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Thursday, July 11, 2024

How a Beam of Pellets Could Blast a Probe Into Deep Space

If you want a spacecraft that can explore beyond the solar system—and you don’t want to wait decades for it to get there—you need one that can really move. Today’s chemical rockets and solar-powered probes are downright poky on interstellar scales. Artur Davoyan has a completely different idea for how to accelerate a spacecraft to extreme speeds: pellet-beam propulsion.

Here’s the gist of how it would work: First, you actually need two spacecraft. A probe blasts off on a one-way trip to deep space, while a second vehicle remains locked in an Earth orbit and fires thousands of tiny metallic pellets at its partner every second. The orbiting craft also either fires a 10-megawatt laser beam at the retreating probe, or aligns a laser fired from the ground at it. The laser hits the pellets, heats them, and ablates them, so that part of their material melts and becomes plasma—a hot cloud of ionized particles. That plasma accelerates the pellet remnants, and this pellet beam provides thrust to the spacecraft.

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Alternatively, Davoyan thinks the probe could get thrust from the pellet beam if the craft were to deploy an on-board magnetic field-generating device to deflect the pellets. In this case, that magnetic action would push the craft forward. 

Such a system could boost a 1-ton probe to speeds up to 300,000 miles per hour. That’s slow compared to the speed of light, but more than 10 times faster than conventional propulsion systems.

It’s a theoretical concept, but realistic enough that NASA’s Innovative Advanced Concepts program has given Davoyan’s group $175,000 to show that the technology is feasible. “There’s rich physics in there,” says Davoyan, a mechanical and aerospace engineer at UCLA. To create propulsion, he continues, “you either throw the fuel out of the rocket or you throw the fuel at the rocket.” From a physics perspective, they work the same: Both impart momentum to a moving object.

His team’s project could transform long-distance space exploration, dramatically expanding the astronomical neighborhood accessible to us. After all, we’ve only sent a few robotic visitors to scope out UranusNeptunePluto, and their moons. We know even less about objects lurking farther away. The even smaller handful of NASA craft en route to interstellar space include Pioneer 10 and 11, which blasted off in the early 1970s; Voyager 1 and 2, which were launched in 1977 and continue their mission to this day; and the more recent New Horizons, which took nine years to fly by Pluto in 2015, glimpsing the dwarf planet’s now famous heart-shaped plain. Over its 46-year journey, Voyager 1 has ventured farthest from home, but a pellet-beam-powered craft could overtake it in just five years, Davoyan says.

He takes inspiration from Breakthrough Starshot, a $100 million initiative announced in 2016 by Russian-born philanthropist Yuri Milner and British cosmologist Stephen Hawking to use a 100-gigawatt laser beam to blast a miniature probe toward Alpha Centauri. (The star nearest our solar system, it resides “only” 4 light-years away.) The Starshot team is exploring how they could hurl a 1-gram craft attached to a lightsail into interstellar space, using the laser to accelerate it to 20 percent of the speed of light, which is ludicrously fast and would reduce travel time from millennia to decades. “I’m increasingly optimistic that later this century, humanity’s going to be including nearby stars in our reach,” says Pete Worden, Breakthrough Starshot’s executive director.

That said, he expects that the futuristic project could take more than a half-century to realize. It poses a few ambitious physics and engineering challenges, including the development of such a massive laser, the construction of a lightsail that can handle that much power without disintegrating, and the design of the minuscule spacecraft and an instrument for communicating back to Earth. There’s an economic challenge as well, Worden points out: determining whether all the pieces can be put together for an “affordable amount of money.” Though the initial funding is for $100 million, they are aiming for a total price tag of around $10 billion, akin to what it cost to build the James Webb Space Telescope, or a few billion more than the Large Hadron Collider. “We’re cautiously optimistic,” he says.

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So Davoyan decided to explore an intermediate option. His project would involve a smaller laser (one a few meters across) and a shorter acceleration distance. If they’re successful, he thinks his team’s concept could be powering deep-space probes in less than 20 years.

Worden feels that such ideas are worth trying out. “I think the UCLA concept and others I’m aware of have really been ignited by the fact that we have started to push the idea that human horizons should include the nearby star systems,” says Worden, who previously served as director of NASA Ames Research Center. He cites research at the Limitless Space Institute in Houston and the Bay Area startup Helicity Space as additional examples. 

Researchers have been envisioning other kinds of advanced deep-space propulsion systems too. These include nuclear electric propulsion and a nuclear thermal rocket engine. Nuclear electric propulsion would involve a lightweight fission reactor and an efficient thermoelectric generator to convert to electrical power, while the nuclear thermal rocket concept involves pumping hydrogen into a reactor, creating the heat energy to give a vehicle thrust.

The benefits of any kind of nuclear system are that they can continue to function fairly efficiently far from the sun—where solar-powered craft would gather less energy—and attain much higher speeds than today’s NASA and SpaceX chemical rockets. “We’ve gotten to the point where chemical systems have topped out their performance and efficiency,” says Anthony Calomino, management lead for NASA’s space nuclear technology. “Nuclear propulsion offers the next era of capabilities for deep-space travel.”

This technology also has applications a little closer to home. For example, a trip to Mars currently takes about nine months. By dramatically shortening the flight time, this kind of craft would make space travel safer by limiting crewmembers’ exposure to cancer-causing space radiation.

Calomino is leading NASA’s involvement in a nuclear thermal program called Demonstration Rocket for Agile Cislunar Operations, or Draco, a collaboration announced in January between the space agency and Darpa, the Pentagon’s advanced research arm. A nuclear thermal reactor wouldn’t be so different from one on the ground or in a nuclear submarine, but it would need to operate at hotter temperatures, like 2,500 degrees C. A nuclear thermal rocket can achieve high thrust efficiently, which means less fuel must be carried on board, which translates into lower costs or more room for science instruments. “That opens up the mass available for payload—therefore enabling NTR systems to carry larger-sized cargo into space or the same-sized cargo farther into space on a reasonable timescale,” Tabitha Dodson, Darpa’s Draco program manager, wrote by email. The team plans to demo the concept later this decade.

Davoyan and his colleagues have most of this year to demonstrate to NASA and other potential partners that their propulsion system could be viable. They’re currently experimenting with different pellet materials and learning how they can be pushed with laser beams. They’re investigating how to design a spacecraft so that the pellet beam transfers momentum to it as efficiently as possible, and to make sure that it pushes—but doesn’t heat up—the spacecraft. Finally, they’re studying possible trajectories to Uranus, Neptune, or other solar system targets. 

If they get a thumbs-up from the agency, they’ll receive $600,000 and another two years to research their concept. That won’t be enough for a large-scale demonstration, Davoyan points out—actually testing a prototype in space will cost tens of millions and would come afterward. R&D takes time. The race to go ultra-fast begins by going slow.

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