Two-and-a-half years ago, a tiny spacecraft soared into the sky aboard a Space X rocket blasting off from Kennedy Space Center in Florida.
Settling into an orbit 426 miles above Earth, the shoebox-sized object—called LightSail 2, designed by the California-based Planetary Society, and crowdfunded by 40,000 private donors chipping in $7 million—unfurled a 20-by-20-foot foil sail as thin as a strand of human hair.
Like many science satellites, LightSail 2 carries a suite of cameras and radio instruments, powered by solar panels. But the point of this mission is the sail, which is specially designed to be sensitive to light. When photons from the sun hit the sail, they nudge LightSail 2 forward with enough force to counteract Earth’s gravity and keep the craft in a stable orbit.
“LightSail 2 is the first small spacecraft to demonstrate controlled solar sailing,” the Planetary Society states on its website. This experimental method of space propulsion is about to go mainstream in a big way, and could very well change the future of space travel.
Three NASA missions—one launching next month, another later this year, and a third around 2025—will use larger-scale solar sails (also called light sails) to efficiently propel spacecraft across the solar system thanks to sunlight. And some scientists hope even bigger solar sails will someday speed probes beyond the solar system.
For missions headed to nearby destinations a few million miles away, like near-Earth asteroids, solar sails have a key advantage over the chemical propellants that most spacecraft use, Julie Castillo-Rogez, principal investigator for NASA’s Near Earth Asteroid Scout mission, told The Daily Beast.
“One is that [a solar sail] can reach very large velocities from constant pushing by solar photons over long periods of time,” Castillo-Rogez said. “At the same time, the sail can be controlled so that its velocity can match the velocity of the target and thus enable extended observation time.”
Solar sails could be even more critical to enabling missions to more distant destinations, like other star systems. “If you don’t want your trip to take tens of thousands of years, you need to go at an appreciable fraction of the speed of light,” Eliot Gillum, a scientist at the SETI Institute in California, told The Daily Beast.
But that’s impossible to do when you’re hauling a bunch of heavy liquid fuel, which is necessary for chemically propelled spacecraft but is also dead weight until you burn it. The trick “is to not bring your fuel with you,” Gillum said. “How do you do that? Light sails.”
As a concept, solar sails have been around for a long time—centuries, actually. Way back in 1608, astronomer Johannes Kepler theorized that a spacecraft could travel on the “heavenly breezes,” although he didn’t know those breezes would take the form of photons. In 1873, mathematician James Clerk Maxwell predicted that light has momentum despite having no mass.
That momentum is key. When light shines on something, it pushes it. Spread a lightweight object over a large area and that force could actually propel the object. Science-fiction writer Arthur C. Clarke featured a solar-sail craft in the 1964 short story “Sunjammer.” Astronomer Carl Sagan made the concept even more famous with a mention on The Tonight Show in 1976.
Thirty-four years later, Japan’s space agency launched the first-ever solar-sail spacecraft: a Venus probe called Ikaros fitted with a crude, non-steerable 46-by-46-foot sail. Ikaros flew by Venus in late 2010 and disappeared out of radio range five years later.
Solar-sail spacecraft have only gotten better since Ikaros. LightSail 2’s designers hoped their mission would last a year in orbit. Nearly 940 days later, it’s still going strong, riding recent waves of elevated solar wind and snapping photos of Earth, all the while further bolstering the proof-of-concept for the solar sail.
Modern materials make it possible. Today’s solar sails are made of Mylar or an insulating material called CP-1. They’re both tough and flexible, even with thicknesses that are just a few microns (thousandths of an inch). An aluminum coating makes them more reflective, increasing the force of the photons that strike them.
As a result, LightSail 2 should still be hovering in space hundreds of miles over our heads when the next solar-sailing mission launches as early as next month.
That mission, Castillo-Rogez’s NEA Scout, aims to catch up with and survey a 50-foot-diameter asteroid named 2020 GE, which is on track to pass within a few million miles of Earth in 3023. The mission is meant to pave the way for more ambitious space-rock surveys in coming years and decades. NEA Scout has a 30-by-30-foot solar sail—twice as big as LightSail 2’s own sail.
Advanced Composite Solar Sail System, NASA’s next solar-sail mission after NEA Scout, has a sail that’s slightly smaller but part of a much lighter frame—one that should make it even more efficient.
The lessons learned from all three of these missions could come together in 2025 with the grandest solar sail demonstration to come: NASA's Solar Cruiser, which could boast the biggest sail yet at 17,689 square feet.
But if the potential of solar sails to transform space travel is enormous, then so are the odds for things to go wrong, especially given how delicate these sails are.
“Parts of the sail membrane might stick together when unfurled, causing a small tear or rip,” Les Johnson, another NEA Scout investigator, told The Daily Beast. “One of the metallic booms might buckle … affecting the sail shape and hence its performance and controllability.” Also, “the momentum induced in the sail from being constantly exposed to sunlight pressure might be more difficult to control than expected.”
If everything goes according to plan, in the span of just six years we’ll have gone from bedsheet-size solar sails to sails the area of several tennis courts. The trend at that point should be clear: Our solar sails are getting bigger while also getting lighter per square foot.
But bigger isn’t always better, especially for extremely long-range missions. To explore beyond the solar system, it might be more useful to build a whole bunch of super-efficient solar-sail craft—and juice them with a laser. The craft and their sails would be small. The laser would be huge.
That’s the plan behind the Starshot project, overseen by University of California, Santa Barbara physicist Philip Lubin and funded to the tune of $100 million by Breakthrough Initiatives (an organization founded by Mark Zuckerberg, Russian-Israeli entrepreneur Yuri Milner, and the late Stephen Hawking to search for alien civilizations). If it ever gets off the ground, Starshot will attempt to launch a fleet of solar-sail probes to the nearest neighboring star system using an array of gigawatt-powered lasers fired from the surface of the Earth.
Calling the proposal ambitious is an understatement. The tiny Starshot probes are supposed to weigh merely a gram each and deploy their own 12-foot-by-12-foot sails. Lasers would aim at the sails, bludgeoning them with photons upon photons and steadily building up momentum until the probes are traveling up to 20 percent the speed of light. That’s fast enough to reach Proxima Centauri b, an Earth-sized planet that scientists believe could be hospitable to life, in around 30 years.
Neither the solar-sail technology nor the laser tech is quite ready for such a mission. When it comes to the sail, “you’ve got to make it lighter,” Lubin told The Daily Beast. And the lasers, while feasible, could cost billions of dollars.
There might be easier, cheaper ways of looking for alien life, but there might not be easier, cheaper ways of sending spacecraft really, really far away really, really fast. But that could change over time as solar sails becoming a faster, more cost-effective way of zipping through space.
If efficiency and control are your goals, ditch the fuel and strap on a sail, just like Kepler imagined 414 years ago.