If the sails and the payload each weigh a gram, such a spacecraft could accelerate to one fifth of the speed of light. This would be 200 times faster than the Voyager probe. Richard Norte of Delft University of Technology, Netherlands, and his colleagues have demonstrated a nanometer-thick photonic-crystal material that could be used as the basis for the sails. They still need to make it about 4500 times larger than the sample that was produced. However, they have mass produced the nanoholes for the highly reflective but lightweight sails at 9000 times lower cost. This means a full size sail could be made for about 25 million euro. This cost breakthrough is key to actually making the sails. The method can be used for other movable lightweight mirrors.

The researchers used a neural network to predict an optimal shape for the holes. The holes are oblong rather than circular. "It looks like a potato," says Miguel Bessa of Brown University, Rhode Island, who led the theory side of the project. Specifically, the team arranged several potato shapes in a repeating five-neighbor pattern, or pentagonal lattice. The potato-shaped arrangement allows the system to respond to a broader range of wavelengths without having to make it thicker and thus heavier.