We're unsure about the best material and don't have the measurements to know.
Ars Technica said:It has been about two years since Yuri Milner announced his most audacious piece of science-focused philanthropy: Breakthrough Starshot, an attempt to send hardware to Alpha Centauri by mid-century. Although the technology involved is a reasonable extrapolation of things we already know how to make, being able to create materials and technology that create that extrapolation is a serious challenge. So much of Breakthrough Starshot's early funding has gone to figuring out what improvements on current technology are needed.
Perhaps the least well-understood developments we need come in the form of the light sail that will be needed to accelerate the starshots to 20 percent of the speed of light. We've only put two examples of light-driven sails into space, and they aren't anything close to what is necessary for Breakthrough Starshot. So, in this week's edition of Nature Materials, a team of Caltech scientists looks at what we'd need to do to go from those examples to something capable of interstellar travel.
The size of the problem
One of our best examples of a light sail was put into space on the IKAROS craft, which was capable of accelerating up to speeds of 400 meters/second. Breakthrough Starshot's craft are expected to travel in the area of 60,000 kilometers/second and accelerate to that speed before leaving the Solar System. So the amount we can learn from the existing craft is fairly limited.
Those speeds—and the acceleration needed to get there—provide a rough idea of the size of the sail we'd need, and the dimensions are pretty impressive: 10 square meters but weighing less than a gram. That, the Caltech researchers calculate, means that the sail will have to average out to being 100 atoms thick yet still be able to transmit the force of acceleration. Graphene is one of the strongest materials we're aware of, so it might work for Breakthrough, but it's also transparent, so it can only act as structural support.
There is some good news in that 100-atom-thick figure. The sail will run into a variety of energetic particles in the interplanetary and interstellar medium, but most of these will be hydrogen and helium. And, based on how deeply those particles penetrate into other materials, there's a good chance that the hydrogen and helium will pass straight through the solar sail. Dust particles create more of a problem, but the authors estimate that they would obliterate about 0.1 percent of the total sail area, and most of that would occur after the acceleration has been completed.
So what we're left with is primarily the challenge of building the material that will reflect light from the solar sail. Or, as we'll see, the collection of interlocking challenges.
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