Sailing into interstellar space

U.S. National Science Foundation grantee astronomers based at The University of Pennsylvania and the University of California, Los Angeles concluded that if spacecraft are to withstand interstellar travel, a laser-powered light sail that billows during acceleration and can endure the light of a million suns is a key requirement. The team, inspired by sailboats and parachutes, undertook the task of designing a prototype interstellar sail.

The principle behind the size, shape and material is to create a sail made of nanoscopically thin material. It would include an array of powerful lasers, carry a microchip-sized probe, and travel at a fifth of the speed of light -- fast enough to travel to Alpha Centauri in about 20 years instead of the 80,000 years it would take a rocket to make the trip.

Preventing the sail from tearing or melting presents a major design challenge. It would need the durability to withstand almost unfathomable light intensity from the lasers that power the vessel. The researchers published two papers in the journal Nano Letters that outline some of those fundamental specifications.

One paper details the construction of the parachute-like sails. "Reaching another star within our lifetimes is going to require relativistic speed, or something approaching the speed of light," corresponding author Igor 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.

"A very tight sail, whether it's on a sailboat or in space, is much more prone to tears. It's a relatively easy concept to grasp, but we needed to do some complex math to actually show how these materials would behave at this scale."

Added Matthew Campbell, lead author on the paper, "Laser photons will fill the sail much like air inflates a beach ball. 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."

The second paper explains how the nanoscale patterning in the light sail dissipates the heat from laser beams a million-fold more brilliant than the sun. The spacing of the holes matches the wavelength of light and the wavelength of thermal emission, enabling the sail to endure the powerful initial push and reduce the amount of time the lasers would need to remain trained on the target.

"If the sails absorb even a tiny fraction of the incident laser light, they'll heat up to very high temperatures," corresponding author Aaswath Raman explained. "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."

Added co-author Deep Jariwala, "A few years ago, even thinking or doing theoretical work on this type of concept was considered far-fetched. 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."