As part of the Breakthrough Starshot Initiative, University of Pennsylvania researcher Igor Bargatin and his colleagues are designing the size, shape and materials for a light sail accelerated to relativistic speeds — speeds approaching the speed of light — by powerful lasers.
Campbell et al. show that the diameter and radius of curvature of a circular light sail should be comparable in magnitude, both on the order of a few meters, in optimal designs for gram-scale payloads. Image credit: Campbell et al., doi: 10.1021/acs.nanolett.1c03272.
Using nanoscopically thin materials and an array of powerful lasers, such a sail could carry a microchip-sized probe at a fifth of the speed of light, fast enough to make the trip to the Alpha Centauri system in roughly 20 years.
“Reaching another star within our lifetimes is going to require relativistic speed, or something approaching the speed of light,” Dr. Bargatin said.
“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.”
Much of the earlier research in the field presumed that the Sun would passively provide all of the energy that light sails would need to get moving.
However, the plan of the Breakthrough Starshot Initiative to get its sails to relativistic speeds requires a much more focused source of energy.
Once the sail is in orbit, a massive array of ground-based lasers would train their beams on it, providing a light intensity millions of times greater than the Sun’s.
Given that the lasers’ target would be a 3-m- (10-foot) wide structure a thousand times thinner than a sheet of paper, figuring out how to prevent the sail from tearing or melting is a major design challenge.
In two new papers published in the journal Nano Letters, Dr. Bargatin and co-authors outlined some of those fundamental specifications.
In the first paper, they demonstrated that their light sails — proposed to be constructed out of ultrathin sheets of aluminum oxide and molybdenum disulfide — will need to billow like a parachute rather than remain flat, as much of the previous research assumed.
“The intuition here is that a very tight sail, whether it’s on a sailboat or in space, is much more prone to tears,” Dr. Bargatin explained.
“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.”
Rather than a flat sheet, the authors suggest that a curved structure, roughly as deep as it is wide, would be most able to withstand the strain of the sail’s 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 Dr. Matthew Campbell, also from the University of Pennsylvania.
“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.”
In the second paper, the researchers provided insights into how nanoscale patterning within the sail could most efficiently dissipate the heat that comes along with a laser beam a million times more powerful than the Sun.
“If the sails absorb even a tiny fraction of the incident laser light, they’ll heat up to very high temperatures,” said Dr. Aaswath Raman, a researcher at the University of California, Los Angeles.
“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.”
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Matthew F. Campbell et al. 2022. Relativistic Light Sails Need to Billow. Nano Lett 22 (1): 90-96; doi: 10.1021/acs.nanolett.1c03272
John Brewer et al. 2022. Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation. Nano Lett 22 (2): 594-601; doi: 10.1021/acs.nanolett.1c03273
