Improved lightsails to take spaceships to other stars

Sailing in space: scientists have been testing sail propulsion systems for spacecraft for around ten years. An American-Dutch team has developed a new technology to produce better so-called lightsails.

A lightsail is a wafer-thin membrane that is stretched in front of the spacecraft and provides propulsion for the spacecraft. The principle is similar to that of sailing on water: on earth, the wind that hits the sail propels a boat. In space, it is sunlight: photons hit the sail and are reflected by it. They transfer their momentum to the sail and thus provide propulsion.

The team from Brown University in Providence in the US state of Rhode Island and Delft University of Technology (TU) in the Netherlands has developed processes to produce extremely thin and highly reflective membranes. The prototype has an edge length of 6 centimeters and is just 200 nanometers thick. The thickness of the first lightsails was in the micrometer range.

Lower weight, better reflectivity

The membrane consists of a single layer of silicon nitride. The surface is provided with a complex pattern of holes. Their diameter is on a nanoscale and is smaller than the wavelength of the incident light. When planning the pattern, artificial intelligence (AI) was used to optimize the shape and placement of the holes with a view to increasing reflectivity and reducing weight.

The holes have a dual function: firstly, they reduce the weight of the material. On the other hand, they improve its reflectivity. The reflectivity determines how much light pressure is generated behind the sail – and therefore the acceleration. Low weight, in turn, requires less force to accelerate. Lower weight therefore means higher speed.

The team led by Miguel Bessa from Brown University was responsible for the design of the sail. Richard Norte and his colleagues from TU Delft developed the manufacturing process. "We developed a new, gas-based etching process that allows us to carefully remove the material under the sails so that only the sail remains," said Norte. "If the sails break, it's most likely during the manufacturing process. Once the sails are hung, they are actually quite robust."

Faster and cheaper to produce

Using the process developed at TU Delft, it takes around a day to produce such a sail and is comparatively inexpensive. According to the researchers, this would have taken 15 years using conventional production methods and would have been significantly more expensive.

Light sails are used to propel spaceships that are sent on interstellar missions. The "Breakthrough Starshot" initiative founded by Russian-Israeli billionaire Yuri Milner plans to use ground-based lasers to accelerate mini spaceships powered by lightsails to 20 percent of the speed of light and then send them to other star systems. With such a drive, a spaceship could reach the Alpha Centauri star system, four light years away, in around 20 years. For comparison: it took the two Voyager probes 35 and 36 years to reach the end of the solar system.

(wpl)