Aiming for Lighter Light Sails
To explore the Universe beyond our Solar System on a reasonable time frame, scientists need spacecraft that can move faster than those that currently exist. One route to faster spacecraft involves powering them by light sails—ultrathin, reflective structures that use laser-driven radiation pressure to propel them to high speeds. If the sails and the payload each weigh a gram, such a spacecraft could accelerate to one fifth of the speed of light, allowing it to venture out to other star systems on timescales of a few tens of years. (For comparison, the fuel-propelled Voyager spacecraft took 50 years to leave our Solar System and would need 10,000 years to get to the nearest star.) Now 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 [1].
Light sails have been an idea since the early 1900s. But the concept received a gust of new attention in 2016 when the Breakthrough Starshot mission was announced. The $100 million project—established by Stephen Hawking, Mark Zuckerberg, and Yuri Milner—is an initiative to build a light-sail-based spacecraft for long-distance exploration. Although sunlight exerts radiation pressure on objects in the Solar System, the project envisions a high-powered laser on Earth that would shine on the light sail for a few tens of minutes, enough time to accelerate the spacecraft to 100 million miles per hour. At this speed, the tiny probe could reach Alpha Centauri—our nearest star—in 20 years. If all went well, the spacecraft would beam back pictures of Proxima b, the planet that orbits Alpha Centauri.
At the time of the announcement, Norte was developing 350-µm-wide mirrors that were thin and highly reflective, two properties any light sail would need to have. But the Starshot mission design calls for 4-m-wide sails—implying a “pretty crazy” amount of scaling up, Norte says. But he and his colleagues decided to give it a go anyway.
The light-sail material developed by the researchers consists of 200-nm-thick silicon nitride film into which a series of roughly 500-nm-sized holes has been etched. The holes make the film more reflective at a wavelength corresponding to the hole size. “A membrane with no holes reflects about 30% of light that hits it, but when you put holes in it, you make it 99% reflective,” Norte says. “It’s basically a perfect mirror.” The more light that is reflected, the more force on the light sail.
Traditionally, holes are etched into such materials using electron-beam lithography. The method comes with high precision, but it’s slow—Norte estimates that it would take 15 years to fabricate a 4-m-wide sail this way. So instead, the researchers turned to optical lithography, a method that could pattern a 4-by-4-m2 sample in a day, Norte says.
In this technique, the silicon nitride membrane is first placed on a silicon wafer. A photoresist is then spun on top of the membrane and an optically opaque mask placed over the top of that. Light is used to weaken the exposed parts of the photoresist, allowing holes to be etched into the underlying membrane. In the final step, the researchers carefully lift the patterned membrane off the silicon wafer.
Norte and his team patterned a 6-by-6-cm2 sample this way. He had previously made a similar-sized sample using electron-beam lithography, but because of the time involved on the machine, it cost $35,000 to make. This new one cost just a few hundred dollars. “The Starshot mission wants to shotgun as many of these nanospacecraft as they can to increase the chances of one getting to Alpha Centauri,” Norte says. “So cost is paramount.”
Grover Swartzlander, a scientist at the Rochester Institute of Technology, New York, who has worked on creating solar sails, says that he is “excited by the large size of the fabricated film.” He notes that existing photonic-crystal reflectors have been too small to allow unambiguous measurements of radiation-pressure force, while this one should be big enough. The lower cost of making these samples could also make them useful for other applications. “If the fabrication costs for these thin-film reflectors can be truly reduced at scale, resulting in off-the-shelf availability, it would not be surprising to find researchers from other fields adopt them for other optics-related applications such as sensing,” he says.
Norte and his colleagues initially considered patterning the light sails with an array of identical circular holes, but such a pattern would reduce the overall effect of the powering laser. As the sail speeds up and moves away from the laser, the wavelength it preferentially reflects will shift because of the Doppler effect, and the sail will subsequently receive less of a push. What is needed instead is a pattern that can handle Doppler-shift changes while remaining highly reflective.
To find the optimal pattern, the researchers turned to a neural network, which predicted an optimal shape that is 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.
The researchers are now working on increasing the size of their sail and looking into ways to test how well it flies. Norte notes that the light sail is just a means to accelerate the nanospacecraft, which will include a microchip, cameras, and other instruments. All those parts need to be miniaturized so that they weigh less than one gram total. “We are really trying to use nanotechnology to go faster and further than we have been able to with traditional spacecraft,” Norte says.
–Katherine Wright
Katherine Wright is the Deputy Editor of Physics Magazine.
References
- L. Norder et al., “Pentagonal photonic crystal mirrors: scalable lightsails with enhanced acceleration via neural topology optimization,” Nat. Commun. 16, 2753 (2025).