Universe Today
Any material used as a light sail is bound by very restrictive physical requirements. It has to be extremely light , can’t melt from the energy applied to it, and must bend, but not break, from that pressure. Various research groups around the world have been working on materials they believe will meet those requirements, and a new paper from researchers at the University of Pennsylvania describes experimental testing of what they believe to be the most functional light sail material yet developed.
Modern day designs of light sails utilize lasers to push the sail along, rather than simply relying on photons from the nearest star to do so. This modern design is part of the design of the Breakthrough Starshot Initiative's plan to get a probe up to 20% of the speed of light and make its way to Alpha Centauri. But, to have a sail that is capable of doing so would require the three properties mentioned above.
The researcher’s solution is a three layer material. Its “core” is made of molybdenum disulfide, which is desirable for its high reflectivity. It’s been difficult to fabricate large, smooth sheets of this material in the past, but the researchers developed a two-stage process to do so. First, they sputter molybdenum directly onto a substrate. Then they “sulferize” the molybdenum by putting it into a chemical vapor deposition chamber at high temperature and introduce hydrogen sulfide gas, which at the elevated temperature of 750C reacts with the molybdenum to create molybdenum disulfide.
On either side of the molybdenum core is a layer of alumina, prized for its thermal emissivity. It is designed to allow the laser light to pass directly through to the molybdenum, which then reflects it back, but the alumina is also designed to be highly emissive at infrared wavelength. In other words, it would allow the heat transferred by the laser to be radiated away - allowing the sail to cool itself.
Another design features is the shape of the sail itself. It is designed to be corrugated with a hexagonal structure. This allows the structure itself to bend at intended points while not causing too much stress on any particular point in the material itself, which might otherwise cause it to break.
According to the experimental results the researchers provided, the new material performed much better than any previous material put forward as a potential light sail, at least in terms of two important metrics. It reflected 50% of the laser light directed at it, while absorbing only around 4% of the energy transmitted to it by the laser. That absorption number is much lower than previous experimental results.
However, while the corrugated design does enable more flexibility in the structure, it also adds some weight. The overall “areal density” of the material is around .7g / m2, compared to .5 g / m2 for a non-corrugated design, and a goal from Breakthrough Starshot of .1 g / m2. While it’s still in the same order of magnitude, there’s a lot more material science work that must be done before a material can get down to that level of density.
Overall this paper represents another step towards the development of a fully functional light sail that might one day push a probe to our nearest star system. Granted a lot of other development has to happen in order for that mission to be a success - sail material is only a small part of it. But as more and more researchers get involved and start to tackle the technical challenges associated with this extraordinary goal, the closer humanity will move to becoming an interstellar species.
Learn More:
M. F. Campbell et al. - Experimental demonstration of corrugated nanolaminate films as reflective light sails
UT - Researchers are Continuing to Scale Up Lightsails That Could Explore the Cosmos
UT - What Should Light Sails Be Made Out Of?
UT - Photonic Lightsails are our Best Shot at Reaching Another Star
