On May 20, if all goes well, the first private spacecraft built to sail on sunlight will soar into the heavens from Cape Canaveral, Fla. The spacecraft is called LightSail and is a project of the Planetary Society, a nonprofit organization that promotes space exploration.

Although they have no mass, the photons in a sunbeam do carry momentum. In sufficient numbers they can push objects around in the vacuum of space. Bounce enough photons off a large reflective spacecraft and light alone can continuously accelerate it without the need for any onboard fuel, much like a sailboat catching a ride on the wind. Such spacecraft are called “solar sails.” This elegant idea goes back more than 400 years, to the German astronomer Johannes Kepler, who noted that a wind seemed to blow comet tails away from the sun, and that it might someday be harnessed to push a celestial vessel through the “heavenly air.”

The sun’s photonic winds may be weak, but they are also continuous, and over time can propel a light, fuel-free spacecraft to speeds far higher than those attainable via traditional chemical rockets. Because fuel is heavy, and mass is the key cost driver of space travel, solar sailing offers a very cheap, very effective way to transport cargo around the solar system. Some of its proponents even say that a sail propelled by intense laser beams may ultimately prove to be one of the few viable ways to travel to other stars.

LightSail is the size of a loaf of bread. That’s small enough for it to piggyback on the Atlas 5 rocket scheduled to launch an X-37B military space plane into low Earth orbit tomorrow. After a monthlong checkout phase, the Planetary Society’s spacecraft will unfurl its silvery Mylar fabric solar sail, testing the sail’s deployment in low Earth orbit. Although at 32 meters wide it is about the size of a boxing ring, the sail is only 4.5 microns thick, the same as a single strand of spider silk. Very large yet very light, the sail will excel at soaking up momentum from sunlight—enough to potentially take it and an attached payload on interplanetary journeys.

This time, however, the spacecraft’s orbit will not be high enough to entirely escape Earth’s atmospheric drag; once its sail deploys that drag will drastically increase, pulling the spacecraft down to a fiery reentry within only a few days. Next year the Planetary Society plans to launch a second LightSail into a higher orbit onboard a SpaceX Falcon Heavy rocket for more extensive tests of solar-sailing propulsion. That flight, if successful, would be the first controlled demonstration of solar sailing in Earth orbit.

“LightSail is technically wonderful, but it's also wonderfully romantic,” Planetary Society CEO Bill Nye said in a statement. “We’ll sail on sunbeams. But wait, there’s more: This unique, remarkable spacecraft is funded entirely by private citizens, people who think spaceflight is cool.” Nye prefers to call LightSail the “people’s satellite.”

According to the Planetary Society, the cost for the entire LightSail project is about $5.5 million, about a million of which the society still needs to raise from donors. That’s a bargain-basement price, considering traditional space missions routinely run into hundreds of millions or billions of dollars, and bodes well for further small-yet-ambitious space missions from nongovernmental organizations and even private citizens.

A star-crossed history

LightSail is not the first attempt at solar sailing nor the first time the Planetary Society has spent millions on spacecraft to demonstrate the technique.

In the mid-1970s two of the society’s co-founders, Bruce Murray and Louis Friedman, worked as part of a team at the NASA Jet Propulsion Laboratory to design a solar sail for a robotic mission to Halley’s Comet. That mission required a truly enormous sail to be launched and deployed from a space shuttle. With some designs calling for a sail nearly a kilometer in diameter, NASA officials canceled the project, balking at its estimated costs and unproved technology. The agency would not robustly revisit solar sailing for years to come, leaving others to pursue the dream of sunlight-fueled space travel.

Friedman, Murray and their collaborator Carl Sagan went on to form the Planetary Society, and raised $4 million to support another solar-sailing mission, a 100-kilogram craft called Cosmos 1. To save money, the project planned for two launches using cheaper Russian rockets, specifically repurposed ballistic missiles launched from submarines. Both launches—a suborbital test flight in 2001 and an orbital attempt in 2005—failed due to rocket malfunctions.

Rather than build yet another large, expensive spacecraft, the society began planning in 2009 what would become the LightSail project, relying on CubeSats—cheap, miniaturized modular spacecraft 10 centimeters on a side. Instead of weighing tens to hundreds of kilograms—mass that requires much larger sail areas and more complex logistics—each LightSail is less than five kilograms. The spacecraft is made up of three strung-together CubeSats, a design that drastically simplifies its construction and reduces its cost. “In essence, Moore’s law has met solar-sailing technology,” says Rex Ridenoure, CEO of Ecliptic Enterprises, the company in charge of LightSail’s integration and testing. “It’s an excellent match for certain mission profiles.”

Thinking big, building small

With the rise of CubeSats NASA took another look at solar sails, and in the 2000s began a CubeSat solar sail project of its own. NASA’s 3.5-meter-wide NanoSail-D was meant to test how solar sails could be used to deorbit defunct satellites. But NanoSail-D was destroyed when its Falcon 1 rocket failed to reach orbit 2008. NASA launched a replacement in 2010 and in 2011 successfully tested the technology in space, although the spacecraft tended to spin and tumble uncontrolled in orbit. Meanwhile, the Japanese space agency had successfully tested a solar sail in 2004, and in 2010 launched IKAROS, a full-fledged interplanetary mission that used a 315-kilogram, 20-meter-diagonal solar sail to fly past Venus later that year. Japan’s IKAROS mission “was not a stunt,” Ridenoure says, but rather “a very explicit technology maturation mission. They are clearly ahead of everyone else in this area.”

NASA is still playing catch-up. The next step in the agency’s solar-sailing plans was to be a mission called Sunjammer, a spacecraft meant to fly out of Earth orbit in 2015 on an unfurled sail nearly 40 meters wide. But NASA canceled Sunjammer last year, citing shortfalls from the contractor building the spacecraft. Instead, the agency is continuing to plan for more modest CubeSat-based solar-sailing missions, developing two slated to launch to the moon in 2018 on the inaugural flight of NASA’s Space Launch System rocket. The agency’s Lunar Flashlight mission will use a solar sail not only to maneuver but also to shine sunlight into dark craters near the moon’s poles, seeking water ice. Its companion, the Near-Earth Asteroid Scout, will use its sail to soar away from the moon to visit a passing space rock. Each is composed of six CubeSats, boasts a nine-meter-wide sail and costs an estimated $15 million—a vanishingly small fraction of the price for a typical NASA interplanetary mission. Constellations of CubeSats augmented with solar sails could be useful for many other applications, such as monitoring space weather or surveying other planets from orbit.

According to Doug Stetson, the Planetary Society’s LightSail project manager, LightSail is proving crucial to NASA’s rekindled interest in solar sails, and his team is sharing its lessons and insights with engineers working on the agency’s new projects. “By coupling a solar sail with a small spacecraft such as a CubeSat, a demonstration mission can be carried out at a cost that is reasonable for a private organization—and it is one way that The Planetary Society can help advance space science,” Stetson says. “The goal is to demonstrate the viability of this approach and encourage other groups and agencies to use it for more advanced exploration missions.”

Even so, don’t expect to see solar sails replace rockets anytime soon, Ridenoure says. For launches, rapid orbital changes and planetary landings, the powerful, immediate kick of rockets can’t be beat. Most of NASA’s spaceflight efforts—launches to the International Space Station, redirecting asteroids to lunar orbit, sending robots and eventually humans to other planets—revolve around these very things. “Solar sailing just isn’t a good match for these mission types,” Ridenoure says. “But for more leisurely missions, solar sailing may have the advantage.”