If Jason Benkoski is right, the path to interstellar space begins in a freight container hidden behind a laboratory-high bay in Maryland. The setup looks a bit like a low-budget sci-fi movie: one wall of the container is lined with thousands of LEDs, an unavoidable metal grid runs down the middle, and a thick black curtain partially obscures the device. This is the solar simulator from the Johns Hopkins University Applied Physics Laboratory, a tool that can shine with 20 solar intensities. On Thursday afternoon, Benkoski mounted a small black-and-white tile on the trellis and pulled a dark curtain around the setup before stepping out of the shipping container. Then he hit the light switch.
When the solar simulator was blisteringly hot, Benkoski began pumping liquid helium through a small embedded tube that snuck across the plate. Helium absorbed heat from the LEDs as it wound 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 just demonstrated solar thermal propulsion, a former theoretical type of rocket engine powered by solar heat. They think it may be the key to interstellar exploration.
“It’s really easy for someone to reject the idea and say, ‘On the back of an envelope it looks good, but if you actually build it, you never get these theoretical numbers,'” says Benkoski, a materials scientist at it. used physics lab and the leader of the team working on a solar heating system. “What this shows is that solar heating is not just a fantasy. It could actually work.”
Only two spacecraft, Voyager 1 and Voyager 2, have left our solar system. But it was a scientific bonus after completing their main mission to explore Jupiter and Saturn. Neither spacecraft was equipped with the right instruments to study the boundary between our planet’s planetary fiefdom and the rest of the universe. Plus, the Voyager twins are slow. By flying at 30,000 miles per hour, it took them almost half a century to escape the influence of the sun.
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. Not surprisingly, a large group of astrophysicists, cosmologists, and planetary scientists are complaining about 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 will determine 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 it is by far the biggest and most important puzzle to figure out how to get to interstellar space in a reasonable amount of time.
Do not pause during the heliopause
The edge of the solar system – called the heliopause – is extremely far away. Since a spacecraft reaches Pluto, it is only a third of the way to interstellar space. And the APL team is studying a probe that will go three times further 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 accomplish this type of mission, they must use a probe that does not look like anything that has ever been built. “We want to make a spacecraft that will go faster, longer 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, researchers from Interstellar Probe met online for a week-long conference to share updates as the study went into its final year. At the conference, teams from APL and NASA shared the results of their work on solar thermal power, which they believe is the fastest way to get a probe into interstellar space. The idea is to run a rocket engine with heat from the sun instead of 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 for me to imagine anything that is going to beat solar heating in terms of efficiency,” says Benkoski. “But can you prevent it from exploding?”
Unlike a conventional engine mounted on the rear 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 of a black carbon foam with one side coated in a white reflective material. Externally, it looks a lot like the heat shield on the Parker Solar Probe. The critical difference is the twisted 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 a nozzle at the end of the tube. The heat shield generates shocks.
430,000 km / h
It’s simple in theory, but incredibly hard in practice. A solar rocket is only effective if it can pull an Oberth maneuver, a track mechanics hack that turns the Sun into a giant sling. The sun’s gravity acts as a force multiplier that dramatically increases the speed of the spacecraft 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 goes. In APL’s mission design, the interstellar probe would pass only one million miles from the sun’s wavy surface.
To put this in 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 reserve it at nearly 430,000 miles per hour. That’s about double the speed that the interstellar probe aims to hit, and the Parker Solar Probe built speed with gravitational assistants from the Sun and Venus over seven years. The interstellar probe will have 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 close to the star. Really close.
Having fun with a solar-sized thermonuclear explosion creates 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 heat reactor during the recent conference. For the APL mission, the probe spent about 2.5 hours in temperatures around 4,500 degrees Fahrenheit as it completed its Oberth maneuver. It’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 will keep the interstellar probe cool while 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 begin to heat up your spacecraft significantly.”
“We do not have many options”
An even bigger problem is how to handle the hot hydrogen flowing through the channels. At extremely high temperatures, the hydrogen eats straight through the carbon-based core of the heat shield, which means that the inside of the ducts must be coated in 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 are not many materials that can meet these requirements,” says Cheikh. “In some ways it is good because we only have to look at these materials. But it is also bad because we do not have many options. ”
The big takeaway from his research, Cheikh says, is that there is a lot of testing to be done on heat shield materials before a solar heating rocket is sent around the sun. But it’s not a deal-breaker. In fact, incredible advances in materials science have made the idea seem feasible more than 60 years after it was first devised by U.S. Air Force engineers. “I thought I came up with this amazing idea independently, but people talked about it in 1956,” says Benkoski. “Manufacturing additives is a key component in this, and we could not do that 20 years ago. Now I can 3D print metal in the lab. ”
Although Benkoski was not the first to float the idea of a solar thermal drive, he believes he is 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 power using sunlight to heat a gas as it passed through embedded channels in a heat shield. These experiments had several limitations. They did not use the same materials or propellant that would be used on an actual mission, and the tests took place at temperatures far 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 have been made to the various 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 things you want to put on a real spacecraft for an Oberth maneuver, ”says Benkoski.
A long way to go
The concept has a long way to go before it is ready to be used on a mission – and with only a year left in the Interstellar Probe study, there is not enough time to launch a small satellite to perform experiments in a low jordbane. But when Benkoski and his colleagues at the APL submit their report next year, they will have generated a wealth of data that forms the basis of space testing. There is no guarantee that national academies will choose the interstellar probe concept as a top priority in the coming decade. But when we are ready to leave the sun, there is a good chance that we will have to use it for a lift on the way out the door.
This story originally originated on wired.com.