Patchwork of advanced materials, powerful lasers, and science Metal Tech News – February 23, 2022
Liquid fuel rockets are adequate for jetting humans to the most popular destinations across our solar system in a reasonable amount of time – about three days to the Moon and six months to Mars. We, however, need a way of traveling near the speed of light to shave enough of the 75,000-year journey to get to Alpha Centauri, our closest solar system neighbor, in a human lifespan.
This futuristic problem may be solved with one of humankind's earliest transportation technologies – the sail – updated with a patchwork of modern materials and engineering to capture a jet stream of photons that pushes a ship through space at nearly the speed of light.
"Reaching another star within our lifetimes is going to require relativistic speed, or something approaching the speed of light," said Igor Bargatin, associate professor in the department of mechanical engineering and applied mechanics at the University of Pennsylvania. "The idea of a light sail has been around for some time, but we're just now figuring out how to make sure those designs survive the trip."
As part of the Breakthrough Starshot Initiative, a $100 million research and engineering program aimed at laying the foundations for an Alpha Centauri flyby mission within a generation, Bargatin and his colleagues are using ultrathin layers of molybdenum disulfide and silicon nitride, ultra-powerful lasers, and equally powerful math to create a sail that can carry a microchip-sized probe at about 20% the speed of light.
Cruising along at these relativistic speeds, the microprobe could get to Alpha Centauri in about 20 years.
It was originally believed that the Sun's solar winds filling a solar sail would passively provide all the energy needed to get a space sailboat up to the speeds required to get to our closest solar neighbor. However, the two-decade sail envisioned by Starshot requires a much more focused source of energy – an enormous array of ground-based lasers shooting beams of light millions of times more intense than the Sun's.
Given that the target of this massive laser bombardment would be a roughly 10-foot-wide structure that is a thousand times thinner than a sheet of paper, figuring out how to prevent this intense blast of photonic energy from blasting a hole through the sail is a major design challenge.
Bargatin and his colleagues propose that the ultrathin light sail would need to billow much like a parachute, rather than being stretched taut like was previously thought.
"The intuition here is that a very tight sail, whether it's on a sailboat or in space, is much more prone to tears," Bargatin said. "It's a relatively easy concept to grasp, but we needed to do some very complex math to actually show how these materials would behave at this scale."
Roughly as deep as it is wide, this curved sail would be most able to withstand the strain of the hyper-acceleration, a pull thousands of times that of Earth's gravity.
"Laser photons will fill the sail much like air inflates a beach ball," said Matthew Campbell, a postdoctoral researcher at the University of Pennsylvania and lead author on a scientific paper on the billowing light sails. "And we know that lightweight, pressurized containers should be spherical or cylindrical to avoid tears and cracks. Think of propane tanks or even fuel tanks on rockets."
Materials such as silicon nitride and molybdenum disulfide, which can be formed into two-dimensional sheets similar to graphene, are part of the solution. These materials, however, must be backed up with some serious engineering and math to withstand the powerful laser blasts proposed by Starshot.
"If the sails absorb even a tiny fraction of the incident laser light, they'll heat up to very high temperatures," said Aaswath Raman, assistant professor in the department of materials science and engineering at UCLA. "To make sure they don't just disintegrate, we need to maximize their ability to radiate their heat away, which is the only mode of heat transfer available in space."
Earlier light-sail research showed that using a photonic crystal design, essentially studding the "fabric" with regularly spaced holes, would maximize the structure's thermal radiation. Building on this concept, the new paper proposes lashing swatches of sail fabric together in a grid reminiscent of a patchwork quilt.
With the spacing of the holes matching the wavelength of light and the spacing of the swatches matching the wavelength of thermal emission, the sail could withstand an even more powerful initial push, reducing the amount of time the lasers would need to stay on their target.
"A few years ago, even thinking or doing theoretical work on this type of concept was considered far-fetched," Jariwala said. "Now, we not only have a design, but the design is grounded in real materials available in our labs. Our plan for the future would be to make such structures at small scales and test them with high-power lasers."
If the science and materials hold up in the lab, Starshot will be one step closer to sailing around the Milky Way galaxy with the first expedition to Alpha Centauri.
Scientific papers detailing the findings of the Breakthrough Starshot Initiative team – "Relativistic Light Sails Need to Billow" and "Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation" – were published at Nano Letters.