How to Sail the Galaxy

Using light sails, a space travel technology in which sails are pushed by light, researchers predict that humans could reach the nearest star system in just 20 years.

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By Sabrina Chen

As we speak, the universe is moving further and further away from us. The speed of the universe’s expansion is accelerating. Every second, 60,000 stars disappear from human perception, their light turning from blue to red as they float away. Even if we could travel at the speed of light, 94 percent of all the galaxies that we can perceive are already unreachable to us forever.

This most certainly feels discouraging toward the future of space-travel. Scientists have been racing to create faster and faster space travel technology since 1957 after the launch of Sputnik, the first satellite. However, recent discoveries regarding a certain form of space travel have been getting some buzz in the science world: the light sail, which refers to any sort of sail pushed by light in space. This would work similar to a sailboat pushed by the wind, except that the wind is light from nearby stars, and the sails have the potential to be a thousand times thinner than a sheet of paper.

Though photons, or particles of light, have no mass, they have the power to push such a sail because they carry kinetic energy. As photons bounce off the sail, they transfer their energy to it by providing a small push. While the push of a single photon is very small, the combined strength of thousands of photons along with the lack of friction in space allows the sail to accelerate forever. Light sails are also designed to reflect light, like a mirror, instead of absorbing it. This is beneficial because light reflected off of an object transfers twice as much momentum as light that’s absorbed.

The main advantage of light sails is that they don't need to carry fuel. Fuel is a major burden in space travel since extreme amounts of it are required to power any spacecraft, weighing it down and thus reducing its speed. Some rocket fuels and engines have detrimental environmental impacts, with various fuel types capable of poisoning the soil, creating holes in the ozone layer, and releasing soot into the atmosphere. Light sails, on the other hand, are not only unaffected by heavy fuel tanks, but are also 100 percent sustainable.

Light sails have been made before. A Japanese spacecraft called IKAROS, which stands for Interplanetary Kite-craft Accelerated by Radiation Of the Sun, used a solar sail to fly successfully for the first time in 2010. Another light sail, appropriately named Lightsail 2, was successfully developed by the United States in 2019. This crowdfunded spacecraft carried a CubeSat—a small satellite around the size of a loaf of bread—into the Earth’s orbit.

The next big venture in light sail technology is being planned by Breakthrough Starshot. The enterprise is part of the larger Breakthrough Initiatives, which are “suite[s] of space science programs investigating the fundamental questions of life in the Universe.” Starshot’s goal is to launch a microprobe carried by a light sail on a one-way trip to the nearest star system, Alpha Centauri. While most previous research on light sails has assumed that they would be passively powered by the sun, Breakthrough Starshot intends on propelling the light sail forwards with a high-powered cluster of lasers built on the Earth. When the probe arrives at Alpha Centauri, it will zip through the star system, gather as much information as it can, and beam the information back to Earth with a laser of its own.

This is an ambitious project. While travel within our solar system has already taken place, venturing outside of our solar system has long been considered nothing more than science fiction. However, if Breakthrough Starshot succeeds in propelling this probe to 20 percent of the speed of light, travel time to Alpha Centauri would be cut from 6,300 years with existing technology to just 20 years, meaning that the journey could take place within a generation.

While Breakthrough Starshot is still in its research phase and has not yet pursued development, the space program published a pair of studies in February 2022 that may reveal the keys to the most effective shape and pattern for a light sail. The papers were published in the scientific journal Nano Letters and were led by three researchers—Igor Bargatin, Deep Jariwala, and Aaswath Raman—who work for Breakthrough Starshot.

The first study, led by Bargatin, revealed that light sails should billow like a parachute instead of remaining flat, as most previous research assumed, in order to make the light sail more resistant to tears. This is crucial because the sails will be under an enormous amount of stress; the Earth-based laser proposed by Starshot is expected to produce light an intensity millions of times brighter than that of the sun. And, similar to a normal sailboat, a tight sail will naturally be more prone to tears than one that can adjust to the intensity of the wind.

The second study, led by Raman, discovered that nanoscale patterning within the sail could dissipate the intense heat from the lasers trained on the sails. As mentioned earlier, light sails are designed to reflect light instead of absorbing it. This is done not only to maximize momentum transfer, but also because light sails can heat up to extreme temperatures and disintegrate if even a tiny fraction of light is absorbed. Raman’s study suggests that embedding the sail with a grid-like pattern along with placing tiny holes in its surface could help dissipate the heat from lasers.

Though light sail technology is still novel, it’s advancing fast. The findings of these two studies will be crucial not only to the development of Breakthrough Starshot’s plan but also to any future research on lightsails. If Breakthrough Starshot does succeed in its mission, then it will gift the world the chance to observe interstellar space before this generation’s time is up. And with all of its fermi bubbles, black holes, paradoxes, and neutron stars, the beauty of the universe is not something to miss out on.