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If Jason Benkoski he’s right, the path to interstellar space begins in a shipping container hidden behind a high bay at the lab in Maryland. The set-up looks like something out of a low-cost science fiction film: one wall of the container is lined with thousands of LEDs, an inscrutable metal trellis runs in the center and a thick black curtain partially obscures the apparatus. This is the solar simulator from the Johns Hopkins University Applied Physics Laboratory, an instrument that can glow with the intensity of 20 suns. On Thursday afternoon, Benkoski mounted a small black and white tile on the truss and pulled a dark curtain around the setup before exiting the container. Then he hit the light switch.
When the solar simulator overheated, Benkoski began pumping liquid helium through a small built-in tube that snaked through the plate. The helium absorbed heat from the LEDs as it wound through the channel and expanded until it was released through a small nozzle. That might 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 sun’s heat. They think it 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 really build it, you’ll never get those theoretical numbers,'” says Benkoski, a Materials scientist at the Applied Physics Laboratory and the team leader working on a solar thermal propulsion system. “What this is showing is that solar thermal propulsion is not just a fantasy. It might actually work. “
Only two spacecraft, Voyager 1 and Voyager 2, have left our solar system. But this was a scientific bonus after completing their main mission to explore Jupiter and Saturn. Neither spacecraft was equipped with the right tools to study the boundary between our star’s planetary fiefdom and the rest of the universe. Plus, the Voyager twins are slow. Trudging along at 30,000 miles an hour, it took 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 clamoring for a dedicated interstellar probe to explore this new frontier.
In 2019, NASA leveraged the Applied Physics Laboratory to study concepts for a dedicated interstellar mission. Late next year, the team will present its research to the Heliophysics ten-year survey of the National Academies of Sciences, Engineering and Medicine, which determines the scientific priorities related to the sun for the next 10 years. APL researchers working on the Interstellar Probe program are studying 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.
The boundary of the solar system, called the heliopause, is extremely far away. When a spacecraft reaches Pluto, it is only one third of the way to interstellar space. And the APL team is studying a probe that would travel three times beyond the edge of the solar system, a 50 billion-mile journey, in about half the time it takes the Voyager probe alone to reach the edge. To carry out that kind of mission, they will need a probe unlike anything ever built. “We want to create a spacecraft that goes faster, farther and gets closer to the sun than it ever has before,” says Benkoski. “It’s like the hardest thing you could do.”
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