Astronomers have waited decades for the James Webb Space Telescope to launch, which promises to peer into space where never before. But if humans actually want to reach our stellar neighborhood, they’ll have to wait a little longer: a probe sent to Alpha Centauri with a rocket would take about 80,000 years to get there.
Igor Bargatin, Associate Professor in the Department of Mechanical Engineering and Applied Mechanics, is trying to solve this futuristic problem with ideas drawn from one of humanity’s oldest transport technologies: sailing.
As part of the Breakthrough Starshot Initiative, he and his colleagues are designing the size, shape and materials for a sail driven not by wind, but by light. Using nanoscopically thin materials and an array of powerful lasers, such a sail could carry a probe the size of a microchip at one-fifth the speed of light – fast enough to make the journey to Alpha Centauri in about 20 years, rather than millennia.
“Reaching another star in our lifetime will require relativistic speed, or something approaching the speed of light,” said Bargatin. “The idea of a light wing has been around for some time, but we’re currently trying to figure out how to make sure those designs survive the voyage.”
Much previous field research has assumed that the sun would passively provide all the energy light sails would need to move. However, Starshot’s plan to bring his sails to relativistic speeds requires a much more focused energy source. Once the sail is in orbit, a massive array of ground-based lasers would train their beams on it, providing light intensity millions of times greater than that of the sun.
Given that the target of the lasers would be a structure three meters wide and a thousand times thinner than a sheet of paper, figuring out how to prevent the glider from tearing or melting is a major design challenge.
Bargatin, Deep Jariwala, Assistant Professor in the Department of Electrical and Systems Engineering, and Aaswath Raman, Assistant Professor in the Department of Materials Science and Engineering at UCLA Samueli School of Engineering, have now published a couple of articles in the journal Nano Letters that outline some of these fundamental specifications.
An article, led by Bargatin, demonstrates that Starshot’s lightweight sails, proposed to be built with ultra-thin sheets of aluminum oxide and molybdenum disulfide, will have to swing like a parachute rather than lie flat, as assumed by much of the previous research.
Rather than a flat sheet, Bargatin and his colleagues suggest that a curved structure, roughly as deep as it is wide, would be more able to withstand the tension of the sail’s hyper-acceleration, a pull thousands of times that of Earth’s gravity.
“The laser photons will fill the sail just like the air swells a ballSays Matthew Campbell, postdoctoral researcher in the Bargatin group and lead author of the first article. “And we know that light, pressurized containers should be spherical or cylindrical to avoid tears and cracks. Think propane tanks or even fuel tanks on rockets ”.
The other paper, led by Raman, provides insight into how the nanoscale pattern inside the sail could more efficiently dissipate the heat that comes with a laser beam a million times more powerful than the sun.
“If the sails absorb even a small fraction of the incident laser light, they will heat up to very high temperatures,” Raman explained. “To make sure they don’t disintegrate, we need to maximize their ability to radiate their heat, which is the only mode of heat transfer available in space.”
Previous light sail research has shown that using a photonic crystal design, essentially studding the sail “fabric” with regularly spaced holes, would maximize the structure’s thermal radiation. The researchers’ new paper adds another layer: sailcloth samples tied together in a grid.
With the spacing of the holes corresponding to the wavelength of the light and the spacing of the samples corresponding to the wavelength of the thermal emission, the sail could withstand an even more powerful initial thrust, reducing the amount of time that the lasers would need to stay on their target.
“A few years ago, even thinking or doing theoretical work on this type of project was considered far-fetched,” said Jariwala. “Now, not only do we have a design, but the design is based on real materials available in our workshops. Our plan for the future would be to build such structures on a small scale and test them with high-powered lasers ”.
