This article was originally published on Supercluster, a website dedicated to telling humanity's greatest outer space stories.
Carl Sagan dreamt of navigating the solar system on sails pushed by gusts of sunlight, streaming out from our host star.
Now, the Planetary Society, led by CEO Bill Nye “the Science Guy,” aims to help make this dream a reality with a satellite no larger than a shoebox. These “solar sails,” which have been tested only a handful of times, may one day carry spacecraft to other planets, or perhaps even other star systems.
The next phase of solar sailing launched on June 24, when a SpaceX Falcon Heavy rocket lifted off the launch pad at Florida’s Kennedy Space Center. Tucked inside its massive nose cone, among two dozen other satellites, sat a tiny spacecraft: a bread-loaf-sized device designed to propel itself by gossamer sails that harness the pressure of sunlight.
The craft, dubbed LightSail 2, could be the beginning of a new era of spaceflight—one in which spacecraft forgo the rocket engines they’ve relied on for decades and elegantly sail on the winds of sunshine. Approximately seven days after launch, LightSail 2 will pop out of its container, extend four 13-foot booms and deploy four mirror-like sheets of Mylar that collectively form a 340-square-foot, kite-shaped sail. The next part is pure cosmic magic.
According to the Planetary Society, LightSail 2 will be the first craft to navigate strictly by light while in Earth’s orbit. LightSail will then tack like a sailboat in order to raise its altitude, which will earn it the distinction of being the first spacecraft to sail to a higher orbit around our planet. This feat of celestial navigation could be one more step toward Carl Sagan’s dream of journeying across the cosmos on reflective sails propelled by beams of light.
If you hold your hands out to the sun, what do you feel? Heat. But hidden among the warmth is a pressure so minute you would never notice it. Light itself exerts a force, like a windy breeze, but it's so subtle you would never notice. On Earth it's roughly equivalent to a paper clip resting in your hands.
But in the near-vacuum of space, even a minute pressure as that can have a major effect as it pushes all the time, hour after hour, day after day. And unlike rocket fuel, there’s a free and practically unlimited supply of sunlight. If we can harness this power, we can use it; all we need are sails. Like seafarers of old, future spacecraft (and space travelers) could propel themselves into the vast cosmic expanse with the help of the sun—or at least that’s how Sagan envisioned it.
Sagan had a grand vision for the future of space exploration, imagining that astro-sailors would take part in intergalactic regattas, racing their spaceships through the universe by harnessing the power of starlight. But he wasn’t the first to envision humanity sailing by light.
Four centuries ago, a comet streaked across the sky, capturing the attention of German astronomer Johannes Kepler. He noticed that the comet’s tail seemed to spread out behind it. Sunlight, he thought, must heat the comet and free material from its surface. This astute observation spurred Kepler to think sunlight might be a useful form of propulsion.
“Provide ships or sails adapted to the heavenly breezes, and there will be some who will brave even that void,” he wrote to Galileo in 1608.
Kepler lived in the midst of a scientific renaissance, during a time in which humanity began to realize its place in the cosmos. Earth was not the center of the solar system, but in fact one of many planets orbiting one of many stars. Since ships—a common form of travel at the time—are driven by winds, it seems only natural that Kepler would envision people sailing through space the same way mariners navigate the seas.
But it would take centuries for Kepler’s dreams to become a reality.
In 1865, James Clerk Maxwell showed the world that light contained packets of energy called photons, and photons had momentum that could be transferred to other objects. If a photon came in contact with an object like a shiny solar sail, it would transfer some of its energy and push the sail forward.
With advances in rocketry following World War II, and with the launch of Sputnik in 1957, the idea of exploring the cosmos suddenly shifted from science fiction to reality, and solar sailing once again seemed like an enticing form of propulsion.
Soon after its inception, NASA began planning a bevy of ambitious space missions, including funding several solar sail studies. In the 1970s, a plan was hatched to rendezvous with a comet—the same comet that Kepler spied in the sky centuries earlier. This spacecraft would be equipped with a solar sail, using the sun’s energy to make its journey. Unfortunately, the plan was scrapped before it could reach the launch pad, but the idea of solar sailing would live on.
Lou Freedman, a NASA engineer who worked on that now-defunct mission, was still convinced that solar sailing was a viable form of propulsion. He (along with Carl Sagan and Bruce Murray, former head of NASA’s Jet Propulsion Laboratory) cofounded the Planetary Society in 1980 in large part to turn solar sailing into a reality.
“Solar sailing in general has been woven into the DNA of the Planetary Society since before we were even founded,” Jennifer Vaughn, the society’s chief operating officer, tells Supercluster. “All three of our founders had a hand in the early stages of solar sailing.”
Figuring out how to build an enormous, ultra-lightweight solar sail and then affordably launch it into space would prove to be a major challenge. But the group persevered, and the society’s first foray into solar sailing finally hit the launch pad in 2005 in the form of a spacecraft called Cosmos 1. Unfortunately the would-be satellite didn’t make it to orbit, crashing down near Russia in the Barents Sea. Ten more years would pass before the Planetary Society achieved their first solar-sailing success.
Mission managers went back to the drawing board and decided to think smaller for their next attempt. Cubesats—which are about the size of a loaf of bread—had burst onto the spaceflight scene, providing a cheaper alternative to traditional, bulky satellites.
According to Bill Nye, the advent of CubeSats actually led to the current square sail design. “A square sail turns out to be a pretty good starting point and a pretty good design,” Nye told Supercluster.
The resulting craft, dubbed LightSail 1, was a scaled-down version of Cosmos 1. This shoebox-sized mini satellite launched in 2015, hitching a ride on an Atlas V rocket and flying to orbit alongside the Air Force’s clandestine X-37B space plane.
Once in orbit, LightSail 1 proved it could successfully deploy its sail in the vacuum of space. But the tiny satellite faced a myriad of problems including software glitches, signal losses and battery issues over the course of its mission. After a tumultuous time in low-Earth orbit, the tiny craft deployed its namesake: four shiny mylar sails, proving you can fit a full light sail in a small package.
Despite its setbacks, as a demonstration of technology, LightSail 1 was a success. To celebrate, it beamed back the ultimate selfie. But it also got people looking up, trying to spot the craft as it soared above the Earth. Nye was even able to spot the tiny satellite glinting in the night as it passed over New York City’s light-polluted sky.
Getting to this point was a huge accomplishment for the Planetary Society, a non-profit formed to help advance space science and exploration. “Space brings out the best in us,” Bill Nye says. “It appeals to all people and all nationalities.”
More than 40,000 members of the Planetary Society have supported this mission (and the development of solar sailing), donating from $5 to more than $1 million to fund the organization.
“It's incredibly rare that one gets an opportunity to participate in the first stages of a new spacecraft propulsion technology,” says Bruce Betts, LightSail mission manager. “There aren’t that many. Being at the initial phases, the first steps of solar sailing, is quite an honor.”
Over the years, while the Planetary Society has been building their sailing ships, others around the globe have set sail as well.
A 2015 test mission called CubeSail, built by the Surrey Space Centre in the U.K., was launched but failed to deploy properly. Three other small sail missions—NASA’s NanoSail-D, Canada’s CanX-7 and Surrey’s InflateSail—did have some success, but just like LightSail 1, their missions were confined to low-Earth orbit.
Japan has had the most success with solar sailing so far, as it’s Interplanetary Kite-craft Accelerated by Radiation Of the Sun (IKAROS) launched into orbit around the sun in 2010. Once in deep space, it spun open a 46-foot-wide square sail and, for the first time in history, began steering and changing its speed with the power of sunshine.
The spacecraft also had solar cells embedded in its kite-shaped sail to generate electricity. The cells were not expected to produce much power during flight but instead helped serve as a test bed for future ion propulsion engines.
IKAROS spent the next three years measuring its acceleration due to light pressure. It also tested various ways to control its motion using liquid crystals (resembling an LCD electronic display) embedded in the sail that could adjust the craft’s reflectivity and change its speed. Electricity flowing through the panels increased reflectivity, enabling the spacecraft to accelerate forward, while turning the flow off made the sunlight pressure more diffuse.
Japanese space officials say that IKAROS was able to adjust both its course and its orientation before the mission’s planned end in 2015. Contact with IKAROS has been lost, but subsequent tracking shows that the solar sail continues to loop around the sun between Earth and Venus, and the shape of its orbit continues to change as a result of changes in light pressure.
But along with its successes, IKAROS data shows that we still have a long way to go before we realize the full potential of solar sailing. The Planetary Society hopes that if all goes according to plan, LightSail 2 will help turn solar sails into a reliable, low-cost, zero-propellant method of flying through space.
From there, the possibilities are nearly limitless. With a light sail, a spacecraft could go to the moon, to asteroids, to Jupiter—anywhere the wind of light blows, using no fuel at all.