After three years in space, the Planetary Society’s LightSail 2 mission burned up in the atmosphere on Thursday, November 17. 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 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 that you can use,” said Bruce Betts, LightSail’s chief scientist and program manager.
LightSail 2 operated in Earth orbit, while future solar sail missions will likely be in deep space. This makes the requirements somewhat different. “We had the challenge of sailing a sailboat in the harbor and out into the ocean,” Betts said. LightSail had an easier time with issues like communications than deep space missions, but had to constantly turn to stay in orbit.
The biggest drawback to using solar sailing for space exploration is that the forces involved are so small that the craft start traveling very slowly. “The downside is that you’re using a boost from the sun that’s actually very small,” Betts explained. “The thrust on our whole sail if it were perfectly reflected, from the sun at a distance from the Earth, is about the same force as a housefly has when it sits on your hand and pushes down.” But the advantage of the method is that this power simply accumulates over time, allowing acceleration up to high speeds without consuming fuel.
Upcoming programs such as NASA’s NEA Scout and the Advanced Composite Solar Sail System (ACS3) will also use solar sails. Ideal targets for such missions are within the inner solar system, as the craft remain close enough to the sun to get enough power from sunlight to keep moving and change orbit.
Another type of exploration that solar sailing enables is entering orbits that would otherwise be impossible using conventional propulsion systems. Missions orbiting the sun, for example, currently have to sit in very specific areas called Lagrange points to be in a stable orbit. Trying to get closer to the sun will 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 is also much room for improvement in solar sailing technologies. Researchers are keen to explore ideas such as using lasers to propel sails and developing 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.
On an extremely long-term basis, one use of solar sailing is the ability to visit other star systems. Interstellar travel is still generations and generations away from being anything like a real possibility, with significant technical challenges standing in the way of exploration beyond our solar system. Chemical propulsion systems like those used in 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 timeline, since they can continue to accelerate as they travel. “It’s the only technology we have so far that shows anything resembling a practical ability to do it someday,” Betts said.
As for more immediate uses, the most likely use of solar sailing is not that it will replace chemical propulsion systems, but that it will be a viable option for use in some specific missions.
“In 10 or 20 years, when people are planning an expedition, they’re going to think, ‘Hey, would solar sailing work for this?’ And for some of them, it will,” Betts said. “It will be part of the real options for a science mission. And in this we contributed with a step in the vanguard of this”.