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