After three years in space, the Planetary Society’s LightSail 2 mission burned up in the atmosphere on Thursday, November 17th. During its mission, the crowdfunded spacecraft made 18,000 orbits of the planet using its giant reflective sail and demonstrated that controlled solar sailing is possible.
LightSail may now be over, but it has opened the door to the use of solar sailing in space exploration. “It doesn’t fit every situation, but now it gives another arrow in the quiver of options for types of propulsion you can use,” said Bruce Betts, Chief Scientist and LightSail program manager.
LightSail 2 operated in Earth orbit, while future solar sailing missions would likely be in deep space. That makes the requirements somewhat different. “We had the challenge that’s analogous to sailing a sailboat in the harbor versus out in the ocean,” Betts said. LightSail had an easier time with issues like communications than deep space missions would but had to constantly turn to keep in orbit.
The biggest drawback in using solar sailing for space exploration is that the forces involved are so small that craft start out traveling very slowly. “The disadvantage is that you’re using a push from the sun that is really, really tiny,” Betts explained. “The push on our entire sail if it were reflecting perfectly, from the sun at Earth’s distance, is about the same force that a housefly has when it’s sitting on your hand, pushing down.” But the advantage of the method is that this force just keeps building up over time, allowing acceleration up to high speeds without using up fuel.
Upcoming programs like NASA’s NEA Scout and Advanced Composite Solar Sail System (ACS3) will also use solar sailing. The ideal targets for these kinds of missions are within the inner solar system, as the craft stays close enough to the sun to get enough force from the sunlight to keep moving and to change an orbit.
Another type of exploration solar sailing enables is entering orbits that would otherwise be impossible using conventional propulsion systems. Missions that orbit around the sun, for example, currently have to sit in very particular regions called Lagrange points to be in a stable orbit. Trying to get closer to the sun would use up too much fuel. But with a solar sail, a spacecraft can orbit closer to the sun and use its sail to maintain its orbit by making constant adjustments.
There’s also plenty of room for improvement in solar sailing technologies. Researchers are eager to explore ideas such as the use of lasers to push at sails and the development of more sophisticated steering systems. Betts compared the LightSail program to learning to crawl, with the next wave of new technologies enabling the equivalent of walking.
In the extremely long term, one use for solar sailing is the potential to visit other star systems. Interstellar travel is still generations and generations away from being anything like a real possibility, with major technical challenges standing in the way of exploring beyond our solar system. Chemical propulsion systems like those used on the Voyager probes, the most distant man-made objects, would take tens of thousands of years to visit the nearest star systems. But spacecraft with solar sails could potentially reduce that timescale as they can continue to accelerate as they travel. “It’s the only technology we’ve got so far that shows anything resembling practical ability to do that someday,” Betts said.
As for more immediate uses, the most likely use of solar sailing isn’t that it will replace chemical propulsion systems but that it’ll be a viable option for use in certain specific missions.
“In 10 or 20 years, when people are planning a mission, they’ll think about, ‘Hey, would solar sailing work for that?’ And for some of them, it will,” Betts said. “It’ll be part of the real choices for a science mission. And that’s what we’ve contributed to with one step in the pioneering of this.”