Billowing spaceship sails could capture laser light like wind

The universe is big, staggeringly, mind-bendingly big. There are trillions and trillions of stars in the observable universe but traveling to even the closest one with our best rockets would take tens of thousands of years.

In order to make a trip to another star really worth it, we’d need to build a craft capable of accelerating to a significant fraction of the speed of light. One of the best ways to do that, might be using solar sails to capture photons either from the Sun or from a ground-based source.

In a recent paper published in the journal Nano Letters, Matthew F. Campbell from the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania, and colleagues, outline the design of sails for just such a craft. They’ve found that in order for solar cells to effectively capture the huge amounts of energy needed to reach relativistic speeds, they’re going to need to billow just like the sails of ocean-faring ships of old.

“A lot of the previous work has assumed the sail would be flat, because a flat sail maximizes reflectivity,” Campbell told SYFY WIRE. “High reflectivity is important because each photon can accelerate the sail more. But unfortunately, flat sails are subject to bending stresses.”

In order to get up to relativistic speeds, the total mass of the probe including the sail needs to be as low as possible. That means making incredibly thin sails. The very same applied force which could push those sails to another star, deforms the material causing it to bend and tear. We can’t very well send a probe to another star if it breaks as soon as we try to launch it.

While curving the sail means a reduction in reflectivity, it also better distributes stress, allowing the sail to maintain structural integrity as it accelerates.

“If your sail is curved, it goes into a different kind of tensile mode. You see these tanks that hold oil or a pressure canister of compressed air, they’re rounded. That’s for a reason. Photons put pressure on the sail just like gas puts pressure on a cylinder. Using a rounded shape allows it to survive the effective photon pressure needed to accelerate to a fraction of light speed,” Campbell said.

The team’s proposed craft would include a small chip only about a gram in mass, capable of gathering data and sending it back to Earth. The sail itself would be comparatively large but lightweight. Campbell compared the size of the sail to something like the size of a dinner table. Somewhere between 1 and 10 square meters in area.

Despite the small size and mass, we would need vast amounts of power to get the probe up to speed quickly. The world’s largest hydroelectric dam, the Three Gorges Dam in China, produces something like 20 gigawatts of power continuously. Launching our solar sail probe would require five times that.

“We wouldn’t use a power station to do it. We’d have an array of capacitors that we’d charge using solar panels. It’s an enormous amount of power and there’s a whole team working on how to do that,” Campbell said.

That power would be used to fire an array of lasers spaced out over several kilometers. Those lasers would fire continuously for between 10 and 20 minutes, accelerating the probe across a distance 10 to 100 times the distance from the Earth to the Moon.

That’s all to launch just a single probe, but there won’t be just one. Shooting a craft across lightyears toward Proxima Centauri is the cosmic equivalent of threading a needle, and who knows what challenges the probe might meet along the way. A successful mission might include hundreds of probes fired one after another.

“I don’t think these probes will be cheap by any means, but we’ll launch a whole bunch of them. I would hope for at least a thousand, in hopes that one or maybe two will make it there,” Campbell said. “There are other benefits to doing fleets as well. It helps with communication. We could have a relay of communications where different chips relay messages to one another back to Earth.”

At present, the team is working on a proof of concept, producing sections of the sail in order to test its properties. They’re using materials like molybdenum disulfide and alumina, both of which might make for good solar sails, owing to their low absorption rates and high reflectivity. Though Campbell is hopeful that over the next couple decades, before they’re ready for launch, even better materials might be made available.

If all goes well, a swarm of tiny probes driven by laser light could make the trip to Proxima Centauri in about 20  years from the time of launch. Getting humans to another stellar system is outside of our current capabilities, but sending our machines is the next best thing.

Photo: NASA, Matthew F. Campbell