Breakthrough Starshot’s concept for a flyby of Alpha Centauri would reach its destination in a single human generation. We’ve discussed sail materials in the last couple of posts, but let’s step back to the overview. Using a powerful ground-based laser, we illuminate a sail on the forward side of which are embedded instruments for communications, imaging and whatever we choose to carry. We need a sail that is roughly 4 meters by 4 meters, and one that weighs no more than a single gram.
As Richard Norte pointed out to the Interstellar Research Group’s Montreal symposium (video here), a US penny weighs 2.5 grams, which gives an idea what we are up against. We need a payload at gram-scale and a sail that is itself no more than a gram. Obviously our sail must be of nanoscale thickness, and able to take a beating, for we’re going to light it up for 10 minutes with that laser beam to drive it to 20 percent of lightspeed. We’re engineering, then, in the realm of nanotechnology, but working on combining our nanoscale components into large objects that can be fabricated at the macro-scale.
This is an uncharted frontier in the realm of precision and microsystems engineering, and it’s one that Norte and his team at Delft University of Technology are pushing into one experiment at a time, with recent funding from the EU and Limitless Space Institute. Things get fascinating quickly at this scale. To make a membrane into a mirror, you puncture your material with nanoscale holes, producing reflectivity at specific wavelengths. The Delft work is with silicon nitride, and in the current thinking of the Starshot team, this material formed as a metagrating is layered between the instruments on the lightsail and the sail body, becoming the means for keeping the sail on the beam and providing attitude control while protecting the instruments.
Image: Delft University of Technology’s Norte, whose lab focuses on novel techniques for designing, fabricating and measuring nanotechnologies needed for quantum optics and mechanics. Credit: Delft University of Technology.
At Delft, as Norte made clear, we’re a long way from achieving the kind of macrostructures that Starshot is looking at, but remember that Starshot is conceived as a multi-decade research effort that will rely on advances along the way. The Delft team is showing us how to make the thinnest conceivable mirror, using machine learning optimization techniques to optimize nanotechnology. The material of choice turns out to be silicon nitride, as we saw in our previous Starshot discussion. Says Norte:
”Of all the material people have used for making photonic crystals, silicon nitride winds up being one of the best. We can make it big, we can make it reflective, we can make it without wrinkles, and it actually has parts per million absorption. This extremely low absorption means we won’t blow this thing up when we shine a laser on it.”
Image: Scanning electron microscope image of a silicon nitride membrane. Credit: Richard Norte/Delft University of Technology. 9:47
The question is how to move to larger mirrors, given that the state of the art when Starshot was announced was at the 350-micron to a side scale. It would be helpful if we could simply ‘stitch’ smaller units together to craft a larger sail, but Norte likens that idea to trying to stitch two soap bubbles together – the structure is so amorphous, filled with the holes of the lattice – that we have to rely on manufacturing techniques that can produce larger wafers rather than combinations of smaller ones.
Scaling up is no small challenge. Norte told the audience at Montreal that his team can now make photonic crystals in the range of 450 mm to the side. The crucial term here is ‘aspect ratio,’ which relates the thickness of a metamaterial to its diameter or width. Interacting with light on the nanoscale means designing around the aspect ratio of these structures to achieve specific nanophotonic effects. Tuning the size and spacing of the holes in the lattice governs the wavelength at which the sail will reflect light.
No less important is the coupling of the laser beam with the sail, because while we are planning to accelerate these sails to speeds that are, by current standards, fantastic, we can only do so by optimizing how they interact with the beam. ‘Alignment resilience’ refers to the reaction of the sail as it is hit by the beam. New ways to arrange the nanoscale holes in the material weigh reflectivity against cost and efficiency, and Norte pointed out that a sail will need to be reflective over a wide range of light, given that it will experience large Doppler shifts in its abrupt change in velocity.
Getting this right will involve considering misalignments between the laser and the sail that can be self-adjusting depending on the design of the mirror lattice, and perhaps faster to accelerate. We seem at the moment to be decades away from being able to make meter-scale photonic crystal lightsails, a daunting thought, but Norte has an exhilarating thought about what we can do today with a sail of the 450 mm size now possible. An Alpha Centauri mission reaches target in centuries, perhaps as few as 200 years. This is assuming a one-gram payload and 70 percent reflectivity.
A wafer size fabrication of 100 mm can be used to build a sail that reaches Voyager 1 distance in 162 days, by Norte’s calculation. Even using the tiny 4.5 cm wafer Delft has already made, we could make that journey in about a year. Using the same 4.5 cm demonstrator alone, we reach Mars with a 1-gram payload in 32 hours, Saturn in 22 days, Uranus in 46 days and Neptune in 74 days. Contrast that with the speed of our fastest flyby probes. Voyager 2 took 12 years, for instance, to reach Neptune.
“It’s a compelling thought,” says Norte. “We can. send microchips to Mars the way we send international mail, just shotgunning them out there in 32 hours. Or we can get them to Titan’s oceans in less than a month. This is possible in nanotechnology today.”
Experimental work at Delft involves developing a sail that can be fabricated in a plasma etcher that allows the team to remove the silicon underneath, suspend the structure, and move it (without breaking vacuum) for lift by a laser. The dynamics of the sail under the beam can be explored, as can the question of beam-riding. Out of all this, Norte said, should come new levels of optical levitation, novel structural engineering, a new generation of sensors and detectors. In other words, new material science ahead.