Interstellar travel: With the perfect sail to the stars
The StarShot project, launched by the Russian billionaire, aims to use lasers to bring tiny spaceships weighing only a few grams to such a speed that they can reach the stars closest to the sun in a generation instead of in a few tens of thousands of years – the time it would take for spaceships to reach them based on current or near-future technology. My readers are familiar with the concept from the Proxima trilogy. For this purpose, these mini-ships have a sail onto which the laser can fire. This sail, about three meters in diameter, must be unimaginably light, yet strong enough to withstand laser fire.
Much previous research in this area assumed that the sun would passively provide all the energy needed for light sails to start moving. However, Starshot’s plan to get its ships to relativistic speeds of one-fifth the speed of light requires a much more directed energy source. Once in orbit, a massive array of ground-based lasers would direct their beams at the sail, delivering light intensity millions of times greater than that of the sun. And that’s with a piece of fabric (ultra-thin layers of aluminum oxide and molybdenum disulfide, to be exact) that’s a thousand times thinner than a sheet of paper. How do you build such a durable sail to withstand a thousand times the acceleration due to gravity? Researchers have now published ideas on how to do this in the journal Nano Letters.
One such paper, submitted by physicist Igor Bargatin, shows that Starshot’s lightweight sails must inflate like a parachute and not remain flat, as previous research has assumed. “The intuition here is that a very tightly stretched sail, whether on a sailboat or in space, is much more prone to cracking,” Bargatin says. Instead of a flat plate, Bargatin and his colleagues suggest that a curved structure about as deep as it is wide would best withstand the stress of the sail’s hyperacceleration. “The laser photons inflate the sail much like air inflates a beach ball,” says Matthew Campbell, a postdoctoral researcher in Bargatin’s group and lead author of the first paper. “And we know that lightweight, pressurized tanks should be spherical or cylindrical to avoid cracking. Think of propane tanks or even fuel tanks in rockets.”
Another paper, led by materials researcher Aaswath Raman, sheds light on how the nanoscale structuring of the sail could most efficiently dissipate heat emanating from a laser beam a million times more powerful than the sun. “When the sails absorb even a tiny fraction of the incident laser light, they heat up to very high temperatures,” Raman explains. “To make sure they don’t just disintegrate, we need to maximize their ability to radiate heat.” Previous research on light sails has shown that using a photonic crystal design, in which the fabric of the sail is made with regularly spaced holes, would maximize the structure’s ability to radiate heat. The researchers’ new work adds another layer of periodicity: they plan to tie the individual tracks of the sail’s fabric together in a lattice. If the spacing of the holes matches the wavelength of the light and the spacing of the fabric tracks matches the wavelength of the heat radiation, the sail could withstand an even stronger initial impact, reducing the amount of time the lasers have to stay on target.
“A few years ago, it was considered far-fetched to think about or do theoretical work on such a concept,” says Deep Jariwala, one of the researchers. “Now we not only have a design, but the design is based on real materials available in our labs. We now plan to fabricate such structures on a small scale and test them with high-power lasers.”