there is simply not much data on their performance, especially extreme temperatures. “There are not many materials that can meet these requirements,” said Sheikh. “In some ways, that’s good, because we just have to look at these materials. But it’s also bad because we don’t have many opportunities. “
The big conclusion from his research, says Cheikh, is that there are many tests that need to be done on heat shield materials before a solar thermal rocket can be sent around the sun. But this is not a breaker of the deal. In fact, the incredible advances in materials science have made 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 talked about it in 1956,” Benkoski said. “The production of additives is a key component of this and we could not do it 20 years ago. Now I can print 3D metal in the lab. “
Even though Benkoski was not the first to float the idea of solar thermal propulsion, he believes he was the first to demonstrate a prototype engine. During their experiments with the channeled tile in the transport container, Benkoski and his team showed that it is possible to generate traction using sunlight to heat a gas as it passes through built-in channels in a heat shield. These experiments had several limitations. They did not use the same materials or fuel that would have been used on an actual mission, and the tests were conducted at temperatures well below what the interstellar probe would have experienced. But most importantly, Benkoski says, the data from the low-temperature experiments match models that predict how the interstellar probe will fulfill its actual mission once adjustments are made to the various materials. “We did it in a system that would never actually fly. And now the second step is to start replacing each of these components with the things you would put on a real Oberth maneuver spacecraft, ”says Benkoski.
A long way
The concept has a long way to go before it can be used for a mission – and with only one year left in the study of the Interstellar Probe, there is not enough time to launch a small satellite to conduct experiments in low Earth orbit. But by the time Benkoski and his APL colleagues present their report next year, they will have generated a wealth of data that lays the groundwork for space tests. There is no guarantee that National Academies will choose the concept of an interstellar probe as a top priority for the next decade. But whenever we are ready to leave the Sun behind, there is a high probability that we will use it for amplification when leaving the door.
This story originally appeared on wired.com.