A sailboat moves forward when the wind blows against the sail, applying pressure and propelling the boat through the water. A solar sail works much the same way, but instead of the wind it’s pushed forward by the energy of light particles and instead of being on a boat it’s on a spaceship .
But how would you build an ultra-light sail capable of reaching ultra-high speeds while being propelled by a massive laser several kilometers wide? That’s the question a new series of papers from researchers at the University of Pennsylvania and the University of California, Los Angeles set out to answer. What they discovered is that the sail will need to inflate like a parachute while being highly reflective so as not to absorb too much laser light.
“If you want to send a probe to another star and you want to use lasers for that, you better make sure the sails bend,” said Igor Bargatin, associate professor in the university’s mechanical engineering department. of Pennsylvania.
The proposed sail is key to the Breakthrough Starshot initiative, which aims to send a one-gram solar sail to our nearest star system, Alpha Centauri, to take a picture of an Earth-sized planet in the habitable zone of the star. To propel the sail at high speed, the scientists propose using a laser array several kilometers wide. The sail would then move at 20% of the speed of light and reach the system more than 4 light years away in just 20 to 30 years.
Starshot is part of the Breakthrough Initiatives and was co-founded by Stephen Hawking and Mark Zuckerberg. The idea of using a giant laser for spacecraft propulsion is also being explored for use in our own solar system, to get a craft to Mars in 45 days.
“It’s a very unusual type of problem and it has such extreme constraints,” said Aaswath Raman, assistant professor of materials science and engineering at UCLA. “It kind of forces us to look at our whole bag of tricks and maybe even come up with ideas that people haven’t considered before.”
The sail proposed by Breakthrough Starshot would be around 10 square meters and weigh around one gram. “The design is extremely slim,” Raman said. “Most of it is actually air or vacuum.”
According to Raman, the word often used to describe a material like this is “nanophotonic”. The laser power is said to be in the range of 100 gigawatts – unprecedented power for a laser beam. “If you absorb even small fractions of that laser intensity,” Raman said. “You will heat up.” Avoiding this overheating guided the way the researchers thought about designing the nanostructure of the sail.
The team considered using a material for the sail called molybdenum disulfide, an inorganic compound that is being used in emerging research into ultra-thin materials. By radiating the heat of the laser light, the sail will be propelled while remaining at cool temperatures.
The researchers also had to think about what shape the sail should take when propelled by the laser light. “If you assume it’s just going to be flat like it’s often assumed in solar sails, then it’s just going to tear,” Bargatin said.
To survive the pressure from the laser, the researchers found that the nanophotonic material had to swell, thereby reducing stress.
“We knew they would bend, we knew it would help but we didn’t know if it would be enough,” Bargatin said. Fortunately, from what he and his colleagues have found, it works “pretty well”.
So, will an ultra-thin and light sail be able to reach 20% of the speed of light and travel to another star system?
“Based on the design, it seems theoretically possible,” said Linxiao Zhu, an assistant professor of mechanical engineering at Penn State who was not involved in the study. However, since all research has been done through simulations, the concept has yet to be proven through experiments.
“I think it’s a very interesting and emerging area,” Zhu said.
“It’s a wonderful idea to explore and research,” Bargatin said. “But, a lot of work still needs to be done if we want to make it a reality.”