An artist’s conception of the Starshot Lightsail spacecraft during acceleration by a ground-based laser array.
By Monisha Ravisetti
For years, physicists have been trying to perfect a way to catapult space probes at a fifth the speed of light. One team is flagging an important section of the blueprint.
Only about 4 light-years away from our solar system lies Alpha Centauri, another bustling space neighborhood. It’s anchored by three stars with the same job as our sun, holds planets analogous to our eight famous orbs and may even have an Earth twin hanging out in the habitable zone.Almost like an alternate reality, the star system is a tantalizing region for space explorers.
There’s just one, glaring issue. With our present technology, spacecraft sent toward Alpha Centauri wouldn’t arrive until somewhere around the year 82022. That’s why, in 2016, late astrophysicist Stephen Hawking and investor Yuri Milner launched Breakthrough Starshot — an initiative to send microchip-size space probes over to Alpha Centauri at 20% the speed of light, reducing the whopping travel time to a mere 20 years.
Their blueprint centers on a lightsail that harnesses the power of photons, aka light particles, beamed from an Earth-based laser, instead of wind like a traditional sail. Though it’d fit right in with the sci-fi tech of Star Trek, the idea gained so much popularity that researchers everywhere began studying how to bring the contraption to fruition, hoping to produce a hyperdrive that blasts around the universe at dizzying rates.
Hailing from the University of Pennsylvania, one such team is tackling a big piece of the puzzle. In a pair of papers published this month in the journal Nano Letters, researchers suggested a way to ensure these innovative spacecraft don’t tear from intense laser pulses during the two-decade-long interstellar voyage. Basically, the researchers propose the sail must “billow” in space’s void like standard boat sails wave amid Earth’s winds.
“Some of the lightsail figures from early on were billowing, some were not, but it was not well studied,” said study author Igor Bargatin, an associate professor in the department of mechanical engineering and applied mechanics at the University of Pennsylvania. “What we did is show you definitely need billowing.
“We realized people haven’t really looked at the mechanics of the problem, and in particular, the possibility of tears,” Bargatin added. “We want to make sure if and when this idea is realized, people pay attention to things that could happen during acceleration.
“We don’t want these sails to fail.”
Interstellar ship parameters
Picture a boat venturing out to sea with a sail attached. The sail will heave with every gust of wind and propel the vessel forward. That propulsion happens because wind hitting the sail bounces off, creating pressure.
Lightsails aren’t all that different.
“When the photons hit our lightsail, they get reflected and they also create pressure,” Bargatin said. “The exact mechanism is a little different because we’re talking light versus actual molecules of air. But they create pressure nonetheless in both cases.” In fact, these devices have already been proven effective to a degree.
In 2010, the Japanese Aerospace Exploration Agency launched a lightsail mission dubbed Ikaros and deemed it a success. In 2019, the experimental LightSail 2 followed suit. Funded by a Kickstarter campaign started by Bill Nye and Neil DeGrasse Tyson, it moved a small satellite in space using pure photon power.
But both Ikaros and LightSail 2 used light emanating from the sun, in contrast to Breakthrough Starshot’s vision of laser beams.
Though sunlight reduces the risk of tears, it’s too weak for Starshot’s endeavor. Plus, Bargatin says, Starshot light pulses must happen within a relatively short period of time because once the lightsail gets too far from Earth, scientists lose their ability to effectively accelerate it.
In short, to reach a fifth the speed of light — so it can access Alpha Centauri in the desired 20 years — within a strict window, lightsails would need extremely strong light pulses possible only with lasers.
“Designed pressures on our lightsails are not huge,” Bargatin said. “They’re about the same as having a penny on your hand.” In scientific terms, the pressure adds up to about 10 pascals, Bargatin says, but consider how we go about our lives without worrying about light pressure at all.
Ten Pascals of light force requires a significant amount of laser power, so unlike Ikaros’ dance with wispy sunrays, lightsails imparted with ultra harsh laser pulses can be badly damaged.
How to build a durable lightsail
According to the researchers, strong laser pulses could create pressure forceful enough to curve and tear the sheet like a taut boat sail may rip if hit by a giant gust of wind.
They believe lightsails must have the ability to “billow” and form a curved shape kind of like a parachute. Both the sail’s length and the radius of curvature, Bargatin explains, should be about 3 meters. In their new papers, the authors outline geometric measurements that ensure optimal billowing.
Even a lightsail protected from tears, however, will encounter other obstacles. To overcome such issues, the major parameter to consider is sail material. The sheets must be strong for durability, lightweight to minimize laser strength, reflect light efficiently for ideal propulsion and shed heat generated from laser pulses.
If the latter bit isn’t taken care of, Bargatin says, the sail could literally meltin space.
“You can come up with a combination of materials. The thicknesses of those materials and curved geometries would allow the sail to survive the pressures that we’re currently designing for,” Bargatin said, noting his team is mostly looking at a material called molybdenum disulfide.
In the grand scheme of things, though, building the massive laser array that’ll beam lightsails forward will be a big hurdle. Researchers working in space-based communication, Bargatin says, are also still figuring out how to retrieve information from the microchip probe attached to the lightsail.
If Breakthrough Starshot’s mechanism works one day, it’ll be a true testament to humanity’s brilliance in the field of science. In an announcement of the organization’s immense goals six years ago, Hawking stated:
“I believe what makes us unique is transcending our limits. Gravity pins us to the ground, but I just flew to America. I lost my voice, but I can still speak, thanks to my voice synthesizer. How do we transcend these limits?
“With our minds and our machines.”