Carl Sagan dreamt of navigating the cosmos on sails pushed by gusts of sunlight, streaming out from our host star.
The Planetary Society, led by CEO Bill Nye “the Science Guy,” aims to make this dream a reality, with the help of a small 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.
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 can have a major effect because it’s pushing all the time, hour after hour, day after day. And unlike rocket fuel, there’s a free and 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: astro—sailors would take part in intergalactic regattas, racing 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 that 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 could 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 could transfer 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, even 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 skies centuries earlier. This spacecraft would be equipped with a solar sail, using the sun’s energy to make its journey. Unfortunately, that 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) co-founded the Planetary Society in 1980 in large part to turn that idea 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-lightweightsolar 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 shoebox—had burst on the 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 recently told Supercluster.
The resulting craft, dubbed LightSail 1, was a scaled-down version of Cosmos 1. This mini satellite launched in 2015, hitching a ride on an Atlas V rocket, riding to orbit alongside the Air Force’s super secret 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 was formed in order 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 people have supported this mission (and the development of solar sailing), donating everything from $5 to more than $1 million to fund the mission.
“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.”
Nye explained that the society’s members were incredibly excited to take part in the mission, with each of them having the opportunity to send their name and photo to space on a special disk affixed to the spacecraft.
“It reminds me very much of a message in a bottle,” says Nye. “It would be great if someone found it and mailed it back to you, but the real reason you put a message in a bottle is for you.” So people around the world have sent in pieces of themselves that will sail the final frontier thanks to LightSail.
The next phase of solar sailing launches on June 24, when a SpaceX Falcon Heavy will leap off the launch pad at Florida’s Kennedy Space Center. Tucked inside its massive nose cone, among two dozen other satellites, sits a tiny spacecraft: a bread-loaf-sized device designed to propel itself by gossamer sails that harness the pressure of sunlight.
This craft, dubbed LightSail 2, could be the beginning of a new era for spaceflight—one in which spacecraft forgo the rocket engines they’ve relied on for decades and elegantly sail with winds of sunshine.
Approximately seven days after launch, LightSail 2 will pop out of its container, extend four long 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 orbit, 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's achieving Carl Sagan's dream of journeying across the vast cosmic sea, propelled by beams of light.
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 so far in terms of solar sailing, as its 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 thanks to 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 its sail, that could adjust the craft’s reflectivity, and its speed.
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 ended, 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 from the sun.
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 solar sail, a spacecraft could keep going to the moon, to asteroids, to Jupiter — anywhere the wind of light blows, using no fuel at all.
Click here for an animation on Solar Sail technology from the Planetary Society and animator Josh Spradling, used as a reference for the animated header of this article.